Abstract:
A switchless, hand-held probe and method for detecting and alerting a user to the presence of an AC voltage on a conductor that minimizes the intermittent activation of the probe&#39;s indicator due to static charge build-up. The probe comprises an antenna, an indicator, detector circuitry and activation circuitry. The probe alerts a user through the use of an indicator to the presence of electrical energy on a conductor. The antenna senses the electrical energy radiated from the conductor. When the electrical energy sensed by the antenna satisfies a particular measurement threshold, a signal is generated by the detector circuitry and received by the activation circuitry. The activation circuitry activates the indicator after a sufficient number of signals are received from the detector circuitry during a given period of time.

Description:
[0001]     This application is a continuation of U.S. patent application Ser. No. 10/101,578 filed on Mar. 20, 2002, which is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to the field of test equipment, and more particularly to a hand-held probe to sense the presence of an alternating current (AC) signal voltage on a conductor.  
       BACKGROUND ART  
       [0003]     Hand-held electrical probes are known in the art for detecting the presence of AC signal potential on a conductor. The probes are either of a contacting type which requires direct electrical contact with an uninsulated portion of the conductor, or of a noncontacting type which senses the resulting electromagnetic field when placed in proximity to the conductor. There is a safety risk associated with the contacting type probe due to the possibility that the user may receive a harmful electrical shock, so that the noncontacting type probe is preferable.  
         [0004]     Since both type probes use a visual and/or audible indicator to annunciate the presence of a detected voltage potential to a user, many of the probes have manual on/off switches that allow a user to deactivate the probe when not in use. The switch, however, presents a number of disadvantages. If the switch is made of metal, it can act as a conduit to shock and injure the user if the switch comes in contact with a “live” wire or circuit. A user may forget to turn the switch on before using it, thereby risking a false negative reading, or a user may forget to turn it off causing the batteries to run down. A manual on/off switch can also break with repeated usage over an extended period of time. Therefore, it is preferable for a user to have a probe that does not rely on the use of a manual on/off switch.  
         [0005]     U.S. Pat. No. 5,103,165 discloses an improved, switchless type probe that may be contacting or noncontacting. In a best mode embodiment the &#39;165 probe housing comprises non-conductive material, such as polyvinylchloride (“PVC”), to reduce the risk of electrical shock for a user, and it eliminates the use of a switch through the use of internal circuitry that automatically activates the probe when it senses an electrical current or voltage, thereby reducing the drain on the batteries. The probe disclosed in the &#39;165 patent provides a visual indicator to alert the user to the presence of an electrical current or voltage, but does not have an audible indicator. The insulated housing of the &#39;165 probe, however, presents a problem with static charge build-up, which causes flickering of the visual indicator and would cause chirping of an audible indicator. The static build-up may occur when the probe is rubbed on cloth, such as a user&#39;s pocket or sleeve. This intermittent flickering and/or chirping may be annoying to the user, and it would also drain the batteries.  
       DISCLOSURE OF INVENTION  
       [0006]     The present invention is to a switchless, hand-held probe for detecting and annunciating the presence of AC voltage signals on a conductor, which incorporates filtering circuitry that minimizes intermittent activation of the probe&#39;s indicator due to static charge build-up. The probe of the present invention comprises an antenna adapted to sense the electrical energy radiated by the AC voltage on the conductor, detector circuitry responsive to the sensed electrical energy at the antenna for providing an electrical signal when the electrical energy at the antenna satisfies certain magnitude and duration thresholds, an indicator to alert a user to the presence of the sensed electrical energy, and activation circuitry connected to the detector circuitry and adapted to activate the indicator only when a sufficient number of signals are received from the detector circuitry during a period of time, thus minimizing the intermittent activation of the probe&#39;s indicator due to static charge build-up and any associated drain on the batteries thereby.  
         [0007]     In the present hand-held probe, the detector circuitry establishes energy thresholds concerning the electrical energy sensed by the antenna. The detector circuitry filters both the magnitude and frequency of the sensed electrical energy. When the sensed electrical energy satisfies the energy thresholds, an electrical signal, which may vary in frequency and duration, is produced by the detector circuitry and received by the activation circuitry. The activation circuitry functions as a switch by activating the indicator only when a sufficient number of signals are received from the detector circuitry during a given period of time. If enough signals are not received by the activation circuitry from the detector circuitry, the indicator will not be activated.  
         [0008]     In one embodiment of the invention, the probe includes an antenna adapted to sense the electrical energy radiated from a conductive member when the probe is positioned adjacent the member; an indicator adapted to alert a user when activated; detector circuitry comprising a first inverter, connected to the antenna, adapted to respond to a range of voltages sensed on the antenna, and a second inverter, connected to the first inverter, adapted to respond each time the output voltage of the first inverter changes; and activation circuitry comprising a capacitor, connected to the second inverter and indicator, adapted to charge on receipt of the signal from the second inverter for activating the indicator when the capacitor is charged sufficiently, and an oscillator adapted to periodically discharge the capacitor.  
         [0009]     These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying Drawing. 
     
    
     BRIEF DESCRIPTION OF DRAWING  
       [0010]      FIG. 1  is a perspective illustration of one embodiment of a hand-held electrical probe according to the present invention;  
         [0011]      FIG. 2  is an exploded view of the embodiment of  FIG. 1 ;  
         [0012]      FIGS. 3A and 3B  are a schematic diagram of circuitry used in the embodiment of  FIGS. 1 and 9 ;  
         [0013]      FIG. 4  is a signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 3A and 3B ;  
         [0014]      FIG. 5  is another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 3A and 3B ;  
         [0015]      FIG. 6  is still another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 3A and 3B ;  
         [0016]      FIG. 7  is still another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 3A and 3B ;  
         [0017]      FIG. 8  is still another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 3A and 3B ;  
         [0018]      FIG. 9  is a perspective illustration of another embodiment of a hand-held electrical probe according to the present invention;  
         [0019]      FIG. 10  is an exploded view of the embodiment of  FIG. 9 ;  
         [0020]      FIG. 11  is a vertical sectional view of the invention of  FIG. 9  taken along the lines as shown in  FIG. 9 ;  
         [0021]      FIGS. 12A and 12B  are a schematic diagram of circuitry used in the embodiment of  FIGS. 1 and 9 ;  
         [0022]      FIG. 13  is a signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 12A and 12B ;  
         [0023]      FIG. 14  is another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 12A and 12B ;  
         [0024]      FIG. 15  is still another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 12A and 12B ;  
         [0025]      FIG. 16  is still another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 12A and 12B ; and  
         [0026]      FIG. 17  is still another signal waveform illustration that is used in the description of operation of the circuitry of  FIGS. 12A and 12B . 
     
    
     DETAILED DESCRIPTION  
       [0027]     Referring to  FIG. 1 , a hand-held electrical probe is generally shown at  100 . The probe  100  includes a probe tip  102  attached to one end of a hollow housing  104  and a cap  106  threadably attached on an opposite end of the housing  104 . Sound holes  107  pass through the probe tip  102  to an audible indicator in the interior so that sound may pass when the audible indicator is activated. A clip  108  is connected to the housing  104  and is used to attach the probe  100  to an object such as a user&#39;s pocket.  
         [0028]     Referring now to  FIG. 2 , the probe  100  is illustrated in an exploded view in more detail. The probe tip  102  has a hollow interior and tip extension  200 . The probe tip  102  is formed of light-transmissive polyvinylchloride (PVC) material. The hollow interior of the housing  104  and probe tip  102  contain a circuit board  202 . The circuit board  202  is mounted in the housing  104  in a manner that is well known to those skilled in the art. The circuit board  202  includes circuitry (described in further detail hereinafter and shown in  FIGS. 3A and 3B ), including an integrated circuit and an electromagnetic field indicator in the form of a light-emitting diode  204  and an audible indicator  206 , or colloquially known as a “buzzer”. In this embodiment, the indicators may be activated by the application of power. An antenna  212 , which is electrically conductive and formed of brass in this embodiment, is connected to the circuit board  202  at one end. The opposite end of the antenna  212  is embedded in the tip extension  200  of the probe tip  102  so as to detect electromagnetic fields without contacting the source of the energy. The circuit board  202  is connected to the antenna  212  in order to receive signals indicating the presence of a detected electromagnetic field.  
         [0029]     In order to reduce the possibility of electrical shock to the user, the probe tip  102 , housing  104 , and cap  106  of the probe  100  are formed entirely of a non-conductive material, such as an impact-resistant plastic, which is polyvinylchloride (PVC) in this embodiment. The probe tip  102  and housing  104  are attached by an adhesive.  
         [0030]     The housing  104  contains two small batteries, shown at reference numerals  214  and  216 , for providing power to the circuitry of the probe  100 . In this embodiment the batteries are each 1.5 volts. A pole at one end of the battery  216  is engaged with a front contact (not shown) attached near the right end of the circuit board  202 . The batteries  214  and  216  have low and high voltage outputs. The front contact is a leaf-type spring in this embodiment. A flexible contact  208  is attached to the left end of circuit board  202  and is connected to the circuitry to complete an electrical circuit through the batteries  214  and  216 . The flexible contact  208  is formed of brass in this embodiment, but may alternatively be formed of another suitable conducting material. The flexible contact  208  may be bent in one direction into contact with an adjacent pole of the battery  214  and is held in such engagement by a coil spring  210 . The flexible contact  208  is also movable into a straight-line position to permit the insertion and removal of the batteries  214  and  216  from the interior of the housing  104 .  
         [0031]     The left end of the housing  104  is provided with internal screw threads  218  that mate with external screw threads  220  of the cap  106  for attachment and removal of the cap  106  from the housing  104 . The coil spring  210  is mounted within the cap  106 , so that when the cap  106  is attached to the housing  104 , the coil spring  210  firmly engages the flexible contact  208  in firm electrical contact with the adjacent pole of battery  214 , and to provide force on the batteries  214  and  216  to push them into the housing  104  so that the pole of the battery  216  is held in firm contact with the front contact of the circuit board  202 , thus completing an electrical circuit between the batteries  214  and  216  and the circuit board  202 .  
         [0032]     As an alternative to the use of flexible contact  208 , internal screw threads  218 , and external screw thread  220 , cap  106  and housing  104  may be attached using an electrical connector having a male end and a female end. The male end of the connector is mounted to circuit board  202  and is electrically connected to circuit  300  (see  FIGS. 3A and 3B ). The female end of the connector is mounted inside the cap  106  and is electrically connected to the coil spring  210 . In this alternative embodiment, the attachment of cap  106  and housing  104  by joining the male and female ends of the connector forms an electrical circuit when the coil spring  210  firmly engages the adjacent pole of battery  214  pushing the batteries  214  and  216  into the housing  104  and in firm contact with the front contact of the circuit board  202 .  
         [0033]     Now referring to  FIGS. 3A and 3B , a schematic diagram of the circuit  300  included on the circuit board  202  as shown and located within the probe tip  102  and housing  104 . The circuit  300  has an antenna  374  to sense the presence of electrical energy radiated from an electrically conductive member, such as a wire, when the probe is positioned near the member. In this embodiment, the circuit  300  functions to alert a user to the presence of electrical energy that has a constantly changing electromagnetic field. The circuit includes two indicators, an audible indicator  302  and a visual indicator in the form of an LED  304 , that alert the user to the presence of certain electrical energy when activated. Furthermore, the circuit  300  includes circuitry connected to the antenna  374  to activate the indicators when the electrical energy sensed by the antenna  374  satisfies a measurement threshold for a certain period of time. In this embodiment, the measurement threshold is satisfied when the radiated electrical energy from the member is between approximately 50 to 400 hertz and is greater than approximately 40-50 volts. Activation circuitry is provided in circuit  300  that receives a signal each time the threshold conditions are met. If enough signals are received during a period of time, the indicators are activated.  
         [0034]     In this embodiment, the circuit  300  uses a 3.0 volt power source, which is provided by the batteries  214  and  216  shown in  FIG. 2 . The batteries  214  and  216  connect to the circuit  300  at the nodes  308  (BAT+) and  310  (BAT−).  
         [0035]     Antenna  374  senses electrical energy on an electrically conductive member when positioned near the member. The antenna  374  is connected to node  314  (N 1 ). Electrical energy sensed by the antenna  374  causes a voltage to be produced at node  314 . It is important that circuit  300  is sensitive only to certain electrical energy. In this embodiment, the configuration of resistor  306  (R 1 ) and inverter  316  (I 1 ) causes the circuit  300  to be sensitive only to the electrical energy that is detected by the antenna  374  above a certain voltage. In the preferred embodiment, resistor  306  is a 330 megaohm resistor and is connected between nodes  314  and  310 . In this embodiment, the resistor  306  causes the circuit  300  to be sensitive to detected voltages that are greater than approximately 40-50 volts.  
         [0036]     The circuit  300  includes an integrated circuit (IC) that contains several inverters shown individually on the schematic diagram of  FIGS. 3A and 3B  at reference numerals  316 ,  318 ,  320 ,  322 ,  324 , and  326 . The IC has the ability to perform many functions otherwise achievable by individual electrical components. The inverters ( 316 ,  318 ,  320 ,  322 ,  324  and  326 ) in this embodiment are CMOS Schmitt Trigger inverters. CMOS type circuits are known for low power consumption. The IC used in this embodiment can come from a variety of different companies such as Fairchild, Motorola, Texas Instrument or Toshiba. The batteries  214  and  216  supply power to the IC by connection at the appropriate pins on the IC as is known in the art. Additionally, the batteries  214  and  216  determine the threshold voltages and output for the inverters. For reference hereinafter, the high threshold voltage to an inverter is ⅔ the voltage of the power supplied to the IC, or 2.0 volts in this embodiment. Therefore, an input high to an inverter is a voltage greater than 2.0 volts in this embodiment. The low threshold voltage is ⅓ the voltage of the power supplied to the IC, or 1.0 volts in this embodiment. An output high from an inverter is approximately 3.0 volts, and an output low from an inverter is approximately zero. Furthermore, the batteries  214  and  216  provide power to the audible indicator  302  and LED  304  when activated.  
         [0037]     Resistor  312  (R 2 ) is a 100 megaohm resistor and is connected between node  314  and the input to inverter  316 . The resistor  312  on the input to the inverter  316  serves to prevent high input current from damaging the IC.  
         [0038]     When the antenna  374  is in close proximity to an electromagnetic field, a voltage is produced at node  314  and at the input to inverter  316 . When antenna  374  is not in the presence of an electromagnetic field (or a very small field), the output of the inverter  316  will be high because of the low voltage present at the input. When a signal with a voltage magnitude greater than the high threshold of the inverter  316  is present on the antenna  374  the inverter&#39;s output will be low. The amount of the signal voltage that appears at the input of inverter  316  is dependent upon the capacitance between the antenna  374  and the signal source (usually a wire), the resistance of resistor  306  and  312 , and the input capacitance of the inverter  316 .  
         [0039]     Capacitor  328  (C 1 ) and resistor  332  (R 3 ) provide a high pass filter and are connected in a configuration between inverter  316  and inverter  318  (I 2 ) in order to provide AC coupling and function as a differentiator. Capacitor  328  is a 0.01 microfarad capacitor that is connected between the output of inverter  316  and node  330  (N 2 ). Resistor  332  is a 470 kiloohm resistor that is connected between nodes  330  and  308 . The time constant of the capacitor  328  and resistor  332  is given by the multiplication of the capacitance value of capacitor  328  and the resistance value of resistor  332 , which is 4.7 milliseconds. This feature functions to enable the circuit  300  to respond to only continuous time varying signals. This feature is helpful during operation, for example, when the distance varies between the probe tip  102  (see  FIG. 1 ) and a wire that is probed, thus, causing a varying sensed signal. This varying of the distance, possibly due to a user moving the probe, could potentially cause the voltage at inverter  316  to vary enough to give an inaccurate reading. The differentiator is described in more detail hereinafter.  
         [0040]     The output of inverter  318  changes in response to certain changes in the input of inverter  316 . A voltage magnitude greater than the high threshold for inverter  316  will cause the output to inverter  316  to go high at certain frequencies. As discussed hereinbefore, this change will affect the voltage present at node  330  if it passes the differentiator configuration of capacitor  328  and resistor  332 . Therefore, the configuration of inverter  316 , resistor  306 , capacitor  328 , and resistor  332  serves to filter voltages present at node  314  (voltages from the antenna  374 ) that are at a certain minimum voltage and a certain frequency. Additionally, that configuration provides a logic input to inverter  318  when the filter conditions are met. When the filter conditions are met, the output of inverter  318  at node  348  (N 4 ) will change from low to high. Otherwise, the output of inverter  318  is low. The operation of inverter  316 , inverter  318 , and the differentiator are described in more detail hereinafter.  
         [0041]     Resistor  334  (R 4 ) is a 1.0 megaohm resistor that is connected between node  330  and the input to inverter  318 . The resistor  334  serves to prevent a high input current from damaging the IC.  
         [0042]     When the output to inverter  318  is high, capacitor  336  (C 2 ), which is a 0.33 microfarad capacitor, is charging. The capacitor  336  is charged through diode  342  (D 1 ), resistor  338  (R 5 ), and resistor  340  (R 6 ) in parallel with resistor  344  (R 7 ). The voltage on the capacitor during the time the capacitor  336  is charging is determined by the following equation (wherein V is the capacitor voltage, Vcc is the battery voltage (3.0 volts), Vc is the voltage existing on the capacitor  336 , t is the time the capacitor is being charged, R is the resistance of resistors  338  and  340 , and C is the capacitance of capacitor  336 : 
 
 V =( Vcc−Vc )*(1− e {circumflex over ( )}(− t ( R*C ))) 
 
 While the capacitor is being charged, its voltage increases exponentially with time. 
 
         [0044]     The anode of diode  342  is connected to node  348 . Diode  342  serves to prevent capacitor  336  from discharging into the inverter  318 . Resistor  344  is a 2.2 megaohm resistor that is connected between node  348  and node  346  (N 3 ). Resistor  344  functions to allow capacitor  336  to slowly discharge back into inverter  318 .  
         [0045]     Resistor  338  is a 47.0 kiloohm resistor that is connected between the cathode of diode  342  and node  350  (N 5 ). Resistor  340  is a 47.0 kiloohm resistor that is connected between nodes  350  and  346 . Resistors  338  and  340  determine the amount of charge added to capacitor  336  when a pulse of voltage is received from inverter  318 .  
         [0046]     The inputs to inverter  320  (I 3 ) and inverter  322  (I 4 ) are each connected to node  346 , and the outputs of inverter  320  and  322  are connected to node  352  (N 6 ). A 1.5 kiloohm resistor  376  (R 12 ) is connected to node  352  and the base of bipolar junction transistor (BJT)  368  to limit the current into BJT  368 .  
         [0047]     The outputs to inverters  320  and  322  are high while the voltage on capacitor  336  is below the high threshold. When the capacitor  336  voltage is above the threshold voltage, their outputs are low. As detailed hereinafter, a low output on inverters  320  and  322  causes audible indicator  302  to sound and LED  304  to illuminate. Inverters  320  and  322  apply power through BJT  368  to the indicators  302  and  304 .  
         [0048]     An oscillator  354  is connected to capacitor  336  through resistor  340  and diode  364  (D 2 ). The oscillator  354  serves as a timer to limit the amount of time that capacitor  336  is charged. The indicators will be activated only on the condition that capacitor  336  is charged beyond a certain threshold voltage during a period of time set by the oscillator  354 . The oscillator  354  is an astable multivibrator that continuously produces an output signal that has a period determined by resistor  356  (R 8 ), resistor  358  (R 9 ), capacitor  362  (C 3 ), inverter  324  (I 5 ), and inverter  326  (I 6 ). The oscillator  354  produces a non-symmetrical square wave with a peak-to-peak voltage of 3.0 volts. The capacitor  336  discharges when the oscillator&#39;s output is low. Therefore, in order for the indicators to activate, the voltage across capacitor  336  must be greater than the threshold voltage before oscillator  354  goes low. The capacitor  336  will charge only when the oscillator is high and the output from inverter  318  is high. In this embodiment, the time period that the oscillator&#39;s output is low is approximately 174 milliseconds, and the time period that the oscillator&#39;s output is high is approximately 290 milliseconds in this embodiment. The oscillator  354  discharges through resistor  340  and diode  364  when the oscillator&#39;s output is low. Diode  364  serves to keep the oscillator&#39;s output from charging capacitor  336  when the oscillator&#39;s output is high.  
         [0049]     In this embodiment, resistor  356 , resistor  358 , and capacitor  362  are 180 kiloohms, 270 kiloohms, and 1.0 microfarads respectively. The oscillator  354  further includes resistor  360  (R 10 ), which is 1.0 megaohm resistor, and diode  366  (D 3 ). Resistor  360  functions to limit the current into the inverter  324 . When the output of the oscillator  354  is low, capacitor  336  is discharged through resistor  340  and diode  364 . This reduces the voltage on capacitor  362  below the low threshold causing the output of inverters  320  and  322  to go high. The time required for capacitor  336  to discharge to the low threshold of inverter  320  and  322  is given by the following equation (wherein Vc is the voltage on capacitor  336  before discharge, R is the resistance of resistor  340 , C is the capacitance of capacitor  336  and V is the low threshold voltage of the inverter): 
 
 T=−R*C*In ( V/Vc ) 
 
         [0050]     Referring now to  FIG. 4 , a voltage versus time diagram illustrates the voltage across capacitor  336  over time while there is a 60 Hz signal at the antenna  374  above the voltage threshold. The voltages at which the inputs to inverters  320  and  322  are at high and low threshold are shown at reference numerals  400  and  404  respectively. The period of time that begins when the voltage goes to the high threshold of the inverter  400  to the time the voltage goes to the low threshold at  404  is the time period that the indicators are activated (as described in further detail hereinafter). As this voltage varies between the low threshold and the high threshold of inverters  320  and  322 , the outputs of these inverters change from high to low and back to high. This occurs at the frequency rate of the oscillator  354 . The period of time that begins at reference number  402  to  406  indicates when the capacitor  336  is discharging. From the time referenced by  406  to  408 , it is indicated when the capacitor  336  is charging. Reference numeral  410  indicates the time period in which the inverter  318  is charging the capacitor  336  with a pulse. Reference numeral  412  indicates the time period in which the inverter  318  is not pulsing and the capacitor  336  is discharging through resistor  344 .  
         [0051]     Referring back to  FIGS. 3A and 3B , the configuration of inverter  320 , inverter  322 , and BJT  368  functions as a switch for activating the indicators. The switch is turned on when the voltage at capacitor  336  reaches the high threshold of inverters  320  and  322 . When inverter  320  and inverter  322  have a high output, no battery power is applied to audible indicator  302  or LED  304 . A low output turns on BJT  368 , thereby applying battery power to node  370  (N 7 ). The BJT  368  is connected as follows: the emitter is connected to node  308 ; the base is connected to resistor  376 ; and the collector is connected to node  370 .  
         [0052]     When battery power is applied to node  370  the indicators, audible indicator  302  and LED  304 , are activated, thereby alerting the user to the presence of a constantly changing AC voltage above a certain voltage threshold. The audible indicator  302  is connected between nodes  370  and  310 . A 150 ohm resistor  372  (R 11 ) and LED  304  are connected in series between nodes  370  and  310 . When powered, the audible indicator  302  and LED  304  activate (switch on and off) at the oscillator&#39;s frequency rate. In this embodiment, the indicators will be on for approximately 290 milliseconds and off for approximately 174 milliseconds.  
         [0053]     Referring to  FIGS. 5-8 , voltage versus time diagrams are provided to illustrate the operation of the differentiator of the present invention. These voltages indicate the voltages present at various points on the circuit  300  during the same period of time.  FIG. 5  represents a sinusoidal signal that is input to inverter  316  at node  314 . Reference numerals  500  and  504  indicate the high thresholds for inverter  316 , and reference numerals  502  and  506  indicate the low thresholds for inverter  316 . Referring now to  FIG. 6 , the output to inverter  316  is shown as a response to the input illustrated in  FIG. 5 . The output to inverter  316  is a pulse whose width is determined by the time it takes for the input signal to cross the threshold points of the inverter. When the voltage at node  314  goes higher than the high threshold, the output of the inverter  316  goes low. When the voltage at node  314  goes lower than the low threshold, the output of the inverter  316  goes high.  
         [0054]     Referring to  FIG. 7 , the output of the differentiator is shown. This figure illustrates the output of the differentiator at node  330  in response to changes in voltage over time at its input, shown in  FIG. 6 . The voltage at node  330  is the voltage input to inverter  318 . Reference numerals  702  and  706  indicate the low thresholds for inverter  318 , and reference numerals  704  and  708  indicate the high thresholds for inverter  318 . The time during which a high output is produced by inverter  318  is given by the following equation (wherein t is the time indicated in  FIG. 7  at reference numeral  700 , R is the resistance of resistor  332 , and C is the capacitance of capacitor  328 ): 
 
2=3*(1− e {circumflex over ( )}( t /( R*C ))) 
 
         [0055]     As shown in  FIG. 8 , due to the differentiator circuit, no matter how long it takes for the signal at node  314  to cross the threshold points of inverter  316 , the pulse width at the output of inverter  318  is determined by this equation.  
         [0056]     The only time that capacitor  336  charges is when the output of inverter  318  is high and the oscillator  354  output is high. Each time the antenna voltage crosses the high threshold of inverter  316 , the output of inverter  318  goes high for a maximum time determined by the time constant of capacitor  328  and resistor  332 . This differentiator along with the accumulating action of capacitor  336 , and the sampling of the oscillator  354  assures that the probe responds only to continuous voltages in the frequency range of approximately 50 to 400 hertz.  
         [0057]     Referring now to  FIG. 9 , another embodiment of a hand-held electrical probe is generally shown at  900 . The probe  900 , which is similar to the probe disclosed in  FIG. 1 , includes a probe tip  902  attached to one end of a hollow housing  904 . A clip  908  is connected to an opposite end of the housing  904  and is used to attach the probe  900  to an object such as a user&#39;s pocket. The opposite end of the housing  904  and clip  908  are adapted to allow for the attachment and removal of a cap  906  from the housing  904  and clip  908 . Sound holes  907  pass through the probe tip  902  to an audible indicator in the interior so that sound may pass when the audible indicator is activated.  
         [0058]     In order to reduce the possibility of electrical shock to the user, the probe tip  902 , housing  904 , and cap  906  of the probe  900  are formed entirely of a non-conductive material, such as an impact-resistant plastic, which is polyvinylchloride (PVC) in this embodiment. The probe tip  902  and housing  904  are attached by an adhesive.  
         [0059]     Referring now to  FIG. 10 , the probe  900  is illustrated in an exploded view in more detail. The probe tip  902  has a hollow interior and tip extension  1000 . The probe tip  902  is formed of light-transmissive polyvinylchloride (PVC) material. The hollow interior of the housing  904  and probe tip  902  contain a circuit board  1002 . The circuit board  1002  is mounted in the housing  904  in a manner that is well known to those skilled in the art. The circuit board  1002  includes circuitry (described previously in further detail and shown in  FIGS. 3A and 3B ), including an integrated circuit and an electromagnetic field indicator in the form of a light-emitting diode  1004  and an audible indicator  1006 , or colloquially known as a “buzzer”. In this embodiment, the indicators may be activated by the application of power. An antenna  1012 , which is electrically conductive and formed of brass in this embodiment, is connected to the circuit board  1002  at one end. The opposite end of the antenna  1012  is embedded in the tip extension  1000  of the probe tip  902  so as to detect electromagnetic fields without contacting the source of the energy. The circuit board  1002  is connected to the antenna  1012  in order to receive signals indicating the presence of a detected electromagnetic field.  
         [0060]     The housing  904  contains two small batteries, shown at reference numerals  1014  and  1016 , for providing power to the circuitry of the probe  900 . In this embodiment the batteries are each 1.5 volts. A pole at one end of the battery  1016  is engaged with a front contact (not shown) attached near the right end of the circuit board  1002 . The batteries  1014  and  1016  have low and high voltage outputs. The front contact is a leaf-type spring in this embodiment.  
         [0061]     Referring now to  FIG. 11 , a vertical sectional view taken along the lines as shown in  FIG. 9  provides more details of the probe  900 . The cap  906  is shown attached to the housing  904  and clip  908 . A rear contact  1108  is housed within the cap  906  along the interior rear surface of the cap  906  as shown and one end of the rear contact  1108  is electrically connected to a coil spring  1110  that is mounted within the cap  906 . The rear contact  1108  is formed of brass in this embodiment, but may alternatively be formed of another suitable conducting material. The opposite end of the rear contact  1108  is adapted to engage the circuit board  1002  to form an electrical contact with the circuitry when the cap  906  is attached to the housing  904  and clip  908 . The attachment of the cap  906 , housing  904 , and clip  908  forms an electrical circuit when the coil spring  1110  firmly engages the adjacent pole of battery  1014  pushing the batteries  1014  and  1016  into the housing  904  and in firm contact with the front contact of the circuit board  1002 .  
         [0062]      FIGS. 12A and 12B  provide another embodiment of the circuitry as shown in  FIGS. 3A and 3B  that functions similar to circuit  300 . While the circuitry described in  FIGS. 12A and 12B  may be included on the circuit board shown in either  FIG. 2  or  FIG. 10 , for convenience, its use shall be described in connection with the probe as described and shown in  FIG. 2 . In this embodiment, circuit  1200  includes two indicators, an audible indicator  1202  and a visual indicator in the form of an LED  1204 , that function to alert the user to the presence of certain electrical energy when activated. The schematic diagram of the circuit  1200  included on the circuit board  202  that is located within the probe tip  102  and housing  104  of the probe  100  (see  FIG. 2 ). The circuit  1200  has an antenna  1274  to sense the presence of electrical energy radiated from an electrically conductive member, such as a wire, when the probe  100  is positioned near the member. The circuit  1200  activates LED  1204  when the presence of electrical energy that is sensed by antenna  1274  satisfies a certain measurement threshold. The audible indicator  1202  is activated by the circuit  1200  when presence of electrical energy is sensed by antenna  1274  that satisfies a certain measurement threshold and that satisfies that measurement threshold for a certain period of time. In this embodiment, the measurement threshold is satisfied when the radiated electrical energy from the conductive member is between approximately 50 to 400 hertz and is greater than approximately 40-50 volts. The activation of the indicators in circuit  1200  does not occur at approximately the same time, as was the case for the circuit found in  FIGS. 3A and 3B . Activation circuitry is provided in circuit  1200  that receives a signal each time the threshold conditions are met. If enough signals are received during a period of time, the audible indicator  1202  is activated.  
         [0063]     By allowing the activation of LED  1204  when the sensed electrical energy satisfies the measurement threshold, but not requiring that the sensed electrical energy satisfy the measurement threshold for a certain period of time, the probe  100  may be susceptible to intermittent activation of LED  1204  due to static charge build-up on the probe&#39;s housing as was discussed previously. This feature will enable a user to test the probe  100  before using it to make sure that it is working and to avoid the potential of a false negative reading caused by a lack of the battery power sufficient to active the indicators. The circuit  1200  allows a user to test the probe  100  by rubbing it on cloth in an effort to buildup a static charge sufficient to activate LED  1204 .  
         [0064]     In this embodiment, the circuit  1200  uses a 3.0 volt power source, which is provided by the batteries  214  and  216  shown in  FIG. 2 . The batteries  214  and  216  connect to the circuit  1200  at the nodes  1208  (BAT+) and  1210  (BAT−).  
         [0065]     Antenna  1274  senses electrical energy on an electrically conductive member when positioned near the member. The antenna  1274  is connected to node  1214  (N 1 ). Electrical energy sensed by the antenna  1274  causes a voltage to be produced at node  1214 . It is important that circuit  1200  is sensitive only to certain electrical energy. In this embodiment, the configuration of resistor  1206  (R 1 ) and inverter  1216  (I 1 ) causes the circuit  1200  to be sensitive only to the electrical energy that is detected by the antenna  1274  above a certain voltage. In the preferred embodiment, resistor  1206  is a 330 megaohm resistor and is connected between nodes  1214  and  1210 . In this embodiment, the resistor  1206  causes the circuit  1200  to be sensitive to detected voltages that are greater than approximately 40-50 volts.  
         [0066]     The circuit  1200  includes an integrated circuit (IC) that contains several inverters shown individually on the schematic diagram of  FIGS. 12A and 12B  at reference numerals  1216 ,  1218 ,  1220 ,  1222 ,  1224 , and  1226 . The IC has the ability to perform many functions otherwise achievable by individual electrical components. The inverters ( 1216 ,  1218 ,  1220 ,  1222 ,  1224  and  1226 ) in this embodiment are CMOS Schmitt Trigger inverters. CMOS type circuits are known for low power consumption. The IC used in this embodiment can come from a variety of different companies such as Fairchild, Motorola, Texas Instrument or Toshiba. The batteries  214  and  216  supply power to the IC by connection at the appropriate pins on the IC as is known in the art. Additionally, the batteries  214  and  216  determine the threshold voltages and output for the inverters. For reference hereinafter, the high threshold voltage to an inverter is ⅔ the voltage of the power supplied to the IC, or 2.0 volts in this embodiment. Therefore, an input high to an inverter is a voltage greater than 2.0 volts in this embodiment. The low threshold voltage is ⅓ the voltage of the power supplied to the IC, or 1.0 volts in this embodiment. An output high from an inverter is approximately 3.0 volts, and an output low from an inverter is approximately zero. Furthermore, the batteries  214  and  216  provide power to the audible indicator  1202  and LED  1204  when activated.  
         [0067]     Resistor  1212  (R 2 ) is a 100 megaohm resistor and is connected between node  1214  and the input to inverter  1216 . The resistor  1212  on the input to the inverter  1216  serves to prevent high input current from damaging the IC.  
         [0068]     When the antenna  1274  is in close proximity to an electromagnetic field, a voltage is produced at node  1214  and at the input to inverter  1216 . When antenna  1274  is not in the presence of an electromagnetic field (or a very small field), the output of the inverter  1216  will be high because of the low voltage present at the input. When a signal with a voltage magnitude greater than the high threshold of the inverter  1216  is present on the antenna  1274  the inverter&#39;s output will be low. The amount of the signal voltage that appears at the input of inverter  1216  is dependent upon the capacitance between the antenna  1274  and the signal source (usually a wire), the resistance of resistor  1206  and  1212 , and the input capacitance of the inverter  1216 .  
         [0069]     Capacitor  1228  (C 1 ) and resistor  1232  (R 3 ) provide a high pass filter and are connected in a configuration between inverter  1216  and inverter  1218  (I 2 ) in order to provide AC coupling and function as a differentiator. Capacitor  1228  is a 0.1 microfarad capacitor that is connected between the output of inverter  1216  and node  1230  (N 2 ). Resistor  1232  is a 100 kiloohm resistor that is connected between nodes  1230  and  1208 . The time constant of the capacitor  1228  and resistor  1232  is given by the multiplication of the capacitance value of capacitor  1228  and the resistance value of resistor  1232 , which is 10 milliseconds. This feature functions to enable the circuit  1200  to respond to only continuous time varying signals. This feature is helpful during operation, for example, when the distance varies between the probe tip  102  (see  FIG. 1 ) and a wire that is probed, thus, causing a varying sensed signal. This varying of the distance, possibly due to a user moving the probe, could potentially cause the voltage at inverter  1216  to vary enough to give an inaccurate reading. The differentiator is described in more detail hereinafter.  
         [0070]     The output of inverter  1218  changes in response to certain changes in the input of inverter  1216 . A voltage magnitude greater than the high threshold for inverter  1216  will cause the output to inverter  1216  to go high at certain frequencies. As discussed hereinbefore, this change will affect the voltage present at node  1230  if it passes the differentiator configuration of capacitor  1228  and resistor  1232 . Therefore, the configuration of inverter  1216 , resistor  1206 , capacitor  1228 , and resistor  1232  serves to filter voltages present at node  1214  (voltages from the antenna  1274 ) that are at a certain minimum voltage and a certain frequency. Additionally, that configuration provides a logic input to inverter  1218  when the filter conditions are met. When the filter conditions are met, the output of inverter  1218  at node  1248  (N 4 ) will change from low to high. Otherwise, the output of inverter  1218  is low. The operation of inverter  1216 , inverter  1218 , and the differentiator are described in more detail hereinafter.  
         [0071]     Resistor  1234  (R 4 ) is a 1.0 megaohm resistor that is connected between node  1230  and the input to inverter  1218 . The resistor  1234  serves to prevent a high input current from damaging the IC.  
         [0072]     The input to inverter  1220  (I 3 ) is connected to node  1248 . A 1.0 kiloohm resister  1280  (R 13 ) is connected between the output of inverter  1220  and the base of bipolar junction transistor (BJT 2 )  1278  to limit the current into BJT 2   1278 . A 150 ohm resistor  1272  (R 11 ) and LED  1204  are connected in series between the collector of BJT 2   1278  and node  1210 . The emitter of BJT 2  is connected to node  1208 . BJT 2  functions as a switch for activating LED  1204 . The switch is turned on when the filter conditions are met, as was discussed previously, and the output voltage of inverter  1218  reaches the high threshold of inverter  1220 . When inverter  1220  has a high output, no battery power is applied to LED  1204 . A low output turns on BJT 2 , thereby applying battery power to resistor  1272  and LED  1204 . This design allows for the activation of LED  1204  when the sensed electrical energy satisfies the measurement threshold, but it does not require that the sensed electrical energy satisfy the measurement threshold for a certain period of time as was the case in the circuit found in  FIGS. 3A and 3B .  
         [0073]     When the output to inverter  1218  is high, capacitor  1236  (C 2 ), which is a 0.33 microfarad capacitor, is charging. The capacitor  1236  is charged through diode  1242  (D 1 ), resistor  1238  (R 5 ), and resistor  1240  (R 6 ) in parallel with resistor  1244  (R 7 ). The voltage on the capacitor during the time the capacitor  1236  is charging is determined by the following equation (wherein V is the capacitor voltage, Vcc is the battery voltage (3.0 volts), Vc is the voltage existing on the capacitor  1236 , t is the time the capacitor is being charged, R is the resistance of resistors  1238  and  1240 , and C is the capacitance of capacitor  1236 : 
 
 V =( Vcc−Vc )*(1− e {circumflex over ( )}(− t /( R*C ))) 
 
 While the capacitor is being charged, its voltage increases exponentially with time. 
 
         [0075]     The anode of diode  1242  is connected to node  1248 . Diode  1242  serves to prevent capacitor  1236  from discharging into the inverter  1218 . Resistor  1244  is a 1.0 megaohm resistor that is connected between node  1248  and node  1246  (N 3 ). Resistor  1244  functions to allow capacitor  1236  to slowly discharge back into inverter  1218 .  
         [0076]     Resistor  1238  is a 47.0 kiloohm resistor that is connected between the cathode of diode  1242  and node  1250  (N 5 ). Resistor  1240  is a 47.0 kiloohm resistor that is connected between nodes  1250  and  1246 . Resistors  1238  and  1240  determine the amount of charge added to capacitor  1236  when a pulse of voltage is received from inverter  1218 .  
         [0077]     The input to inverter  1222  (I 4 ) is connected to node  1246 , and the output of inverter  1222  is connected to node  1252  (N 6 ). A 500 ohm resistor  1276  (R 12 ) is connect to node  1252  and the base of bipolar junction transistor (BJT)  1268  to limit the current into BJT  1268 .  
         [0078]     The output to inverter  1222  is high while the voltage on capacitor  1236  is below the high threshold. When the capacitor  1236  voltage is above the threshold voltage, the inverter&#39;s output is low. As detailed hereinafter, a low output on inverter  1222  causes audible indicator  1202  to sound. Inverter  1222  applies power through BJT  1268  to the audible indicator  1202 .  
         [0079]     An oscillator  1254  is connected to capacitor  1236  through resistor  1240  and diode  1264  (D 2 ). The oscillator  1254  serves as a timer to limit the amount of time that capacitor  1236  is charged. The indicator will be activated only on the condition that capacitor  1236  is charged beyond a certain threshold voltage during a period of time set by the oscillator  1254 . The oscillator  1254  is an astable multivibrator that continuously produces an output signal that has a period determined by resistor  1256  (R 8 ), resistor  1258  (R 9 ), capacitor  1262  (C 3 ), inverter  1224  (I 5 ), and inverter  1226  (I 6 ). The oscillator  1254  produces a non-symmetrical square wave with a peak-to-peak voltage of 3.0 volts. The capacitor  1236  discharges when the oscillator&#39;s output is low. Therefore, in order for the indicator to activate, the voltage across capacitor  1236  must be greater than the threshold voltage before oscillator  1254  goes low. The capacitor  1236  will charge only when the oscillator is high and the output from inverter  1218  is high. In this embodiment, the time period that the oscillator&#39;s output is low is approximately 103.5 milliseconds, and the time period that the oscillator&#39;s output is high is approximately 290 milliseconds in this embodiment. The oscillator  1254  discharges through resistor  1240  and diode  1264  when the oscillator&#39;s output is low. Diode  1264  serves to keep the oscillator&#39;s output from charging capacitor  1236  when the oscillator&#39;s output is high.  
         [0080]     In this embodiment, resistor  1256 , resistor  1258 , and capacitor  1262  are 180 kiloohms, 100 kiloohms, and 1.0 microfarads respectively. The oscillator  1254  further includes resistor  1260  (R 10 ), which is 1.0 megaohm resistor, and diode  1266  (D 3 ). Resistor  1260  functions to limit the current into the inverter  1224 . When the output of the oscillator  1254  is low, capacitor  1236  is discharged through resistor  1240  and diode  1264 . This reduces the voltage on capacitor  1262  below the low threshold causing the output of inverter  1222  to go high. The time required for capacitor  1236  to discharge to the low threshold of inverter  1222  is given by the following equation (wherein Vc is the voltage on capacitor  1236  before discharge, R is the resistance of resistor  1240 , C is the capacitance of capacitor  1236  and V is the low threshold voltage of the inverter): 
 
 T=−R*C*In ( V/Vc ) 
 
         [0081]     In this embodiment of the circuit, the period of the output signal of oscillator  354 , as compared to the same signal in the circuit shown in  FIGS. 3A and 3B , was reduced in order to increase the rate at which the audible indicator  1202  is activated when the electrical energy sensed by antenna  1274  satisfies the measurement threshold for a certain period of time. This is accomplished by reducing the value for resistor  1258  from 270 kiloohms to 100 kiloohms. The reduction in the period of the output signal of oscillator  354  also results in the need to adjust the values of various other components in the circuit to insure that the probe will operate properly when exposed to continuous voltages at lower frequencies within the frequency range of approximately 50 to 400 hertz. The maximum time that the output of inverter  1218  goes high must be increased by adjusting the time constant of capacitor  1228  and resistor  1232  in order to allow capacitor  1236  to charge sufficiently at lower frequencies, and capacitor  1236  must be allowed to discharge more quickly back into inverter  1218  by reducing the value of resistor  1244 .  
         [0082]     Referring now to  FIG. 13 , a voltage versus time diagram illustrates the voltage across capacitor  1236  over time while there is a 60 Hz signal at the antenna  1274  above the voltage threshold. The voltage at which the input to inverter  1222  is at high and low threshold is shown at reference numerals  1300  and  1304  respectively. The period of time that begins when the voltage goes to the high threshold of the inverter  1300  to the time the voltage goes to the low threshold at  1304  is the time period that the indicator is activated (as described in further detail hereinafter). As this voltage varies between the low threshold and the high threshold of inverter  1222 , the output of the inverter changes from high to low and back to high. This occurs at the frequency rate of the oscillator  1254 . The period of time that begins at reference number  1302  to  1306  indicates when the capacitor  1236  is discharging. From the time referenced by  1306  to  1308 , it is indicated when the capacitor  1236  is charging. Reference numeral  1310  indicates the time period in which the inverter  1218  is charging the capacitor  1236  with a pulse. Reference numeral  1312  indicates the time period in which the inverter  1218  is not pulsing and the capacitor  1236  is discharging through resistor  1244 .  
         [0083]     Referring back to  FIGS. 12A and 12B , the configuration of inverter  1222  and BJT  1268  functions as a switch for activating the audible indicator  1202 . The switch is turned on when the voltage at capacitor  1236  reaches the high threshold of inverter  1222 . When inverter  1222  has a high output, no battery power is applied to audible indicator  1202 . A low output turns on BJT  1268 , thereby applying battery power to node  1270  (N 7 ). The BJT  1268  is connected as follows: the emitter is connected to node  1208 ; the base is connected to resistor  1276 ; and the collector is connected to node  1270 .  
         [0084]     When battery power is applied to node  1270  the audible indicator  1202  is activated, thereby alerting the user to the presence of a constantly changing AC voltage above a certain voltage threshold. The audible indicator  1202  is connected between nodes  1270  and  1210 . When powered, the audible indicator  1202  activates (switches on and off) at the oscillator&#39;s frequency rate. In this embodiment, the indicator will be on for approximately 290 milliseconds and off for approximately 103.5 milliseconds.  
         [0085]     Referring to  FIGS. 14-17 , voltage versus time diagrams are provided to illustrate the operation of the differentiator of the present invention. These voltages indicate the voltages present at various points on the circuit  1200  during the same period of time.  FIG. 14  represents a sinusoidal signal that is input to inverter  1216  at node  1214 . Reference numerals  1400  and  1404  indicate the high thresholds for inverter  1216 , and reference numerals  1402  and  1406  indicate the low thresholds for inverter  1216 . Referring now to  FIG. 15 , the output to inverter  1216  is shown as a response to the input illustrated in  FIG. 14 . The output to inverter  1216  is a pulse whose width is determined by the time it takes for the input signal to cross the threshold points of the inverter. When the voltage at node  1214  goes higher than the high threshold, the output of the inverter  1216  goes low. When the voltage at node  1214  goes lower than the low threshold, the output of the inverter  1216  goes high.  
         [0086]     Referring to  FIG. 16 , the output of the differentiator is shown. This figure illustrates the output of the differentiator at node  1230  in response to changes in voltage over time at its input, shown in  FIG. 15 . The voltage at node  1230  is the voltage input to inverter  1218 . Reference numerals  1602  and  1606  indicate the low thresholds for inverter  1218 , and reference numerals  1604  and  1608  indicate the high thresholds for inverter  1218 . The time during which a high output is produced by inverter  1218  is given by the following equation (wherein t is the time indicated in  FIG. 16  at reference numeral  1600 , R is the resistance of resistor  1232 , and C is the capacitance of capacitor  1228 ): 
 
2=3*(1− e {circumflex over ( )}( t /( R*C ))) 
 
         [0087]     As shown in  FIG. 17 , due to the differentiator circuit, no matter how long it takes for the signal at node  1214  to cross the threshold points of inverter  1216 , the pulse width at the output of inverter  1218  is determined by this equation.  
         [0088]     The only time that capacitor  1236  charges is when the output of inverter  1218  is high and the oscillator  1254  output is high. Each time the antenna voltage crosses the high threshold of inverter  1216 , the output of inverter  1218  goes high for a maximum time determined by the time constant of capacitor  1228  and resistor  1232 . This differentiator along with the accumulating action of capacitor  1236 , and the sampling of the oscillator  1254  assures that the probe responds only to continuous voltages in the frequency range of approximately 50 to 400 hertz.  
         [0089]     Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that various changes, omissions, and additions may be made to the form and detail of the disclosed embodiment without departing from the spirit and scope of the invention, as recited in the following claims.