Voltage wand

In many applications for checking electrical circuits, machinery, and power sources, in the home, in the workshop or laboratory, or in the field, it is desirable to quickly determine if a voltage is present. The voltage wand comprises a high input impedance C-MOS amplifier circuit attached to a conductive probe for detecting the presence of an AC, pulsed DC or DC voltage. The C-MOS amplifier circuit is connected to a light emitting diode which lights to indicate the presence of a voltage. To detect AC or pulsed DC, the probe is brought into proximity but not contact with the voltage source and the voltage field is picked up by the probe. To detect DC, a circuit is completed through the voltage source and voltage wand.

BACKGROUND OF THE INVENTION 
The invention relates generally to instruments for electrical measurement 
and more particularly to instruments for detecting the presence of AC, 
pulsed DC and DC voltages. 
There are many situations, both industrial and in the home, when it is 
desirable to be able to detect the presence of an AC, pulsed DC or DC 
voltage. In the home, checking the 110 volt house voltage may be necessary 
in locating and repairing household appliances. The telephone ring circuit 
is powered by AC voltage as are 70 volt audio lines. The car ignition 
system is a pulsating DC system. An AC chopper may be utilized as a power 
source, e.g., in a battery operated forklift. 
Thus there are a variety of potential users for an instrument to detect the 
presence of an AC, pulsed DC or DC voltage, from the homeowner to the 
engineer. The users would particularly include electricians, automotive 
mechanics working on automobile electrical systems, and air conditioning 
repairmen. The instrument will be useful for locating and diagnosing 
problems and lowering repair costs. In particular, an instrument for 
detecting the presence of voltages would promote safety. The instrument 
could be used to detect ungrounded fixtures and to indicate that high 
voltage lines were live. 
It is desirable to have an instrument which provides a quick check of 
whether a voltage is present or not before more extensive measurements are 
undertaken. The most effective type of instrument would have the 
characteristics of small size and ease of use. The instrument would 
ideally not have to make physical contact with the conductor, since in 
many cases the conductor might not be readily accessible, or might be 
sheathed with insulation, or it would be safer not to directly contact an 
unknown source of potentially high voltage. The instrument should also be 
low cost to be available to the many potential users. 
In general instruments are not available which meet all these requirements. 
Conventional voltmeters are available for measuring voltages, i.e., the 
potential between two points in a circuit. These are complicated 
instruments and require physical contact with the voltage source being 
measured. Other instruments are available to provide continuity checks in 
a circuit; however, these devices also require contact between different 
points in the circuit. Examples of these types of instruments are 
described in U.S. Pat. No. 3,704,411 to Jamieson issued Nov. 28, 1972 
relating to a portable device for testing electrical appliances; U.S. Pat. 
No. 4,150,330 to Hudson et al issued Apr. 17, 1979 describing a fuse 
tester and flashlight; U.S. Pat. No. 3,227,948 to Cheshire issued Jan. 4, 
1966 for a multipurpose flashlight structure; U.S. Pat. No. 3,753,090 to 
Tomek issued Aug. 14, 1973 relating to a combination flashlight and 
continuity tester; and U.S. Pat. No. 2,916,699 to Eisenberg et al issued 
Dec. 8, 1959 for a combination probe and continuity tester. Other 
apparatus is available which is large, bulky and complicated, is not 
portable, and requires an on/off switch to prevent rapid draining of 
batteries. 
Accordingly, it is an object of the invention to provide an instrument for 
detecting the presence of AC, pulsed DC, or DC voltages. 
It is also an object of the invention to provide an instrument which 
detects AC or pulsed DC voltages without any physical contact with the 
conductor. 
It is another object of the invention to provide an instrument which is 
simple, small in size, portable, easy to use, and low cost. 
SUMMARY OF THE INVENTION 
The invention is an instrument for detecting the presence of an AC, pulsed 
DC, or DC voltage. The instrument has a conductive probe which is 
connected to the input of a C-MOS device. The output of the C-MOS device 
can be used to directly drive a light emitting diode (LED) or is input 
into a transistor whose output drives the LED. The C-MOS device and 
transistor are powered by batteries. In operation for an AC or pulsed DC 
voltage, the probe is covered with a nonconductive cap and brought into 
proximity with the voltage. The probe picks up the voltage field and the 
signal is amplified to drive the LED, turning the light on to indicate the 
presence of a voltage. In operation for DC, the probe must be brought into 
contact with the conductor and the person must touch a clip connected to 
the negative side of the amplifier circuit and also the opposite side of 
the voltage source to complete the circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is an instrument for detecting the presence of AC or pulsed 
DC voltage without physical contact with the conductor by picking up and 
amplifying the AC voltage fields produced. The invention can also be used 
to detect the presence of a DC voltage by completing a circuit through the 
instrument to the conductor. The instrument detects an AC voltage or 
pulsed DC voltage by means of a conductive probe which picks up the 
voltage field surrounding the conductor and by means of an amplifier, a 
C-MOS integrated chip, which is sensitive to the voltage field picked up 
by the probe, amplifying the signal to drive an indicator means. 
The invention is illustrated schematically in FIG. 1. The main component of 
the invention is a very high input impedance C-MOS digital device 10 which 
is powered by battery means 12. The input of C-MOS device 10 is connected 
to the probe through high impedance limiting resistor 14. The output of 
C-MOS device 10 is connected to the input of a transistor 16 whose output 
is connected to a light emitting diode (LED) 18. The transistor 16 and LED 
18 are also connected to the battery means 12. A reversed biased diode 20 
is also connected across the input of the C-MOS device 10. The negative 
side of the circuit is also connected to an external conducting clip for 
operation in detecting DC voltages. The C-MOS device 10 is responsive to 
the AC or DC voltage picked up by the probe and produces an output to 
drive the transistor. Transistor 16 is a current amplifier to drive LED 
18. The advantage of the C-MOS device 10 is that it is an extremely high 
input impedance amplifier, about 10.sup.12 ohms. Since an integrated chip 
is a digital device, with the characteristic of being on or off, it is 
superior to an analog device which uses more current. The high impedance 
limiting resistor 14, typically 10 megohms, is used to protect the C-MOS 
device 10 and for the safety of the operator when the instrument is used 
to detect DC voltages. The diode 20 is connected to the input of C-MOS 
device 10 in a reverse bias mode to function as a high impedance resistor 
to provide a leakage path back to the negative side of the circuit to 
stabilize the C-MOS device 10, i.e., to hold the digital circuit in one 
state (low) so LED 18 will not always be on, draining the battery means 
12. The C-MOS device 10 has a low idle current so when LED 18 is off no 
current is drawn providing a long battery life for battery means 12 and 
eliminating the requirement for an on/off switch. 
One embodiment of the invention is illustrated in FIGS. 2 and 3. A voltage 
wand 28 comprises a nonconductive, substantially cylindrical, casing 30. 
The casing 30 contains a pair of batteries 32 and 34 in series which are 
electrically connected to amplifier assembly 36. The amplifier assembly 36 
is shown schematically as a package that contains the C-MOS device 10, 
limiting resistor 14, transistor 16 and diode 20 as shown in FIG. 1. The 
conducting probe 38, which is connected to the input of C-MOS device 10 
through limiting resistor 14 in the amplifier assembly 36 extends through 
the narrow front piece 40 attached to casing 30. The probe 38 is a metal 
rod or wire or other conductor. A removal cap 42 of nonconductive material 
attaches to the front piece 40 to cover the probe 38. A piece of 
conductive foam 44 is held in the front of the cap 42 and makes electrical 
contact with the probe 38. The conductive foam 44 is mounted in the cap 42 
and so is removed from the probe when the cap 42 is removed. The LED 46 is 
electrically connected to the amplifier assembly 36 and extends through 
the casing 30. A metallic clip 48 is mounted to the casing 30. A 
conductive contact 50 extends through the casing and electrically connects 
the clip 48 to the negative side of the circuit, e.g., the casing of 
battery 32. 
The C-MOS device 10 can be selected from CD-4050, CD-4049, MC-4050, MC-4049 
and CD-4107; almost any other C-MOS device can be utilized. The batteries 
32 and 34 are typically two 11/2 volt batteries; generally a minimum of 3 
volts is required, however up to 18 volts can be utilized. Two connections 
of the C-MOS device 10 are connected to the batteries while the input of 
C-MOS device 10 is connected to the probe and the output is connected to 
transistor 16. The transistor 16 can be a 2N222 or PN222; however, 
generally any NPN transistor can be used. The LED utilized is a Sanyo 
1368-10.P; however, other LEDs can be utilized. If a smaller LED or a 
higher output C-MOS device 10 is utilized, the transistor 16 may be 
eliminated altogether. Alternatively, other indicator means than a LED 
could be utilized. 
The voltage wand 28 is assembled in the casing 30, such as a pen casing or 
other casing of nonconductive material. This provides a convenient 
physical size so that the voltage wand can be carried in the pocket of the 
operator. The elements can be permanently potted into the casing 30 or 
removably inserted through an end cap 52, which would permit replacement 
of the batteries 32 and 34. However, since the device places little drain 
on the batteries, the batteries will have a long lifetime and the voltage 
wand can be essentially a throw-away device. 
In operation, when checking for an AC or pulsed DC voltage the cap 42 is 
kept on the casing 30, bringing the conducting foam 44 into contact with 
the probe 38. The tip 42 is placed near the voltage to be detected. The 
conductive foam 44 provides a larger surface area for the probe 38 to pick 
up the AC voltage, making the orientation of the probe less critical. The 
voltage signal is picked up by the probe, amplified by the circuit, 
turning the LED on to show the presence of a voltage. 
To detect a DC voltage the cap 42 is removed, thereby also removing the 
conductive foam 44 from the probe 38. The probe 38 is placed into contact 
with the conductor. The operator must hold the clip 48 and also touch the 
negative side of the voltage source to complete the circuit. The 10 megohm 
limiting resistor prevents the operator from being exposed to a high 
current. Alternatively, the clip can be connected to the voltage source by 
means of a jumper wire, eliminating the need for the operator to use his 
body to complete the circuit. With the probe making contact with the 
conductor the return path is through the clip, through the operator, to 
the negative side of the DC source. DC voltages as low as 3 volts can be 
detected. 
For safety reasons and for practical purposes, for the types of circuits 
and sources that are likely to be checked with this instrument, the normal 
range for detecting AC voltages is 70 to 500 volts, and for DC voltages it 
is 3 to 48 volts. However, the instrument can function outside these 
ranges, which are only suggested practical limits for the operation. A low 
AC voltage may be detected in the DC mode of operation. 
The instrument has an additional self-check feature. By rubbing the tip 42 
on the operator's sleeve static electricity will be produced which will 
cause the LED to light up, showing that the instrument is operating. 
The circuits as described is designed to detect the presence of positive 
voltages. However, by a simple change, connecting the LED 18 between the 
amplifier output and the positive side of the battery and the diode 20 
between the input and positive side of the battery, the instrument can be 
configured to detect negative voltages. 
Changes and modifications in the specifically described embodiments can be 
carried out without departing from the scope of the invention which is 
intended to be limited only by the scope of the appended claims.