Abstract:
The present invention provides for a highly reliable circuit, suitable for controlling the delivery of electrical power to a second electrical outlet, based on the power consumption of a first outlet. This circuit has application in home, small workshops and on job-sites where the activation of a power tool, necessitates the activation of a dust collecting device. Because of the compact size, other household applications are also possible. Briefly, a microcontroller monitors the current used by the first outlet. When that current exceeds a first predetermined value, the controller enables the second outlet after a first predetermined period of time. When the current drops below a second predetermined value, the controller disables the outlet after a second predetermined period of time.

Description:
[0001]     This application claims priority of U.S. Provisional application No. 60/690,779 filed Jun. 15, 2005, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     There are many instances where the use of one electrically powered device necessitates the need or desire to use a second device. One such instance exists in the industrial or home workshop. Often, after a power tool is activated, it is desirous to also activate a vacuum or other electrically powered particle collector. Most power tools have a 1¼″ or 2″ dust port for a shop vacuum connection in order to collect sawdust and wood chips. For example, the operator activates a power tool, such as a drill, a miter or table saw, and some time shortly thereafter, the vacuum automatically engages. Once the power tool has been disengaged, the vacuum automatically disengages a short time thereafter. In this way, the operator is freed from the inconvenience of having to activate and deactivate the vacuum before and after the use of a power tool. Similarly, there are other situations where this solution would be desirous. For example, a user, upon activating his television, may desire that his stereo or cable box automatically activate. Similarly, the activation of one&#39;s personal computer could be used to automatically engage the associated monitor, printer or other peripheral devices.  
         [0003]     Many of the circuits designed to address this problem suffer from one or more shortcomings. For example, some circuits activate a second electrical outlet immediately upon detecting current flow through a first outlet. This can cause circuit breakers to trip due to the large simultaneous startup currents of the two devices. To correct this, some circuits utilize an analog delay mechanism, such as an RC time constant to offset the activation times of the two devices. While this alleviates the problems associated with simultaneous activation, such circuits are inflexible, only capable of a predetermined delay, which is only alterable through changes to the components on the circuit board. Also, analog components, such as op-amps and the like, can be less reliable than digital circuitry.  
         [0004]     The present invention overcomes the shortcomings of the prior art through the use of a digital microcontroller controlled circuit, with increased reliability and which allows the turn-on and turn-off delays to be easily programmed and modified.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides for a highly reliable circuit, suitable for controlling the delivery of electrical power to a second electrical outlet, based on the power consumption of a first outlet. This circuit has application in home and small workshops where the activation of a power tool, necessitates the activation of a dust collecting device. Because of the compact size, other household applications are also possible. Briefly, a microcontroller monitors the current used by the first outlet. When that current exceeds a predetermined value, the controller enables the second outlet after a first predetermined period of time. When the current drops below that predetermined value, the controller disables the outlet after a second predetermined period of time.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates a schematic diagram of a representative embodiment of the present invention;  
         [0007]      FIG. 2   a  represents a first embodiment of the enclosure for the present invention; and  
         [0008]      FIG. 2   b  represents a second embodiment of the enclosure for the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]      FIG. 1  illustrates a schematic diagram of a representative embodiment of the present invention. Input plug  10  represents the connection from the present invention to an electrical power source. This plug can be integral to the enclosure, as shown in  FIG. 2   a,  or at the distal end of a power cord, as shown in  FIG. 2   b.  One input terminal from plug  10 , preferably the neutral line, is connected directly to one of the contacts of unswitched output  20  and one of the contacts of switched output  30 . The other input terminal from plug  10 , or the “hot” line, is connected to the second contact of unswitched output  20  through transformer T 1 . In the preferred embodiment, transformer T 1  has a windings ratio of 500:1, such that the current produced at the secondary winding is 500 times less than that passing through the primary winding. This small alternating current is then converted into a DC voltage, such as by using diode D 1 , resistors R 3  and R 4 , and capacitor C 4 . This DC voltage is made available to the control circuitry, such as microcontroller U 1 . In the preferred embodiment, a 8 bit microcontroller is used. The microcontroller of the preferred embodiment has an internal clock generator, at least one general purpose output and at least one analog to digital converter. Other preferred features include a programmable timer, such as a 555 timer, internal random access memory (RAM), and read only memory (ROM), preferably in the form of EEPROM or FLASH ROM. The incorporation of these features reduces the parts count and cost, and improves the reliability of the circuit. In the preferred embodiment, the microcontroller is programmable, such as during manufacture and optionally during actual field use.  
         [0010]     Returning to  FIG. 1 , the DC voltage, which corresponds to the alternating current passing to the unswitched output  20  is sampled by microcontroller U 1 . When this voltage exceeds a first predetermined threshold, which is programmed into the microcontroller, the software determines that the unswitched output is in use. In the preferred embodiment, this first threshold is defined so as to correspond to about 1 Amp, although the invention is not limited to this value. Other applications may necessitate the need for a threshold which may be higher or lower than this value. For example, if the preset invention is used to enable electrical power to flow to an associated monitor or printer when a personal computer is turned on, a different threshold value may be appropriate. After waiting a first predetermined period of time, the microcontroller asserts one of its general purpose outputs. In the embodiment shown in  FIG. 1 , microcontroller U 1  drives the output corresponding to pin  2  low when this predetermined time period has elapsed. This signal is connected to the gate of triac Q 1 , thereby enabling it. Triac Q 1  then supplies the necessary current to triac Q 2 , thereby enabling it and allowing current to pass to switched output  30 . Triac Q 2  is the power triac, responsible for supplying the power to the auxiliary outlet when appropriate.  
         [0011]     At a later time, if the voltage presented at the A/D input of microcontroller U 1  drops below a second predetermined threshold, the microcontroller will deassert its general purpose output after a second predetermined time period has elapsed. This has the effect of disabling the triacs at the line voltage zero crossing and thereby turning off the switched output  30 . This second predetermined threshold can be the same as the first threshold, but is preferably lower, thereby providing some amount of hysteresis. In the preferred embodiment, this second threshold is 0.8 Amps, although the invention is not limited to this or any particular value. This delay period is variable, and can be beneficial in scenarios where the switched outlet is used to power a vacuum. In this case, after the saw is turned off, the vacuum continues to operate long enough to clean any remaining debris.  
         [0012]     Returning to  FIG. 1 , diodes D 2  and D 3 , resistor R 5  and capacitor C 3  form the power supply used by microcontroller U 1 . Diode D 2  is preferably a zener diode, with a reverse breakdown voltage of 5.6V. It is connected directly to the “hot” line, thus the voltage supplied to the microcontroller is exactly that of the “hot” line. Capacitor C 3  is in parallel with this zener diode, and maintains this voltage. Diode D 3  is in series with diode D 2  and capacitor C 3  to insure that current does not flow from the neutral line through the zener diode during periods when the neutral line is at a higher potential than the “hot” line. Thus the junction between diode D 2  and resistor R 5  is 5.6V below the level of the “hot” line at all times.  
         [0013]     Jumper Block J 1  allows for maximum flexibility of operation. For example, the user or manufacturer can choose to insert one or more jumpers in jumper block J 1 . During initialization, the microcontroller U 1  monitors the voltage levels at several of its general purpose inputs to detect the presence or absence of these particular jumpers. The specific combination of jumpers detected can be used to determine the desired values of different parameters, including but not limited to turn-on current, turn-off current, turn-on delay time, and turn-off delay time. For example, the presence of a specific jumper may be used to set the delay times to their maximum duration, while the absence of that jumper may signify that minimum durations should be used. Thus, without any modifications to the circuitry, or to the software program, the functionality of the device can be modified, based on the particular application in which the device is to be used. In another embodiment, jumper block J 1  can be used in conjunction with a computer, such as a laptop computer, to facilitate programming of the microcontroller after its shipment to customers.  
         [0014]      FIG. 2  illustrates two physical embodiments of the present invention. In  FIG. 2   a,  the input terminal  120  and two outputs  100 , 110  are encased in a small enclosure  130 , with the input terminal adapted to be inserted into an electrical outlet. The use of digital electronics minimizes the power consumed by the present invention, allowing it to safely operate in a small enclosure. In the preferred embodiment, the enclosure  130  measures 2.5″×4″×2″. Output  100  is an unswitched electrical outlet, while output  110  is switched based on the current flow through output  100 , as described above.  
         [0015]      FIG. 2   b  shows a second embodiment of the present invention. In this embodiment, an extension cord  250  is used, terminating in an electrical plug  240  adaptable to be inserted into an electrical outlet. The enclosure  240  contains four outputs. In  FIG. 2   b,  two of these outputs  200 ,  210  are traditional outputs, similar to those found on an extension cord or bus strip. Output  220  is the unswitched output as described in  FIG. 1 , while output  230  is the switched output. The invention is not limited to only this configuration. The enclosure  240  can be configured in a variety of ways. For example, two pairs of switched/unswitched outlets can be provided. Alternatively, one unswitched output can also be used to control a plurality of switched outputs.  
         [0016]     As noted above, this circuit is adapted to operate in conjunction with a wide range of devices. For example, for traditional workshop uses, the circuit is operable withmost power tools, such as those with a rating of 125 Vac/15 Amp. Modification to the circuit can be made to accommodate other operating voltages and current requirements. Similarly, the circuit can be used in conjunction with televisions, stereos, computers and other electrical household devices.  
         [0017]     In typical workshop applications, the unswitched outlet is preferably used in conjunction with a saw, such as a miter, table, circular or band saw, a sander or a router. The switched outlet is then used to operate a wet/dry vacuum, a dust collector, or a power feed attached to several other devices.  
         [0018]     In residential applications, the unswitched outlet is preferably used in conjunction with a personal computer, while the switched outlet controls the associated monitor. Alternatively, the unswitched outlet may be used with a television, while the switched outlet is used to control the cable box, stereo or other electronic component.