Proximity Alarm

Abstract

A control device in a proximity alarm system gives warning when an object enters an electrostatic field located around an energized power line. The alarm system itself comprises antenna means for sensing the presence of the electrostatic field thereby inducing an alternating current potential therein. A switching circuit and an induced signal control circuit are electrically connected to each other. The switching circuit includes antenna sensitivity control, test circuitry and alarm circuitry. The induced signal control circuit includes amplifier means, half-wave voltage doubling means, oscillating means, power supply voltage regulating means and frequency filtering means. In a specific embodiment, the switching circuit and the induced signal control circuit are located in a casing composed of a non-magnetic material. The induced signal control circuit is encapsulated in a resinous material and is removably mounted within the casing.

Description

BACKGROUND OF THE INVENTION

Proximity alarms which give warning when an object enters an electrostatic field are well known in the prior art. More particularly, this invention relates to systems for signalling the close approach of an object to an energized electrical transmission line. The proximity alarms disclosed in U.S. Pat. Nos. 3,125,751 and 3,168,729 are examples of prior art alarms which have been developed for such usage. However, there are several problems which are attendant in these prior art alarms which render them unusable in the field and clearly not adaptable to commercial mass production techniques.

It has been found that the alarm disclosed in U.S. Pat. No. 3,168,729 is designed to respond to the presence of electromagnetic fields. This fact has presented numerous problems when such a device is subjected to environments in which it is used. The earlier alarm disclosed in U.S. Pat. No. 3,125,751 could be triggered by other sources of electrical energy if the field strength at the alarm was sufficiently high. Thus, even though the object is not dangerously close to radio towers, the radiations from the high powered radio towers could activate the alarms. Likewise, lightning in the vicinity of the alarm would produce radiant energy causing it to be triggered. Furthermore, in the earlier type of alarm, no test circuitry was ordinarily provided even though testing for the proper operation of the system is extremely important.

In several modifications of the earlier prior art proximity alarms, it was found that the available devices could not withstand a vibration or a high heat environment. Constant vibration due to the environment of use caused fatigue failures in the field. This resulted in frequent breakdown thereby causing frequent replacement problems. It was further found that the prior art devices were adversely affected by the extreme temperature changes taking place in the environment around the device. Such temperatures are generally in the range of from about -75.degree. to about +200.degree. F.

These proximity alarms are generally used with vehicles having projections. The alarm system is designed to operate off the vehicle power supply. It was discovered that the power supply to the alarm system tended to vary sharply over a period of time. It was found that this fact seriously affected the sensitivity and effectiveness of the total alarm system.

Another basic problem associated with the prior art devices was found to exist in the large variance in response characteristics from one device to the next. In other words, the prior art units did not have consistent response characteristics in the field thereby resulting in many obvious disadvantages. This is an extremely difficult problem to overcome and one that seriously affects the efficiency of use in the field. In addition, the mass production of the device for supplying the demand was greatly hampered. It was found that the problem of mass producing substantially identical units was an impossibility when considering the feasibility of using the prior art proximity alarms.

PURPOSE OF THE INVENTION

The primary object of this invention is to provide a proximity alarm system which may be used in the field over an extended period of time and may be mass produced with a high degree of repeatability in construction and response characteristics.

Another object of this invention is to produce a proximity alarm which is not adversely affected by an intense heat environment or a substantial vibration environment.

A further object of this invention is to provide a proximity alarm device which overcomes all of the problems found to exist in conjunction with known prior art devices.

SUMMARY OF THE INVENTION

This invention is directed to an alarm system including antenna means for sensing the presence of an electrostatic field which thereby induces an alternating current potential therein. A switching circuit and an induced signal control circuit are electrically connected to each other. The switching circuit has an antenna sensitivity control, test circuitry and an alarm circuitry. The induced signal control circuit includes amplifying means, half-wave voltage doubling means, oscillating means, power supply voltage regulating means and frequency filtering means.

The specific arrangement of a switching circuit and an induced signal control circuit provides a beneficial way of isolating the problems associated with prior art devices. None of the specific prior art devices disclose the use of a power supply voltage regulating means or a frequency filtering means within such an induced signal control circuit.

In a specific embodiment of this invention, the induced signal control circuit is encapsulated in a resinous material and disposed in a casing composed of a non-magnetic material. The switching circuit is also disposed in the casing adjacent the encapsulated signal control circuit.

Other specific embodiments of the alarm system made in accordance with this invention include the use of variable resistance means electrically connected to transistor means for establishing and maintaining predetermined induced signal response characteristics in the transistor means. While some prior art devices simply distinguish among the types of dangers to which it responds, the present alarm system is designed to eliminate extraneous alternating current frequency from the induced signal control circuit. It is through the specific circuit means set forth herein and the manner in which the control device is constructed that provides a unique approach and substantially improved results in working characteristics under severe environmental conditions.

BRIEF DESCRIPTION OF DRAWINGS

Other objects of this invention will appear in the following description and appended claims, reference being made to the accompanying drawings forming a part of the specification wherein like reference characters designate corresponding parts in the several views.

FIG. 1 is a front elevational view of a control device made in accordance with this invention,

FIG. 2 is a side elevational view partially in section of the device shown in FIG. 1,

FIG. 3 is a back elevational view of the device of FIG. 1 with the cover removed,

FIG. 4 is a bottom elevational view of the device of FIG. 1,

FIG. 5 is a circuitry diagram of the induced signal control circuit made in accordance with this invention, and

FIG. 6 is a circuitry diagram of the switching circuit made in accordance with this invention.

DESCRIPTION OF SPECIFIC EMBODIMENT

More specifically, an alarm device as shown in FIG. 1 includes a casing 100 on which is mounted a warning lamp 81 which lights intermittently when a voltage is induced in a sensor cable (not shown). The sensor cable is attached to the casing 100 at the sensor connection 11a. The system is turned on when the battery switch 52 is in the on position. The sensor cable is mounted on an object such as the boom of a crane which might enter an electrostatic field. The manner in which the sensor cable or antenna is mounted onto the moving object is well known in the prior art and does not form a part of this invention. When the sensor cable is moved sufficiently close to a high voltage power line, the electrostatic field surrounding the power line induces a voltage in the sensor cable. The sensitivity of the system to the electrostatic field which induces the voltage is adjusted through the sensitivity switches 14 and 23. In this specific embodiment, there is a coarse sensitivity control 14 and a fine sensitivity control 23. The battery switch 52 is connected to the battery of the system and the alarm device is operable when the switch 52 is in the on position. It may be desirable to have an exterior alarm attached to the system. In this instance, the exterior alarm switch 82 is placed in the on position.

Various types of warning devices may be used. The warning light 81 mounted in the casing 100 is a primary warning signal in this specific embodiment. Audible sounding devices may be used for warning the driver and other ground personnel when the alarm device is activated. Other exterior and remote lights may also be used in conjunction with the warning system.

The warning device of this invention can be adjusted to actuate the alarm at any desired distance from one foot to several hundred feet depending upon the voltage of the energized power line. When the strength of the electrostatic field around the power line exceeds the minimum desired working distance, the alarm mechanism such as warning light 81 and an external light will blink or a horn will sound. The rate of blinking will increase or the oscillating of the audible alarm will increase as the object such as a boom or extension moves close to the energized power line. The alarm system will continue to warn until the object is withdrawn from the danger zone.

Test switch 12 is located in the casing 100 for the purpose of testing the switching circuitry or the signal control circuitry of the alarm system. A sensor test light 80 will be turned on when the test switch 12 is placed in the on condition, if the sensor cable (not shown) is in working order.

A bank of connectors 101 is disposed on the bottom portion of the device of this embodiment for attaching the various alarms and battery sources that are used to supply energy to the alarm system.

The control device made in accordance with this invention has two basic circuit portions mounted in the casing 100. The control device circuitry includes a switching circuit portion and an encapsulated signal control circuit portion. The switching circuit portion is shown in detail in FIG. 6 and the signal control circuit portion is shown in FIG. 5. The switching circuit includes test circuitry, sensitivity circuitry and alarm circuitry disposed in the casing 100. As shown in FIG. 3, the coarse sensitivity control 14 includes a bank of capacitors. The fine sensitivity control 23 is composed of a variable resistance. Toggle switches are used for the battery switch 52 and exterior alarm switch 82. A pushbutton device is used to form the test circuitry switch 12. The relay 41 is mounted in the casing and is in a normally de-energized state. The wires have been removed from the drawing as shown in FIG. 3 for clarity.

An encapsulated module 10 is removably mounted within the casing 100 and includes the signal control circuitry which is shown in detail in FIG. 5. The encapsulating process for the signal control circuit 10 is accomplished in a standard prior art manner. The circuitry as shown in FIG. 5 is placed in a housing or box construction and filled with a resinous material. The housing is made of a plastic material in this embodiment. The module 10 is mounted on the support member 103, as shown in FIG. 2. Any type of fastening means may be used. The switching circuitry and the encapsulated signal control circuit 10 are disposed adjacent each other within the casing 100 and connected through the use of a plug means 24a and 24b.

It has been found that the casing 100 should be constructed of a non-magnetic material in an effort to shield the highly sensitive circuitry used in conjunction with the construction of this alarm control device. In this specific embodiment, an aluminum alloy is used to shield against electrostatic pickup and possible electromagnetic effects. The construction as described hereinabove provides a very rugged device heretofore unavailable in the prior art. The use of the particular circuit in the encapsulated module 10 has overcome the problems associated with fatigue failure due to vibration. In addition, the control device of this invention has become suited to use under very high temperature conditions. The use of sealing means between the cover 102 and the casing 100 provides a dust barrier which is a further significant improvement over any of the prior art devices known and used heretofore.

The basic circuitry of the control device of this specific embodiment is shown in FIGS. 5 and 6. A sensor 11 consisting of some type of antenna means is mounted along the portion of the object which might come into contact with electrostatic field around a power line. For example, the antenna means would be mounted on the boom of a crane, on the fork portion of a fork lift truck, or on the ladder portion of a ladder truck. When the sensor cable is moved sufficiently close to a high voltage power line, the electrostatic field which surrounds the power line induces a voltage in the antenna means or sensor 11. The induced voltage is applied to the capacitor voltage divider network generally designated 14. The induced voltage is applied to contact 13b of the test switch 12. The voltage divider network 14 is also referred to as the coarse sensitivity control. The capacitor divider network 14 includes a capacitor 15 and a bank of capacitors 16 through 21.

In this specific embodiment, a 5 Megohm variable resistor 23 is placed across the capacitor bank thereby constituting the fine sensitivity control. The fine adjustment of the sensitivity is accomplished by selecting any desired percentage of induced input voltage across the capacitor bank and connecting it into the input of the field effect transistor 25. The switching circuit as shown in FIG. 6 is electrically connected to the encapsulated signal control circuit as shown in FIG. 5 through the use of pins 1 through 8 located in the plug means 24a and 24b. One of the plug means portions 24a and 24b is male and the other is female. The input of the induced voltage is accomplished through pins 1 and 2 into the field effect transistor 25.

An important feature of this invention resides in the use of a variable resistance 60 connected to the source 25a. The transistor 25 used in this specific embodiment is very sensitive to any contamination around its leads. A breakdown will occur in the input impedence by virtue of any building up of a conducting surface such as oxidation layers or moisture. A primary problem associated with prior art devices has been the collecting of such types of contamination and the deposit of minerals which effectively changes the input impedence of the transistor means used in the circuitry. A basic problem has been discovered in the mass production of the control device of this invention wherein commercially available transistors are very different from each other. This basic problem is found to exist in the establishing of a consistent proper biasing potential when commercially available transistors are used in the production of such a control device. For this reason, the variable resistance 60 is used to provide the proper biasing potential for the transistor 25 thereby improving the repeatability with respect to sensitivity established in each alarm control device being produced.

The transistors 25 and 26 constitute a two stage direct coupled amplifier. The input voltage which is induced in the sensor 11 is amplified by the transistors 25 and 26 and subsequently applied to the emitter-follower transistor 27. One of the basic problems associated with the prior art devices has been the difficulties associated with extraneous alternating current frequencies. The inclusion of capacitor 62 has solved this basic problem by limiting the range frequencies to which the control device will respond. It is essential that the alarm is not triggered when regular transmitters are used at the work site or when the work is being done in the vicinity of a radio transmitter of any sort. The capacitor 62 acts as a very low impedance shunt or bypass. The impedance is effective to pass frequencies of about 60 cycles per second up to about 450 cycles per second. Once beyond this range, said impedance begins to roll off thereby providing a decrease in response. It has also been found necessary to incorporate the use of a further capacitor 67 to overcome the radio frequency problem. The use of this particular shunting capacitor 67 allows the bypass of all lower frequency components to the case through the pin 8 attached to the ground.

The amplified input voltage is taken from the emitter-follower 27 and applied to a half-wave voltage doubler means and a filter. The half-wave voltage doubler means consists of the diodes 28 and 29. The amplified input signal is coupled across the capacitor 70 to the junction of the diodes 28 and 29. On the positive half cycle, the diode 29 will be conductive and diode 28 will be open. During this cycle, the capacitor 70 will charge to the full peak value of that positive wave form. On the next half cycle, or negative half cycle, the diode 29 will be in the off condition and diode 28 will be biased forwardly. That is, the negative half cycle will cause current flow to diode 28 and the voltage that was charged previously in capacitor 70 will also be applied through diode 28 thereby applying twice the voltage across the diode 28. This diode configuration thereby produces a doubled half-wave pulse which is subsequently sent through the filter consisting of resistor 30 and capacitor 31. The filter means establishes the doubled half-wave to a DC level proportional to the amplitude of the input voltage. The capacitor 31 substantially eliminates any ripple that may be apparent in the half-wave rectified signal. The capacitor 31 charges upon the half-cycle and then discharges so that only a DC voltage potential that is fully filtered is supplied to the transistor 32. The voltage at this point in the circuit varies from about 0 to about 3 volts.

The proportional DC voltage is applied to a normally conducting transistor 32. When the induced signal is present, the voltage comes to the base 32c in a negative direction and will eventually cause the transistor 32 to turn off. When the transistor 32 turns off, the capacitor 36 starts to charge up to a higher level and the uni-junction transistor 37 begins to fire. The negative half-wave is used to cut off the normally conducting transistor 32 thereby allowing the uni-junction transistor 37 to begin oscillation. The oscillation of the uni-junction transistor 37 is established at a frequency determined by the conductive state of transistor 32. In this specific embodiment, the maximum frequency of oscillation is 5 pulses per second. In this way, the oscillating means is completely compatible with the audible alarms that are used.

Another feature of this invention resides at the point where a variable resistance 34 is used in conjunction with the oscillating transistor 37. A basic production problem occurs when uni-junction transistors such as those used in the circuitry of this control device are purchased in bulk. All of these uni-junction transistors have different firing points. They vary considerably with regard to their sensitivity. The transistors having sensitive firing points will oscillate at a much higher frequency. In that instance, a larger amount of resistance must be inserted in combination therewith to slow the oscillation down. It has been found that the combination of using a variable resistance 34 to standardize the oscillating frequency of the transistor 37 together with encapsulation of the entire circuit provides a totally standardized, highly efficient and workable device in the field. This result has heretofore been unknown with respect to alarm devices used to give warning when an object approaches an electrostatic field around an energized power line.

Another feature of this invention is directed to the use of a pulse stretcher means to insure a solid signal for driving the alarms used with the control device of this invention. As the pulse from the uni-junction transistor 37 is applied to the transistor 38, there is a very rapid charge of the capacitor 40 through the diode 39. Once the pulse is gone, the transistor 38 returns to a non-conducting state and the capacitor 40 has to discharge slowly through the resistance 41. In other words, the transistor 38 is normally non-conducting until it is pulsed by the output from the uni-junction transistor 37. The fast charge existing in the capacitor 40 and the slow discharge thereof effectuates the stretching out of the pulse. The resulting pulses are approximately 80 milliseconds in duration. These pulses are applied to the pulse amplifier 43 through the resistance 42.

The amplified output of transistor 43 is applied to a high current transistor switch 45 and pin 5 of the plug means 24b. This circuitry connection allows current to flow through the alarm relay 46 across pins 5 and 6 from pin 7 through resistance 47 to pin 6 of plug 24b. Current continues through alarm relay 46 to pin 5 through high current transistor 45 and then to pin 4. Diode 48 serves to dampen high voltage spikes which may be produced by the coil relay 46.

When the alarm relay 46 is energized, contacts 49, 50 and 51 are closed. Contact 49 connects the ungrounded vehicle battery terminal to the external terminal of the boom light and the alarm spares. The contact 51 connects the alarm light 81 across the crane battery supply at pins 4 and 7 of plug 24a. The flow of current is effected only when the switch 52 is in the on or closed position. In this embodiment, the relay 46 is a 6 volt relay. When this relay is used in conjunction with resistor 47, a louder clicking sound will result than if a 12 volt relay is used.

A very important feature of this invention is directed to the problem which was discovered to exist only after prior art devices were used in the field. It was found that the output voltage from the generator and battery systems of vehicles such as cranes varies drastically. It is determined that such output voltage variation in the power supply of the vehicle itself adversely affected the frequency of oscillation for the oscillating means 37. It is necessary to hold this voltage constant and thereby effectuate a highly efficient and standardized circuitry which would operate properly in the field. In this particular embodiment, the voltage regulating means accepts voltages from about 10 volts to about 16 volts and regulates it down to about 8 to 8 1/2 volts. The zener diode 54 used in combination with the transistor 53 constitutes the voltage regulating means across the supply voltage of the vehicle which includes the voltage across the generator and battery system thereof. It was the discovery of this particular problem that led to the solution proposed by the inventors and resulted in a sound alarm control device which is extremely efficient and highly reproducible. None of these advantages were available in the known prior art devices. The capacitor 56 acts as a filter to prevent noise from the vehicle battery power supply from entering the circuitry of the control device. The resistors 71 and 72 and capacitor 68 provide additional filtering against such noise.

The testing circuitry includes the test button 12 and contacts 13a, 13b and 13c. The contacts 13a, 13b and 13c are moved from their lower positions as shown in FIG. 6 to their upper closed positions when the button 12 is pressed. Contact 13a connects the base terminal of the shunt transistor 32 to ground. This manually cuts off conduction in the transistor 32 and causes the oscillator 37 to oscillate at its maximum rate of 5 pulses per second. The oscillation of transistor 37 checks the relay alarm light 81 and the external outputs which may be connected to audible alarms. At the same time the button 12 is pressed, contacts 13b and 13c make a path to allow battery current to flow from the on-off switch 52 through the sensor 11 from terminal A to B, through 13b to the sensor power test lamp 80. The closing of this circuitry tests the continuity of the sensor cable. If there is a break in the sensor cable somewhere along the vehicle, the lamp 80 will not be turned on.

ADVANTAGES OF THE INVENTION

Known prior art devices used in the field were highly subject to fatigue failure and extremely inefficient. At the same time, there was a high sensitivity to stray or extraneous radio frequency signals which would inadvertently cause the alarms in the prior art systems to be activated. While some of the prior art devices were designed to be sensitive to the electrostatic field around an energized power line, they were also affected by the electromagnetic field. Where an alarm system is sensitive to the electromagnetic field which varies with the load on the line, normal variations charge the sensitivity of the alarm device without the operator knowing it. Therefore, there is a basic advantage in having a control alarm system that is sensitive only to the electrostatic field.

Where there is a DC energized power line, it is possible that the capacitor coupling of the alarm system made in accordance with this invention will be usable. That is, most of the industrial DC sources are actually non-filtered and have a high component of AC current in them. As a result, it is possible to use the alarm system of this invention in so-called DC direct current environments in which the alternating current component is sufficiently high to detect.

Another basic advantage of the alarm system made in accordance with this invention is that it is mass-producible due to its use of very simple and straightforward components made in an encapsulated module unit. The various uses of voltage regulating means, filtering means and variable resistance means allow the units to be more repeatable in the field. That is, the best that could be accomplished with prior art devices was to effect a 50 percent variation from one control device to the next with respect to the sensitivity to the electrostatic field. In other words, it would be necessary to adjust the unit as much as 50 percent before the appropriate sensitivity would be attained. However, with the control device of the present invention, it is possible to obtain an adjustment down within the 10 percent range and there is a substantial reduction of the variations between units.

The basic use of a switching circuit portion in combination with encapsulated signal control circuitry portion has overcome many of the failures which have been caused by fatigue due to vibration environment. In addition, the basic mechanical integrity of the control device has been improved substantially and in a manner that allows the device to be commercially acceptable as compared to the prior art devices which were not commercially acceptable.

While the basic circuitry of the alarm device has been described in its operation to warn of an electrostatic field, it is possible that other types of dangerous conditions may be indicated in an alarm system made in accordance with this invention. For example, it may be possible to provide a means which will energize the relay when the movement of the vehicle and its projections come into a dangerous condition. That is, when the boom of a work vehicle is raised too high or put in an otherwise dangerous condition, a switch could be closed and the alarm be made to sound.

While the proximity alarm has been shown and described in detail, it is obvious that this invention is not to be considered as being limited to the exact form disclosed, and that changes in detail and construction may be made therein within the scope of the invention, without departing from the spirit thereof.

Claims

Having thus set forth the nature of this invention, what is claimed is:

1. In a proximity alarm which gives warning when an object enters an electrostatic field, the combination comprising:

a. a casing composed of a non-magnetic material,

b. a switching circuit portion including test circuitry and alarm circuitry disposed in said casing,

c. a signal control circuit portion responsive to any induced signal received from the electrostatic field to form an oscillating response signal effective to operate an alarm,

d. said signal control circuit portion being encapsulated and disposed adjacent the switching circuit and being removably mounted in said casing, and

e. plug means for electrically connecting the switching circuit portion to the signal control circuit portion.

2. In an alarm system as defined in claim 1 wherein

the signal control circuit portion includes variable resistance means electrically connected to transistor means for establishing and maintaining predetermined induced response characteristics within said transistor means.

3. In an alarm system as defined in claim 1 wherein

the signal control circuit portion includes amplifier means having transistor means for receiving any induced signal emanating from the electrostatic field and a variable resistance means electrically connected to the transistor means for establishing a predetermined biasing potential within said transistor means.

4. In an alarm system as defined in claim 3 wherein

said transistor means is a field effect transistor and said induced signal is received at the gate of the transistor and

said variable resistance means is electrically connected to the source of the transistor.

5. In an alarm system as defined in claim 3 wherein

the signal control circuit portion includes filtering means having means electrically connected to the output of the transistor means for filtering extraneous alternating current frequency from the system.

6. In an alarm system as defined in claim 5 wherein

said filtering means includes means for applying the filtered extraneous alternating current frequency to the casing thereby effecting grounding thereof.

7. In an alarm system as defined in claim 1 wherein

the signal control circuit portion includes oscillating means having a uni-junction transistor and a variable resistance means electrically connected to the uni-junction transistor for establishing a predetermined firing point in said transistor.

8. In an alarm as defined in claim 1 wherein

the signal control circuit portion includes oscillating means and power supply voltage regulating means which controls varying voltage supply to the alarm system thereby maintaining the frequency of the oscillating means at a predetermined level.

9. In an alarm system as defined in claim 1 wherein

the signal control circuit portion is responsive to an induced voltage signal and includes means for amplifying the induced voltage signal, means for converting the amplified induced signal to a doubled half-wave form, oscillating means for producing short duration pulses in response to said double half-wave form, and means for stretching the width of the short duration pulses from the oscillating means to provide satisfactory energizing of the alarm circuitry.

10. A proximity alarm system which gives warning when a projection from a vehicle enters an electrostatic field located around an energized power line comprising:

a. alarm means electrically connected to a normally de-energized relay,

b. antenna means for sensing the presence of the electrostatic field,

c. means for receiving and amplifying an induced signal sensed by the antenna means,

d. means for converting the amplified induced signal to a doubled half-wave form,

e. oscillating means for producing short duration pulses in response to said doubled half-wave form,

f. means for stretching the width of the short duration pulse from the oscillating means to provide satisfactory energizing of the alarm means,

g. means for energizing the relay with the stretched pulse thereby actuating the alarm means,

h. means for providing power supply voltage from the vehicle to the alarm system, and

i. means for regulating varying power voltage supplied to the alarm system at a predetermined level.

11. An alarm system as defined in claim 10 wherein

said amplifier means includes filtering means for filtering extraneous alternating current frequencies from the system.

12. An alarm system which gives warning when an object enters an electrostatic field comprising:

a. antenna means for sensing the presence of said field which thereby induces an alternating current potential therein,

b. a switching circuit including means for controlling the sensitivity of the antenna, circuit means for testing the alarm system, and alarm circuit means,

c. a signal control circuit responsive to any induced signal received from the electrostatic field to form an oscillating response signal effective to operate the alarm circuit means, and

d. means for electrically connecting the switching circuit to the signal control circuit.

13. An alarm system as defined in claim 12 wherein

said signal control circuit is encapsulated in a resinous material and disposed in a casing composed of a nonmagnetic material and

said switching circuit is disposed in said casing adjacent the encapsulated signal control circuit.

14. An alarm system as defined in claim 13 wherein

said encapsulated signal control circuit is removably mounted in said casing and

said circuit connecting means includes a plug means having a plurality of connecting pins.

15. A proximity alarm control device comprising:

a. a casing composed of a non-magnetic material,

b. means for receiving an induced input voltage from an electrostatic field around energized power lines,

c. a switching circuit disposed in said casing,

d. said switching circuit including circuit means for testing the device, alarm circuit means and means for providing a power supply voltage to the control device,

e. a signal control circuit responsive to the induced input voltage from the electrostatic field to form an oscillating response signal effective to operate the alarm circuit means,

f. said signal control circuit being encapsulated and disposed adjacent the switching circuit and being removably mounted in said casing, and

g. plug means for electrically connecting the switching circuit to the signal control circuit.

16. An alarm device as defined in claim 15 wherein

the signal control circuit includes variable resistance means connected to transistor means for establishing and maintaining predetermined induced signal response characteristics in said transistor means.

17. A control device as defined in claim 15 wherein

the induced input voltage receiving means is the base of a transistor means and

the signal control circuit includes amplifier means having a variable resistance means electrically connected to said transistor means for establishing a predetermined biasing potential within said transistor means.

18. A control device as defined in claim 17 wherein

said transistor means is a field effect transistor and said induced voltage is received at the gate of the transistor, and

said variable resistance means is electrically connected to the source of said transistor.

19. A control device as defined in claim 17 wherein

the signal control circuit includes filtering means having means electrically connected to the output of the transistor means for filtering extraneous alternating current frequency from the signal control circuit.

20. A control device as defined in claim 15 wherein

the signal control circuit includes oscillating means having a uni-junction transistor and a variable resistance means electrically connected to the base of the uni-junction transistor for establishing a predetermined firing point thereof.

21. A control device as defined in claim 15 wherein

the signal control circuit includes oscillating means and

said power supply voltage regulating means control varying voltage in the supply to the alarm circuit means to maintain the frequency of the oscillating means at a predetermined level.