Proximity Alarm Circuit For Entrance Annunciators

Abstract

A household alarm circuit, when switched on during night hours, responds to the touch or proximity of a human body to the house door knob or lock by sounding the existing door bell or chime. The alarm circuit utilizes the low voltage supply for the door chime and has a proximity sensing lead for connection to the striker plate of the door latch. When the door knob or lock is approached or touched, the alarm circuit repeatedly sounds the door chime. The alarm circuit includes a rectifier for connection to the AC door chime supply, a transistor cut off by a potential change transmitted to its gate by the sensing lead, an SCR switch closed by the transistor, and a relay in series with the SCR switch with a contact for closing the door chime circuit.

Description

BACKGROUND

The present invention is concerned with the problem of burglars and like intruders attempting to force or otherwise gain entrance through a doorway. Such an intruder may first examine or try to force the door lock or knob and thus touch or come close to the knob and lock assembly and the strike plate for the door latch.

Adjacent most entrance doors is a door bell, chime or like annunciator installation having a button close to the door lock. Such an installation customarily includes a stepdown transformer whose primary is connected to the house 115 volt AC supply, and whose low voltage secondary is wired to the door annunciator button.

It is the object of the present invention to utilize an existing door annunciator, and the existing low voltage power supply whose connection to the annunciator is completed by a button or key, and to cause the annunciator to sound by means of a proximity circuit responsive when such an intruder moves into proximity to the door.

SUMMARY OF THE INVENTION

According to the present invention such a proximity circuit for connection between the key terminals of an annunciator circuit carrying alternating current comprises electronic valve means for sensing an adjacent mass to alter the conductive state of the valve means, a storage capacitor, a rectifier for connection to the key terminals and rectification of the alternating current, and having a charging circuit to the storage capacitor, and controlled switching means connected to the rectifier having a power supply connection from the capacitor and input means connected to said valve means, the switching means being responsive to the change of the change of state of the valve means to close a circuit between the key terminals thereby to actuate the annunciator and disable said charging circuit allowing discharge of the storage capacitor and interruption of power supply to the switching means until the charging circuit is restored, whereby successive charge and discharge cycles of the storage capacitor cause repeated actuation of the annunciator.

DRAWING

For the purpose of illustration typical embodiments of the invention are shown schematically in the accompanying drawing in which,

FIG. 1 is a schematic diagram of a proximity alarm circuit connected to an existing door chime installation; and

FIG. 2 is a schematic diagram showing a modification of parts of the alarm circuit of FIG. 1.

DESCRIPTION

Shown at the lower right in FIG. 1 of the accompanying drawing is a conventional residential door chime installation including a transformer T connected to household 115 volt alternating current lines A and C and a secondary providing 16 volts AC at terminals t 16. The transformer and associated wiring have an interent distributed capacity Cd to ground and to other household wiring having an effect to be described hereinafter. The transformer 16 volt terminals t 16 are connected to the coil K1 of a door chime which typically has two chime notes, one sounded on energization of the solenoid and the other sounded on deenergization. A door button switch S1 for energizing the chime solenoid is usually mounted on the exterior of a door jam near the door knob and lock mechanism. This normally open switch has two screw terminals or the like t1 and t6 which carry the 16 volt AC for the chime.

The remaining ALARM CIRCUIT portion of FIG. 1 has a shielded input cable SH with a lead L adapted to be electrically connected to a metallic mass such as the securing screw SC of a strike plate P for a door latch. The alarm circuit obtains its power from the chime button or key terminals t1 and t6 which are normally near the latch plate P.

The alarm circuit of FIG. 1 has a conventional diode rectifier bridge B whose inputs have connections to the 16 volt key terminals t1 and t6. In one connection is one contactor S1a of a ganged switch S1 having three positions, 1, 2 and 3, in the first of which power to the alarm circuit is turned off, and in the second and third of which power is on. An arc suppressor consisting of a 33 ohm resistor R2 and an 0.05 microfarad capacitor C1 is connected across the bridge inputs. The approximately 23 volts peak of full-wave rectified bridge output is applied to a floating ground bus G, and through a 470 ohm ripple filtering resistor R1 to a positive bus +. The unfiltered ripple voltage is also applied through a 39 picofarad coupling capacitor C2 to the base of a transistor Q2 (type 2N3416). The transistor Q2, a relay coil K2 and a silicon controlled rectifier (SCR) switch Q3 (G.E. type C103Y) are connected in series between the positive and floating ground busses. The anode and cathode of SCR Q3 carry the main, switched current in the just described series, and are herein termed primary electrodes in contrast with the control electrode, gate g. The base b of transistor Q2 is normally floating, biasing the transistor to conductive state. But the gate g of the SCR Q3 is normally held at or near floating ground by a 1 kilohm bleeder resistor R8, maintaining the SCR switch open so that no current flows through the relay coil K2, and its contact k1, which parallels push button S1, is open as shown.

A main storage capacitor C3 (100 microfarads) is normally charged fully by the power supply, and has a minor discharge path through an 18 kilohm resistor R4 and the drain-to-source circuit d-s of a field effect transistor Q1 (2N5457). Transistor Q1 is normally biased to conduction by resistive connection of its gate g to floating ground, and its drain-to-source potential drop is low so that its drain d is near floating ground.

When the ganged switch S1 is in position 2 or 3, if part of a human body such as the hand comes close to, or touches, the strike plate P or the door knob or lock electrically connected to the plate and its screw SC by the latch, then the capacity of the body to ground or household wiring, or the electrostatic charge of the body, raises the potential at the gate g of the FET Q1 and drives the FET toward cut off, raising the potential at the drain d. This voltage rise is passed through a diode D6 (type 1N4001) to the gate g of the SCR switch Q3, latching the switch closed and completing the path through the relay coil K2 to transfer its contacts with a snap or toggle action. Accumulation of a charge at the SCR gate is prevented by a one kilohm bleeder resistor R8. Sensitivity of the proximity alarm circuit is adjusted by a 5 megohm potentiometer R7 in series with an 0.001 microfarad capacitor C4 having a 10 megohm bleeder resistor R5, and with a 10 megohm resistor R3 which isolates high voltage in the alarm circuit from the strike plate. An 0.2 microfarad spike filtering capacitor C5 parallels the drain-source circuit of transistor Q1.

After latching of the SCR Q3 the storage capacitor C3 then has its main discharge through the approximately 2500 ohm resistance of the coil K2, energizing the relay and closing its contact k1. The door chime circuit is thereby completed through the chime solenoid K1 and is closed sounding the first chime note, for convenience called ding. Closing of the relay contact k1 also short circuits the bridge B, momentarily interrupting rectified power supply to the main storage capacitor C3. This capacitor continues to discharge for a period of a few tenths of a second as determined primarily by the 2500 ohm resistance of the coil K2 until its voltage drops below the drop out voltage of the relay. The relay then de-energizes, opening its contact k1 and de-energizing the chime solenoid K1 causing the chime to sound the second note, dong. A second contact k 2 of the relay K1 may simultaneously energize an external alarm X.

Meanwhile a second storage capacitor C6, which was previously charged through the relay coil K2 and a diode D5 (1N4001) begins a relatively slow discharge through the base-to-emitter circuit of the now conducting transistor Q2, thereby holding Q2 conductive and the SCR switch Q3 latched closed although the original body influence at the strike plate P may have been removed. The duration of C6 discharge is preferably several seconds, determined primarily by the 100 microfarad capacitance of C6 and the 10 kilohm resistance of R6. Discharge of capacitor C6 back through the relay coil K2 is prevented by the diode D5.

While the slower discharge of the second storage capacitor C6 holds the relay coil path closed, the main storage capacitor recharges rapidly from the power supply in a few hundredths of a second (as determined by the 470 ohm filter resistor R1) until the pull in voltage of relay K2 is reached, transferring relay contact k to sound a ding, and again shorting out the rectifier bridge B, dropping out the relay K2 to sound a dong. The charge-discharge cycle of the main storage capacitor C3, and the ding dong sounding is repeated for several seconds until the secondary storage capacitor C6 approaches full discharge. Then, to insure cut off of transistor Q2, a negative excursion of the unfiltered ripple voltage at the bridge B positive output is coupled through the capacitor C2 to the gate g of transistor Q2, driving the transistor to cut off and unlatching the SCR switch Q3 by interruption of its holding current.

The operation described above is with a second contactor S1b of the ganged switch S1 in position 2'. This operation is modified by transfer to position 3'. The bridge B then remains connected to the chime button terminals t1 and t6, but the delayed opening of the relay energization path (Q2, Q3) upon discharge of the second storage capacitor C6 is eliminated by the shunting of transistor Q2 when the switch section S1b is in position 3'. In this position the shunt S1b continuously maintains holding current on the SCR switch Q3 once the SCR switch is latched. The main storage capacitor C3 will therefor continue the ding dong cycling until the ganged switch is thrown to position 2' or 1'.

As shown in FIG. 2 a solid state relay circuit may be used in lieu of the electromagnetic relay circuit of FIG. 1. In the circuit of FIG. 2 the bridge B is connected to the screw terminals t1 and t6 of the chime button S1 and supplies rectified power to the positive and ground busses + and G as in FIG. 1. The strike plate screw SC is connected by a lead L to the gate g of a FET Q14 (2N5457). As in the circuit of FIG. 1 body coupling to the gate g cuts off the FET raising the potential at its drain d. Sensitivity of the FET is adjusted by a 100 kilohm potentiometer R19. As long as body coupling is present, a storage capacitor C13 (100 microfarads) charges through R18 and diode D17 (1N4001) to nearly the potential of the positive bus. As C13 charges, the base-emitter voltage of a type 2N3638 transistor Q13 rises until Q13 starts to conduct. The voltage at the collector of Q13 then rises to about 7 to 9 volts at which point an avalanche diode Q12 (2N4990) breaks over at a very low voltage drop. The 7 to 9 volts is then applied through resistor R14 to the junction of resistor R14 and R15, but drops somewhat due to the increased current drawn through resistor R14 and the 2.2 kilohm emitter resistor of Q13. Since the resulting voltage across R15 then exceeds the trigger voltage at its control c, the silicon-controlled-rectifier Q11 abruptly conducts. As with closing of relay contact k1 in FIG. 1, conduction of the SCR Q11 short-circuits the chime terminals t1 and t6 (in this case through the bridge) energizing the chime solenoid to sound a ding and momentarily interrupting power supply to the busses + and G. SCR Q11 is held conducting by a secondary storage capacitor C12 (100 microfarad) previously charged from the power supply through R11 and now discharging through a diode D16 (1N4001) in series with a resistor R12 (220 ohms), through resistor R12 and SCR Q11. The time constant of this discharge circuit is a few tenths of a second after which SCR Q11 ceases conducting due to a lack of holding current restoring power supply and de-energizing the chime to sound a dong.

The avalanche diode Q12 has also opened due to loss of voltage at the positive bus + and supply of current through transistor Q13. But when power is restored after dong, continued discharge of capacitor C13 enables Q13 to conduct again, avalanching diode Q12. The ding-dong cycle is repeated for several seconds until capacitor C13 is discharged as determined primarily by the time constant of capacitor C13 and resistors R14, R15 and R16. Capacitor C13 thus performs the same function as capacitor C6 in FIG. 1, the holding or time delay for automatic reset. The time delay function may be eliminated by closing a switch S11. When switch S11 is closed, diode D11 has its cathode slightly higher than its anode and is back biased, non-conducting because of a small drop across resistor R11 caused by current conducted through transistor Q14 and resistor R18. In this state capacitor C13 cannot charge through diode D11. However, an alarm initiated by body capacity or leakage to ground at transistor Q14 gate g causes SCR Q11 to conduct. This pulls the junction of SCR Q11 anode and resistor R12 down to nearly ground potential. Diode D11 then conducts, and the residual charge on C12 allows equalization of charge between capacitors C12 and C13 until a dong occurs. Then diode D11 ceases to conduct and the process repeats with the next ding, maintaining just enough voltage on capacitor C13 to keep transistor Q13 conducting for each toggling of the chime until reset manually.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims. I claim:

Claims

1. A proximity circuit for connection between the key terminals of an annunciator circuit carrying alternating current comprising:

electronic valve means for sensing an adjacent mass to alter the conductive state of the valve means,

a storage capacitor,

a rectifier having two terminals for connection to the key terminals and for rectification of the alternating currant, and having a charging circuit to the storage capacitor, said two rectifier terminals comprising the only connection from the proximity circuit to the annunciator circuit, and

controlled switching means connected to the rectifier having a power supply connection from the capacitor and input means connected to said valve means, the switching means having a switching element connected across the rectifier and across the connection to the key terminals and being responsive to the change of state of the valve means to close a circuit between the key terminals each time the valve means alters state thereby to actuate the annunciator and disable said charging circuit allowing discharge of the storage capacitor and interruption of power supply to the switching means until the charging circuit is restored,

whereby successive charge and discharge cycles of the storage capacitor cause repeated actuation cycles of the annunciator.

2. A circuit according to claim 1 wherein said rectifier comprises a bridge having alternating current inputs for connection to said key terminals and direct current outputs.

3. A circuit according to claim 2 wherein the storage capacitor is connected between the bridge outputs.

4. A circuit according to claim 2 wherein the valve means is connected between the bridge outputs.

5. A circuit according to claim 2 wherein the switching means is connected between the bridge outputs.

6. A circuit according to claim 2 wherein the switching means is connected between the bridge inputs.

7. A circuit according to claim 6 wherein the switching means comprises a relay coil connected between the bridge outputs with a contactor connected between the inputs.

8. A circuit according to claim 1 wherein the switching means comprises a first solid state switch having primary electrodes connected between the bridge outputs.

9. A circuit according to claim 8 wherein the switching means includes a relay coil in series with said primary electrodes.

10. A circuit according to claim 8 wherein the switching means comprises a second solid state switch having a control electrode connected to one of the primary electrodes of the first solid state switch.

11. A circuit according to claim 1 wherein the switching means comprises a switching device and a second electronic valve for controlling closing of the switching device, characterized by a second storage capacitor connected to the second valve and having a second charging path from the rectifier and a discharge path through the second valve with a substantially longer time constant than the discharge cycle of the first said storage capacitor, the second valve means being responsive to discharge of the second storage capacitor to hold the switching device closed thereby to cause repeated cycling of the first storage capacitor during the time constant of the second capacitor.

12. A circuit according to claim 11 wherein said switching device comprises a solid state latching switch.

13. A circuit according to claim 12 wherein the latching switch and the second electronic valve are in series.

14. A circuit according to claim 8 characterized by a second capacitor connected to a second electronic valve in series with the first solid state switch, said second valve forming a discharge path for the second capacitor with a time constant substantially longer than the discharge cycle of the storage capacitor, and said second valve being responsive to discharge of the second capacitor to complete the power supply connection to the first solid state switch, thereby to cause repeated cycling of the storage capacitor during the time constant of the second capacitor.

15. A circuit according to claim 14 characterized by a manual switch operable to disable said second electronic valve and allow unlimited cycling of the storage capacitor.

16. A circuit according to claim 1 wherein said electronic valve means comprises an output electrode connected to the input means of the controlled switching means, input means, and a conductive lead connected to said input means.

17. A circuit according to claim 12 wherein the switching means comprises a second solid state switch connected between the bridge outputs and having a control electrode connected to the valve means and output electrodes connected between the bridge outputs.

18. A circuit according to claim 16 wherein the valve means is connected between the bridge outputs.

19. A circuit according to claim 16 wherein the switching means comprises a first solid state switch having primary electrodes connected between the bridge outputs.

20. A circuit according to claim 16 wherein said switching device comprises a solid state latching switch.