IES960079A2 - Alarm circuit - Google Patents

Abstract

An alarm circuit contained within, for example, an external bell box includes an alarm device such as a siren 15 or strobe light 16 which is activated in response to an alarm signal received by the circuit. The alarm circuit further includes a light emitting diode LED1 and a photodiode PD1 positioned to receive light from the light emitting diode. When the light path between the light emitting diode and the photodiode is interrupted, for example by the presence of foam injected into the bell box in an attempt to disable the alarm, the resultant change in conductivity of the photodiode is detected and the alarm is raised.

Description

ALARM CIRCUIT This invention relates to an alarm circuit, in particular but not exclusively to an alarm circuit which is housed in a so-called bell box located on the exterior of a building and controlled from a remote alarm panel normally inside the building.

External bell boxes for alarm systems are prone to tampering by injecting foam into the box so that the operation of the bell circuit or the sounding of the bell is impeded. Previous attempts to mitigate this problem have included incorporating a mechanical pendulum in the bell box which stops swinging if foam is introduced into the box. There are many problems associated with such mechanisms and it is an object of the present invention to mitigate such problems.

Accordingly, the present invention provides an alarm circuit including an alarm device and a tamper detect circuit comprising a light source and a photosensor positioned to receive light from the light source, the light source being energised in normal operation of the alarm circuit and the tamper detect circuit having means to detect a change in the electrical characteristic of the photosensor when the path of light from the light source to the photosensor is interrupted and for activating the alarm device in response to such change.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing whose single figure is a block circuit diagram of an alarm circuit which is housed in an external bell box.

OPEN TO PUBL;C INSPECTION UNOER SECTION 28 AND RULE 23 JNL No ......OF _ ^79 Ο07φ The alarm circuit 10 is mounted on a PCB housed within a conventional bell box (not shown). Power to the circuit is 12v d.c. which is derived from a remote alarm panel via terminals 11, 12 on the PCB. A 12v d.c. backup battery is also mounted in the bell box and connected to the alarm circuit through terminal 13, 14.

The bell box housing contains a siren circuit 15 and optionally a strobe light 16 of conventional construction. The alarm is triggered by the receipt at a terminal 17 of an alarm signal from the remote alarm panel, the alarm signal pulling the normally high input to an inverter 18 low. The output of the inverter 18 thus goes high which triggers the siren circuit 15 and (if present) the strobe light 16. A timer unit 19 may be included to ensure that the siren/strobe are not activated for longer than a specified duration.

A switch 20 is provided which is biassed open but is normally held closed by the cover of the bell box housing, so that when the cover is removed the switch opens. In its normal closed position the switch 20 connects the 12v power supply to a capacitor C2 which charges up and remains charged all the while the switch 20 is closed.

The capacitor C2 is connected to the input of an inverter 21 via a resistor R5, and thus maintains the input of the inverter 21 high. Consequently, the output of the inverter 21, and corresondingly the input of a further inverter 22, is held low. This holds the output of the inverter 22 high, so reverse biassing a diode 23 connected to the input of the inverter 18. In this state the circuit comprising the switch 20, capacitor C2, inverters 21 and 22 and diode 23 have no effect upon the input of the inverter 18 and cannot trigger the alarm.

However, if the switch 20 is allowed to open, for example by unauthorised removal of the bell box cover, the capacitor C2 discharges until the voltage on it reaches a level determined by the voltage drop across a resistor R6. This pulls the input to the inverter 21 low, so that the output of the inverter 22 also goes low. This forward biasses the diode 23, so pulling the input to the inverter 18 low in a similar manner to a trigger signal from the alarm panel. Thus the siren sounds to indicate that the bell box has been tampered with.

This fact is signalled back to the remote alarm panel in the following manner. A transistor T2 has its base connected to the input of the diode 23 via a resistor R3, and hence the transistor is normally conducting when the switch 20 is closed. Thus the voltage at a tamper terminal 24, connected to the remote alarm panel, is low. However, when the output of the inverter 22 goes low, indicating a tamper, the base of the transistor T2 also goes low and turns the transistor off. This allows the voltage at the tamper terminal 24 to go high, and this condition is detected at the remote alarm panel.

A tamper signal at the terminal 24 is also produced if the 12v supply from the remote alarm panel is cut. In this case the capacitor C2 also discharges, causing the input to the inverter 18 to go low as described above.

It is to be noted that a diode 25 isolates the backup battery power from the capacitor C2 so as to prevent the battery maintaining the charge on the capacitor if the power from the panel is cut.

In order to detect if foam is injected into the bell box 9600?j in an attempt to disable it, a light emitting diode LED1 and a photodiode PD1 are provided spaced apart on the PCB in relative positions such that light from the LED 1 can fall on the photodiode PD1. In normal operation of the alarm circuit, the LED1 is energised and the light falls on the photodiode PD1. This causes a reverse bias current to flow in the photodiode PD1 which prevents a capacitor Cl from charging up. Since the capacitor Cl is connected to the base of a further transistor T1 via a resistor Rl, the latter is turned off and remains off all the while light from the light emitting diode LD1 falls upon the photodiode PD1.

However, if the light path between the LED1 and the photodiode PD1 is broken, for example by injecting foam into the bell box, then the photodiode PD1 will cease to conduct and the capacitor Cl will charge up. This turns the transistor T1 on which pulls the input to the inverter 21 low via the resistors R2 and R5. As descibed previously, this causes the input to the inverter 18 to go low, thereby sounding the siren 15 and activating the strobe light 16. Also, a tamper signal is sent back to the remote alarm panel via the tamper terminal 24 in the manner previously described.

Claims (5)

1. An alarm circuit including an alarm device and a tamper detect circuit comprising a light source and a photosensor positioned to receive light from the light source, the light source being energised in normal operation of the alarm circuit and the tamper detect circuit having means to detect a change in the electrical characteristic of the photosensor when the path of light from the light source to the photosensor is interrupted and for activating the alarm device in response to such change.

2. An alarm circuit as claimed in claim 1, wherein the light source is a light emitting diode and the photosensor is a photodiode.

3. An alarm circuit as claimed in claim 1 or 2, wherein the alarm circuit is normally powered from a remote alarm panel and has a local backup battery, the alarm circuit further having means for detecting if the power from the remote panel is cut and for activating the alarm device in response to such detection.

4. An alarm circuit as claimed in claim 1, 2 or 3, wherein the alarm circuit is contained in a bell box with a removable cover, the alarm circuit further having means for detecting if the cover is removed and for activating the alarm device in response to such detection.

5. An alarm circuit as claimed in claim 1, substantially as described herein with reference to the accompanying drawing.