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A SIMPLE ALARM CIRCUIT

There are four basic parts to an alarm circuit. First, there is the sounding device, usually a bell; second there is the power supply, which may be a battery or a mains supply. Next, there are the switches which are actuated by the intruder, often referred to as sensors, and finally a control which in its simplest form is just a master switch to switch the whole circuit on or off.

A simple circuit consists of bell, battery, control switch and parallel sensors. The master switch, bell and battery are connected in series, and several sensors in parallel are also connected in series with the bell and battery. These sensors may take the form of door switches that operate when a door is opened, or a pressure pad filled under a carpet that closes the circuit when trodden on. Any number of sensors can be added to the circuit, and the bell will ring when any one of them is operated.

As it stands there is a very serious limitation to this circuit. If the bell rings when a door is opened, it can be stopped by simply shutting the door and thus opening the switch contacts. Alternatively, if the alarm is actuated by a pressure mat, it will stop as soon as the intruder steps off the mat. Obviously, such an arrangement is of little use as a security device.

What is required is a means whereby the alarm once started latches on and any subsequent alteration of the sensor conditions will have no effect at all on its continued sounding. Only switching off at the master switch will then silence it.

This latching facility is an essential part of any alarm system and usually consists of part of the control unit circuit, although it can also be contained in the bell housing.

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