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Study Guide

Simple Alarms


1. Alarms using an electro-magnetic buzzer


To use the circuit shown below (left), the diode should be permanently illuminated by a beam of light. When the light beam is cut, by an intruder, the conductivity of the diode decreases and the alarm sounds. The alarm will continue to sound, even when the light returns, until the reset switch is pressed. The value of resistor R is found by experiment but will probably be between 100k and 560k (start with 180k.)

If you want an alarm which sounds when the diode is exposed to light (for example, in order to protect a bag, drawer, suitcase etc) you will need to use the modifications to the circuit shown in the diagram on the right.


The NOR gate is one quarter of a 4001 chip. Transistor: BC109C or similar.


Vero diagrams

1. Alarms using an electro-magnetic buzzer : Dark Alarm

2. Alarms using an electro-magnetic buzzer : Light Alarm

Alarms using oscillator driving high impedance earphone

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