This simple circuit when connected to a metal locker or cupboard generates an alarm when touched by an undesired entity.
The transistor gets saturated by just touching its base. Here is a simple, low cast touch switch with alarm based on this idea. The 50 Hz hum present in our body is the key to this circuit. BC 549 transistor has been chosen for this circuit. The circuit is basically a burglar alarm with timer. Timer circuit is a monostable multivibrator. Initially voltage at pin 2 of IC 1 is low.
If the sensor, which is the base of the transistor T1 is touched by the finger (even the slightest, touch will do) transistor T1 is saturated. Now the voltage at pin 2 of IC 1 is greater than 1/3 Vcc. As a result IC 1 gets a trigger pulse at its trigger input pin 2. Therefore its out goes high for a predetermined time period. The period can be adjusted by varying the value of resistor R1 or capacitor C1 as T=1.1*R1*C1 sec.
The siren circuit is build around IC 2 (UM 3561). Since the output of IC 1 is high, the IC 2 gets power supply. The output from IC 2 is amplified by transistor to drive an 8-ohm, 500mw load-speaker.
source: www.electrosofts.com
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