555 Timer Modeling

The
555 IC timer chip is widely used as both a monostable and astable
multivibrator. Originally introduced by Signetics using bipolar
technology, the 555 is now available from several manufacturers in
bipolar and CMOS technology.

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Touch Door Alarm Circuit Using 555 Timer IC

This is a design simple alarm that can be used to provide a audible alarm when someone touches the door knob or handle of your room. The door knob or handle must be made of metal for the circuit to work. The main chip in the circuit is a 555 timer which will be triggered if a hand comes close to or touches the door knob.


The circuit attaches to the door knob at the end of the 1 Mega ohm resistor. This is operation of the circuit. Once the timer is triggered the LED will light and the UJT will output a tone to the speaker. The timers will time out in 5 seconds. The sensitivity of the trigger can be changed by changing the 1 Mega ohm resistor to another value. The 5 second time out can be adjusted by changing the value of the resistor connected between pin 8 and pin 7. The output tone can be changed by changing the RC values on the base of the UJT.

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