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Crank Doorbell Circuit Schematic

2016-02-20 10:29  
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Here is an interesting hobby circuit of a “crank” door bell! The circuit is built around a timer 555 and a musical piezo-buzzer. The circuit works off 9 volt battery supply. One 9volt PP3/6F22 compact battery is enough for powering the whole gadget.

Timer IC (NE555) is wired in monostable mode to produce a time out o some seconds. The output of the timer is fed to the input of the piezo sounder through a diode D1. Capacitor C2 works as a buffer. RC components R2, C1 sets the monotimeout of the timer IC NE555. An ordinary low voltage small dc motor is here used as the crank switch.

In prototype, one small “disc motor” from a discarded cd drive (optical deck) was used. Even a slight rotation of the motor shaft will produce sufficient counter emf to enegize the LED(S) inside the ac opto-coupler PS2505. As a consequence, phototransistor inside PS2505 conducts and there by it pull down the trigger input (pin2) of IC NE555. The result is a loud but pleasing musical tone output from the piezo sounder for a finite duration, here around seconds.

One can easily replace the musical buzzer with an EM relay for switching external loads. A 9 volt/600 Ohm relay is preferred for this modification. The circuit can also be used as a two wheeler security aid which senses the wheel rotation of a parked vehicle and sounds an alarm to alert the owner. With a little skill, one can easily attach the crank mechanism (like the dynamo-wheel assembly) to any vehicle. However, for burglar alarm application, a latching output circuit is highly recommended.

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