Door KnocK Alarm With Timer


This low-cost circuit
uses the piezoelectric element of a piezobuzzer as the input sensor. The
piezoelectric element plate is fixed at the centre of the door wing by
using a cello tape. Apply a small quantity of adhesive at the edges
between the plate and the door. Extend wires about 1-1.5 metres from the
piezoelectric to the circuit. IC NE555 (IC1) is configured in
monostable mode. When it gets an input pulse its output goes high for a
period set by VR1, resistor R5 and capacitor C3. IC UM66 (IC2) is used
as a melody generator. When the door is knocked at, the piezo plate
generates an input pulse, which is amplified by transistor T1.

The amplified signal triggers the timer IC NE555 and its output pin 3
goes high to enable the melody generator. Music is heard from the
speaker LS1. After the set time period, the melody sound stops. Assemble
the circuit on a general-purpose PCB and
enclose in a suitable case. Fix the piezo element at the door and place
the speaker in a central room inside the house using long wires. The
circuit works off 5-12V DC. The music time can be adjusted through VR1
by changing the R-C time constant of the timer.


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