Circuit 85 – Smoke alarm (Deaf person).

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Circuit 85 – Smoke alarm (Deaf person)


When normal smoke alarm sounds it will activate the add on circuit which in turn will switch on a vibration motor which will arouse the deaf person.

Circuit diagram


R1 = 1k ohm                                R2 = 470 ohm

R3 = 470 ohm                                R4 = 47k ohm

R5 = 10k ohm                                VR1 = 100k ohm

C1 = 100μF                                C2 = 470 μF

IC1 = 555 Timer                        Transistor (TIP121)

LED = 5mm Standard                 Smoke alarm (Commercial)

Motor (12V)                                THY1 = Thyristor (TIC106D)

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RLA-1 = Relay (DPDT)                D1 = D2 = 1N4001


The heart of the circuit is a 555 timer chip configured in astable mode.

This means the out put at pin 3 is constantly changing, i.e. the output goes high (9V) for a specified time and then low (0V) for a  specified time before again switching high.

The frequency is controlled by the size of R1, VR1, and C1.

Formula to calculate the frequency:-

From values of R1, VR1, and C1.  The frequency of the output is set to 1 hertz


This means the red LED is ...

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