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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)

...read more.



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 lit then unlit each half second, also the yellow LED lit then unlit for half second durations.

Lets look at the operation of LEDs.

Consider the output voltage at pin three to be 0V.

...read more.


If more than 1.2V is applied to the base, the transistor switches on.  Now current can pass easily from collector to emitter, so current can flow from the top rail, through the relay, the transistor, and back to the battery.

When the relay is activated the motor (vibration) is switched on.

Thus as the output from pin 3 (555) goes high and then low alternately, the transistor will

switch on and off, which in turn will activate and deactivate the relay causing the motor to rotate and stop.

Latching mechanism

The output from the primary alarm system is connected via a diode to the gate of a thyristor.  When a positive voltage is applied to the gate the thyristor will switch on and provide a path for the current to return to the battery, so the circuit can now operate.

...read more.

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