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Circuit 85 &amp;#150; Smoke alarm (Deaf person).

Extracts from this document...

Introduction

Circuit 85 – Smoke alarm (Deaf person)

Function

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.

Middle

Operation

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

(approximately).

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.

Conclusion

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.

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