This is high temperature fire alarm circuit, when the temperature rise exceeds the limit, it will sound the siren rang out. It is easy to use with transistor circuit to detects heat. Then, a loud noise of the sirens with IC packages.
How it works
We uses the combination between Q1 (BC109), which is silicon NPN transistor and Q2 (AC188) which is PNP germanium transistor To detect heat in case of fire. Then, the siren will sound warning beeps. The collector lead of Q1 is connected with Base lead of Q2, while the collector lead of Q2 is connected with the relay-RY1.
The second section circuit include with IC1-UM3561. Which is siren generator IC or a machine gun at pin 5 and pin 6 of IC is connected to 3 volts power supply. When relay is running, pin 2 to ground. The resistor-R2 across pin 7 and pin 8 to limit the frequency of oscillator. The output will send to right of both transistors Q3 and Q4 which is connection as darlington compound to boost up signal from IC1 resistor R4, which is connected in series with Zener Diode to limit voltage to apply to IC1. The LED1 connected with resistor R1 in series and parallel with R4 . Which will light up when the sound generator working.
How to test
Functional testing of circuit can point out burnt on the transistor Q1. This will give the resistance the joints of the legs emitter – collector decline due to higher temperatures. And it started to conduct current. While Q2 is also current conductor. Since its base leg connects to collector leg of Q1 resulted in a relay run. And switch on the siren circuit causing the alarm beeps.
Parts you will needs
Resistors ¼ W +-5%
ZD1___Zener diode 3V 0.5W
LED1____LED red color
IC1______UM3561 Three Siren Sound Generator
RY1____Relay 6V, 100 ohms
Speaker 8 ohms, 1W