This heat detector circuit design uses uses a complementary comprising npn metallic transistor pair T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1.
|Circuit diagram of Automatic heat detector : Click on image to enlarge|
This temperature sensor circuit uses a popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren.The 7 and 8th pin of the IC usesd to set the inbuilt oscillator frequency. The output of the IC is pin 3.
Two darlington pair transistors (BC147&BC187) are included in this circuit to amplify the signals from the IC since the ic UM3561 is not capable to drive the speaker.
Testing the circuit
To test the working of the circuit, bring a burning matchstick close to transistor T1 (BC109), which causes the resistance of its emitter-collector junction to go low due to a rise in temperature and it starts conducting. In the same time, transistor T2 will also conducts because its base is connected to the collector of transistor T1. As a result, relay RL1 energizes and switches on the siren circuit to produce loud siren.