This circuit uses a complementary pair
comprising npn metallic transistor 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.
The second
part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator
IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC
are connected to the +3V supply when the relay is in energised state, whereas pin 2 is
grounded. A resistor (R2) connected across pins 7 and 8 is used to fix the frequency of
the inbuilt oscillator. The output is available from pin 3. Two transistors BC147 (T3) and
BEL187 (T4) are connected in Darlington configuration to amplify the sound from UM3561.
Resistor R4 in series with a 3V zener is used to provide the 3V supply to UM3561 when the
relay is in energised state. LED1, connected in series with 68-ohm resistor R1 across
resistor R4, glows when the siren is on.
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. Simultaneously, transistor T2 also conducts because
its base is connected to the collector of transistor T1. As a result, relay RL1 energises
and switches on the siren circuit to produce loud sound of a fire-brigade siren.
