Automatic Heat Detector

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 energized state, whereas pin 2 is grounded.

Circuit

Circuit diagram

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 energized 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
firebrigade siren. Note: We have added a table to enable readers to
obtain all possible sound effects by returning pins 1 and 2 as suggested
in the table.

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