Automatic heat detector

 The components need to prepare this:

• Transistor
• Resistor & capacitor
• Loudspeaker
• IC UM3561
• Relay
• Zener diode
• Diode
• LED

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 re-lay 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 firebrigade.
EMP Lab 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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