6727951-automatic-heat-detector.pdf
DESCRIPTION
building servicesTRANSCRIPT
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ELECTRONICS FOR YOU MARCH 2001
C I R C U I T I D E A S
S.C. DWIVEDI
sound from UM3561. Resistor R4 in se-ries with a 3V zener is used to providethe 3V supply to UM3561 when the re-
SUKANT KUMAR BEHARA
lay is in energised state. LED1, con-nected in series with 68-ohm resistorR1 across resistor R4, glows when thesiren is on.
To test the working of the cir-cuit, bring a burning matchstickclose to transistor T1 (BC109),which causes the resistance of itsemitter-collector junction to go lowdue to a rise in temperature and itstarts conducting. Simultaneously,
transistor T2also conducts be-cause its base isconnected to thecollector of tran-sistor T1. As aresult, relay RL1energises andswitches on thesiren circuit toproduce loudsound of a fire-brigade siren.
Lab note.We have added a
table to enable readers to obtain all pos-sible sound effects by returning pins 1and 2 as suggested in the table.
Pin Designation Sound EffectSEL1 SEL2No Connection No Connection Police Siren+3V No Connection Fire Engine SirenGround No Connection Ambulance SirenDo not care +3V Machine Gun
This circuit uses a complementarypair comprising npn metallictransistor T1 (BC109) and pnpgermanium transistor T2 (AC188) to de-tect heat (due to outbreak of fire, etc)in the vicinity and energise a siren. Thecollector of transistor T1 is connectedto the base of transistor T2, while thecollector of transistor T2 is connectedto relay RL1.
The second part of the circuit com-prises popular IC UM3561 (a siren andmachine-gun sound generator IC), whichcan produce the sound of a fire-brigadesiren. Pin numbers 5 and 6 of the ICare connected to the +3V supply whenthe relay is in energised state, whereaspin 2 is grounded. A resistor (R2) con-nected across pins 7 and 8 is used to fixthe frequency of the inbuilt oscillator.The output is available from pin 3.
Two transistors BC147 (T3) andBEL187 (T4) are connected inDarlington configuration to amplify the