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. This
circuit warns the user against fire accidents. It relies on the
smoke that is produced in the event of a fire. When this smoke
passes between a bulb and an LDR, the amount of light falling on
the LDR decreases. This causes the resistance of LDR to increase
and the voltage at the base of the transistor is pulled high due
to which the supply to the COB (chip-on-board) is completed.
Different COBs are available in the market to generate different
sounds.
The choice of the COB depends on the user. The signal generated by
COB is amplified by an audio amplifier. In this circuit, the audio
power amplifier is wired around IC TDA 2002. The sensitivity of
the circuit depends on the distance between bulb and LDR as well
as setting of preset VR1. Thus by placing the bulb and the LDR at
appropriate distances, one may vary preset VR1 to get optimum
sensitivity.
An ON/OFF switch is suggested to turn the circuit on and off as
desirable.
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