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.