Working of second and third sections is known to many of the hobbyists- second is merely a multivibrator, `whereas third is an audio amplifier.
This device comprises a triggering section, an oscillating section, and an amplification section.
SCR] may be triggered either by bringing the gate of SCRI in contact with positive pole of the battery momentarily or by flashing a light on the light dependent resistor (LDR).
By adjusting potentiometer. VRI, the sensitivity of the instrument can be controlled. To switch off the alarm the battery will have to be disconnected momentarily.
This signal is amplified by the third section and produces an audio alarm through the speaker. This circuit works on 9V.
Even a light from a cigar can trigger the silicon controlled rectifier. On power (negative voltage) being applied, the second section produces oscillations.
When light falls on the LDR, its resistance is reduced, thereby a current flows through the gate of SCR]. This gate current triggers SCR] and the oscillating and amplification sections of the alarm get negative voltage.
These two sections commonly get negative voltage when SCRI is triggered.
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