This circuit activates a speaker output detecting the absence of light on LDR (Light Dependent Resistor). It is particularly well suited to burglar alarm system.
Contrary its normal use as an astable or monostable multivibrator. The transistor BC 148 is an NPN type transistor, which is use as a audio preamplifier & transistor BEL 188 is a PNP type transistor, which is use as an amplifier.
These both transistors is working together as a Complimentary Pair Direct Coupled Amp. For oscillation of the circuit Feedback Amp. Transistor BEL 188 is collector giving of .02 mfd or 12K Resistor starts to pre transistor BC 148.
The LDR is placed between bases of the preamp. Or negative supply of the circuit.
There is only 3V DC supply is required for operating the circuit.
R1 – 270K
R2 – 12K
C1 – .02MFD
T1 – BC 148
T2 – BEL 188
8E SPK. – 1
3V Battery Holder – 1
LDR – 1