Circuit Light Siren
The LDR and R1 & R2 form a
voltage divider whose output voltage is light dependent. As the light level increases, the junction of the LDR and R1 increases.
When it reaches 6.8V, Q1 turns on and subsequently triggers the alarm. Pot R2 calibrates the divider for the required light level threshold. The voltage out of the divider may be measured with a voltmeter so you can see what is happening as you debug the circuit.
Q1 is an NPN transistor applied as a comparator. Never heard of a transistor comparator before well now you have not exactly high performance with limited gain and 600mV offset, but fully functional and perfect for this application. Q1 has a minimum current gain (hFE) of 40 at low current levels.
When the LDR is fully off, the Vbe of Q1 is -6.2V. Most
transistors are rated for 6 to 7V reverse bias on the base to emitter junction.
When light falls on the LDR, its resistance decreases and the voltage at the junction of the LDR and R1 increases. When the voltage exceeds 6.8V, Q1 turns on. The collector current of Q1 is the base current of the darlington driver and turns it on. This is called a high-side driver because it works against the positive bus.
When voltage appears across the siren module, the voltage across and current through R5 reverses polarity and forces the voltage at the input voltage divider slightly higher thus causing it to “snap” on positively. When three transistors are wired like this, the DC gain is so high that a minute amount of leakage can cause circuit malfunction. R3 and R6 provide paths for leakage current thus insuring reliable operation.