krys WROTE:I already constructed the circuit. My problem is that when I connect the circuit to a 9V dc source, the buzzer buzz non-stop. I think it's because of the surrounding light. How should I resolve this problem? I mean...how should I make the circuit only trigger by abnormal light instead of surrounding light such as the sun or the moon?
Ok I'll tell you how to trouble shoot the circuit.
First remove the buzzer if it is making too much noise. Connect in the power supply say 9V. Get a digital meter ready. Pin 2 of the op-amp 741 should be half of the supply voltage ie 9/2=4.5V . Then measure pin 3 of the op-amp. On normal ambient light, the LDR should be below 1000 ohms. So adjust R9 until the voltage on pin 3 falls just below 4.5V. If you are not able to get it below 4.5, get another pot of 1K and connect it in series with R9. temporarily short R9 with a wire soldered with 2 crocodile clips. Adjust the 1K pot until the LED goes off. Then cover the LDR with your hand and the LED will be lighted again.
The reason you have to keep R9 because if the alarm is to be used at night, you might need it as the ambient light is lower. So I guess you didn't read the web-site carefully as it was all mentioned in your web site.
Notes
To setup the alarm ,power up the circuit and adjust R9 so that LED D1 goes off.
The LDR can be housed in a dark tube to increase sensitivity.
The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with R9 for fine adjustment.
Allen