Description Figure shows the circuit diagram of the 'Visitor Alarm' . Here I have used a LDR and a mono stable Multi Vibrator for making this circuit . When who breaking the LED ray, the LDR has high resistance so the transistor will be OFF and the pin number two of the IC 555 is negative then the IC 555 will be triggering so the output is ON. When who don't breaking the LED ray, the LDR has low resistance so the transistor will be ON and the pin number two of the IC 555 is positive after that the IC 555 doesn't triggering so the output is OFF. Circuit Idea Making Of LDR and LED Cover both items with a black insulation tape. See the below images for further details. LED LDR Arrangement Of LDR and LED Parts List Component No: Value R1 10K R2 10K R3 680R R4 10K C1 10MF C2 103pF Q1 BC548 D1 White LED Or LASER LDR U1 NE555 B1 6V Buzzer