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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.
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Arrangement Of LDR and LED
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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 |