Laser Door Alarm
If you’ve already made the Laser Burglar Alarm (which is highly recommend doing FIRST), then you’re probably wondering how to make the circuit act in the opposite way… meaning how do you make it so that the buzzer sounds when the light is turned off?
This circuit requires more patience and parts, but it’s totally worth it. It uses the same parts as the previous experiment (plus a few more) with a couple of extra twists and turns in the circuit to let the buzzer know when it’s time to turn off. Use this in doorways or as an invisible trip wire trigger across hallways.
- Red laser pointer
- 9V battery or two AA’s in a AA battery pack
- 7 alligator clip leads
- CdS Cell
- 9V Battery OR 2 AA’s in a battery holder
- NPN Transistor 2N3904 or 2N222A
- 4.7k-ohm resistor
- Buzzer (3-6V)