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how to build a laser tripwire or switch - YouTube

1. Connect the one end of the relay coil to the collector of the transistor.
2. Connect the other end of the coil to +V end of the power source.
3. Connect your resistor and LDR to the base of the transistor.
4. Connect the resistor to +V and the LDR to ground.
5. Connect the emitter to ground.
6. ????
7. PROFIT!!!!

also get a board for this...

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