A short cut of the light that falls on the LDR causes the relay to close its contacts and lock. The circuit is from a 1979 documentation but can be assembled with BC548 or BC549 transistors and even the operational one admits equivalent. The relay is 6 or 12 V, depending on the power supply and the LDR is of the common round type. Sensitivity can be increased with the use of lenses and the adjustment trimpot can be 47k.
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