In this circuit the signal from the phototransistor is amplified by a Darlington stage and then applied to a trigger stage that controls a relay. The relay trips when the sensor is illuminated. The sensor can be any common phototransistor and the relay is a sensitive type for 12 V voltages with a coil of the order of 220 ohms. Circuit sensitivity is determined by adjusting the 100k ohms trimpot.
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