Light dependent switch

Using this circuit diagram can be made an electronic switch that allows actuation of a relay. This control circuit is triggered using a light source which will act a Photoresistor.
P1 potentiometer will adjust so that the relay is not activated if the LED is off, but is activated once LED lights. In the scheme, the values of P1, R1 and R2 are suitable for use with a LDR which has a resistance of about 6 k. Measure the resistance with a multimeter, the value of R1 should be chosen equal to (approximately) with that, and that of P1 by about two times higher. The values of R2 and R3 should be approximately equal to the sum of P1 and R1.