The l.e.d.s are turned on by a light-sensitive controller, formed around op.amp IC1,configured as a comparator. The ambient light level is detected by a light dependent resistor, R1. Its sensitivity is controlled by preset VR1 and the resulting voltage across it is fed into the non-inverting input (pin 3) of IC1. The inverting input (pin 2) is fed, via resistor R2, by the voltage generated by IC2.