The circuit shown in the figure activates a small indicator lamp that will start to flash when the ambient lighting is reduced. The trigger point of the circuit is determined by P1 and the blinking frequency is set to P2. The sensor is a common phototransistor that, for greater efficiency, must be mounted on a tube and pointed at the place where you want to monitor the light. This sensor cannot receive the light from the lamp itself, which is fed so that there is no feedback, capable of unstable the circuit. For lamps up to 50 mA, a BC548 can be used and for lamps from 200 mA to 1 A, a power Darlington, such as the TIP120, mounted on a heat radiator must be used. Resting consumption is very low. Consumption, when activated, depends only on the lamp used. Thus, to supply larger lamps, a source or battery must be used.
