When the day is getting black to a certain extent, the circuit will automatically open led projector light. This is due to photosensitive components mainly.
The general photosensitive component is cadmium sulfide (abbreviated as CdS) photosensitive resistance, also known as the CdS cell. The value of the photosensitive resistance is changing with the light intensity. When the light intensity is large, the resistance is almost zero, with good performance of electrical conductivity. On the contrary, if there is no light, the resistance is large, almost non-conductive. You can connect the CdS components and an electric relay directly to form a simple circuit. Such a circuit can make a strong electromagnet coil power in the light and disconnect the currents in the weak light. However, the CdS components are usually unable to provide enough current to trigger the relay if only by means of light. So you need a transistor to amplify currents over CdS component. A typical circuit is shown below:
The transistor is equivalent to a switch. It has three terminals: the base, the emitter and the collector.
When there is a small current flowing from the emitter to the base, there will be a large current flowing from the emitter to the collector. In other words, in this circuit, if the base grounded, “switch” (emitter to collector paths) was open. When the photosensitive component is received light, the transistor opens and the solenoid coil power relay is triggered, which induced that the street lights turn off. In the evening darkness, the light-sensitive component was high-impedance, so no current flows through the base and relays aren’t triggered, so street lights are lightened.