Why Street Lights Can Open Automatically When It's Getting Dark
When the day is getting black to a certain extent, the circuit will automatically open the street lights. This is due to photosensitive components mainly.
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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: