NE555 light-operated switch

The device uses the on and off of the light path to control the switch of the light, and adds a delay circuit to improve the stability and practicality of the light-controlled switch.

The circuit principle is shown in the figure. This circuit consists of a transmitter (VD1 high brightness light emitting diode LED), a receiver (photoresistor LR), a delay circuit (NE555) and a switch control part (relay K1). When a person approaches the table, the light emitted by the LED is blocked, and the internal resistance of the photoresistor becomes high resistance.

The base potential of transistor VT1 decreases and the collector potential increases. Capacitor C1 is charged through the diode, and the emitter of transistor VT2 outputs a high-level signal (this level signal must be greater than or equal to 4V, i.e. 2/3VCC). Then the 7-pin discharge tube of NE555 is turned on, so that relay K1 is attracted and the desk lamp lights up. When people leave. The light path between the light-emitting diode and the photoresistor is unobstructed, the base potential of VT1 increases, the collector potential decreases, and capacitor C1 discharges, causing the emitter potential of VT2 to slowly decrease. When it drops to 2V, the 7-pin discharge tube of NE555 is cut off, the relay is released, and the desk lamp goes out. This circuit is set to delay for 1 minute, which can prevent the inconvenience caused by the short-term departure of people or occasional body movement that makes the light path unobstructed, causing the relay to release and the desk lamp to go out.