Homemade USB interface desk lamp

Circuit and working principle:

The USB desk lamp circuit diagram is shown below. In order to avoid possible electromagnetic radiation, the power supply of the circuit uses capacitor current limiting, and then sends it to the rectifier circuit to provide the 5V DC operating voltage required by the control circuit. When the computer is turned off and not in use, even if the control circuit has AC power, the output pin ③ of IC2 is at low potential, so the triac will not conduct. Optocoupler IC1 separates the computer and desk lamp control circuits. Capacitor C1 is used to bypass variations that may be present in the USB port. R1 is used to limit the input current.

When the computer is turned on, the 5V DC voltage in the USB port is applied to the optocoupler IC1, and IC1 is immediately turned on. After it is turned on, its output pulls down the potential of pins ② and ⑥ of IC2. Then the output of IC2 becomes high, the gate G of the triac BT136 is triggered through R3 and D4, BT136 is turned on, completing the power supply circuit of the desk lamp, and the desk lamp is turned on and emits light.

When the computer shuts down. The input voltage connected to the optocoupler disappears immediately, causing the potential of pins ② and ⑥ of the timer chip to become high and unable to perform its normal function. At this time, its output pin ③ becomes low, and the desk lamp automatically goes out.