电压监视器电路，Voltage Monitor

Parts List

D1 LED
R1 1.2k Resistor
R2 10k Var. Resistor
U1 UA741 OP AMP

When the input voltage is 0 the LED glows. The LED stops glowing when the voltage rises to the level determined by R2. Reverse + and - pins to reverse operating mode. To set voltage at which LED goes off, (1) Set 0V at input. (2) Set input voltage at desired level. (3) Adjust R2 to point right after LED goes out.