Voltage Monitor

Circuit diagram

 

Parts

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

Description

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.

author: Paul Stilo
circuit from http://circuitarchive.150m.com/