Ultra-simple Voltage Probe

Detects 1.8 to 220 Volts DC or AC

Minimum parts counting

Circuit diagram

Ultra-simple Voltage Probe-Circuit diagram


  • D1 5 or 3mm. Red LED
  • D2 5 or 3mm. Green or Yellow LED
  • LP1 220V 6W Filament Lamp
  • P1 Red Probe
  • P2 Black Probe

Device purpose:

This circuit is not a novelty, but it proved so useful, simple and cheap that it's worth building. When the positive (Red) probe is connected to a DC positive voltage and the Black probe to the negative, the Red LED illuminates. Reversing polarities the Green LED illuminates. Connecting the probes to an AC source both LEDs go on. The lamp limits the LEDs current to 40mA @ 220V AC and its filament starts illuminating from approx. 30V, shining more brightly as voltage increases. Therefore, due to the lamp's filament behaviour, any voltage in the 1.8 to 220 Volts range can be detected without changing component values.


A two colors LED (Red and Green) can be used in place of D1 & D2.

author: RED Free Circuit Designs
circuit from http://www.redcircuits.com/