Live-line Detector

Posted in Tools and measuring, Other sensor, on 2015-09-13

Circuit diagram:

Live-line Detector


  • C1____________100nF 63V Polyester or Ceramic Capacitor
  • D1_____________Red LED (any type)
  • IC1____________4017 Decade counter with 10 decoded outputs IC
  • P1_____________SPST Pushbutton
  • B1_____________3V Battery (two 1.5V AA or AAA cells in series etc.) Sensing probe__3 to 15 cm. long, stiff insulated piece of wire

Circuit operation:

If the unit is brought close to a live conductor (insulated, and even buried in plaster) capacitive coupling between the live conductor and the probe clocks the counter, and causes the LED to flash 5 times per second, because the 4017 IC divides the mains 50Hz frequency by 10. When remote from a live line, the unit stops counting, the LED resulting permanently off.


  • Sensitivity can be varied using a more or less long sensing probe.
  • Due to 3V operation, the LED's current limiting resistor can be omitted.

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