Detects the presence of a live mains conductor

Parts:

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

Diagram:

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.

Notes:

• 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.