Minimum parts counting
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
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.