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
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.
