Live-line detector is a circuit that detects the presence of a live mains conductor circuit operation: capacitive coupling between the live conductor and the probe will clock the counter when this unit was around a live conductor. And then the counter causes the LED to flash 5 times per second. It can occur due to the 4017 IC divides the mains 50Hz frequency by 10. This unit will stop counting, when remote from a live and then the LED resulting permanently off.
Notes: Length sensing probe can affect the sensitivity. The LED’s current limiting resistor can be omitted, because it’s a 3V operation [Source: electronics-lab.com]