This Power On Indicator circuit is highly useful to detect the presence of AC in Mains powered instruments such as Lab equipments and battery chargers which are to be switched on continuously. This Non Contact Power Monitor lights an LED when current starts to flow through the power cord of the instrument.
This circuit can detect AC in the power cord from a distance of one foot or more. The AC frequency is available around the power cord which appears as AC hum. The circuit senses the AC hum and indicates the same through the LED. IC CD4017 is the Johnson decade counter with ten outputs. Its clock input pin14 is highly sensitive and can detect the e.m.f. The outputs of IC becomes high one by one as its input pin 14 receives pulses. IC counts only if the reset pin 15 is grounded. In the circuit only one output (pin 11) is used to light the LED. The fixed inductor L1 accepts e.m.f from the power cord and sends it as pulses to the input of IC. This makes the outputs high very fast so that LED blinks.
Place the circuit near the power cord of the instrument. No setting is necessary and LED will light up indicating power. If LED is off, it indicates that current is not flowing through the power cord.