Power On Indicator Circuit

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.

Power On Indicator Circuit diagram

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.


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  • JFM

    What type of inductor is required? Could you suggest a digi key or mouser part number?
    Many Thanks

  • D.Mohankumar

    The circuit is well tested no such problems found

  • Sivanand

    u need to modify the circuit in such a way that it will pickup only the ac frequency 50/60 hertz..otherwise it will amplify all those switching noise from electronic ballasts/ SMPS etc and render the circuit ineffective.

  • D.Mohankumar

    It is possible.This circuit is intended to show the ability of the counter IC 4017 to detect e.m.f

  • Sivanand

    The circuit requires a 9Volt power supply to power it …the whole circuit can be simplified by using a current sense transformer with very high turns ratio eg 1:1000 or even 1:2000 such inductors are available commercially and are used in Current leakage circuit breakers…

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