This simple circuit can be used for detecting 50 Hz current inside a conductor without physically connecting with the conductor. You just have to bring the L1 coil near the conductor and the LED will light up indicating a current inside the wire conductor.
The following circuit depicts a simple way of locating a cable or wiring that may be carrying some current.
The detection coil is made of a telephone pick-up consisting of a suction cup. Alternatively you can use a 100 uH coil, te ones which are used in buzzer circuits.
How the Circuit Works
A very small voltage in L1 is introduced by the magnetic field from a current-carrying conductor. This small voltage is then amplified by the A1 and A2 opamps.
A maximum amplification of A1 and A2 of signals around 50Hz is being ensured by the value of capacitor C2 to C5. During the positive halve-waves of the mains current, D1, i.e. the LED will light up.