Alternating Current Detector (ART402E)

   The circuit shown in figure 1 can detect the circulation of alternating currents in electrical installations from values as low as 300 mA without the need for contact. The circuit can also be adapted to be used as shield for microcontrollers.





   The pickup coil may have several shapes depending on the type of application desired. One possibility is to use the primary winding of a small power transformer from which the core has been withdrawn. Another possibility, shown in the same figure 4, is to make a "U" of non-magnetic magnetic material (ferrite) and to wind 500 to 1000 turns of very thin (28 or thinner) enameled wire. In this case, the conductor wire that is wanted to detect the current.

   The gain of the circuit is determined by R3 which can be increased as a function of the sensitivity obtained with the coil used. The recommended maximum value for this component is around 2.2 M ohms.

  If a trimpot is used instead of R3, the circuit's trigger point can be precisely adjusted. The MC1458 operational amplifier is an operational double with characteristics similar to 741. Two 741 ICs can then be used instead.

   Circuit power can be supplied with voltages from 9 to 12 V and the relay used depends on this voltage. Sensitive types with coils up to 50 mA are recommended.



CI-1 - MC1458 or equivalent - dual operational amplifier

Q1 - BC548 - NPN general purpose transistor

D1, D2, D3-1N4148 - general purpose diodes


Resistors: (1/8 W, 5%)

R1, R2 - 47 k ohms

R3 - 220 k ohms

R4 - 100 k ohms

R5 - 1.2 k ohms

R6 - 560 ohms



C1 - 4.7 uF x 16 V - electrolytic

C2 - 10 uF x 16 V – electrolytic



L1 - Coil pickup - see text

K1 - Relay according to supply and coil up to 50 mA

Printed circuit board, wires, solder, ferrite core, etc.


Circuit Bench