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 One possibility is to use the primary winding of a small power transformer from which the core has been withdrawn.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.
Semiconductors:
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
Capacitors:
C1 - 4.7 uF x 16 V - electrolytic
C2 - 10 uF x 16 V – electrolytic
Several:
L1 - Coil pickup - see text
K1 - Relay according to supply and coil up to 50 mA
Printed circuit board, wires, solder, ferrite core, etc.
