The current sensor uses a Hall effect ACS756 (50 amp version). Heavy duty leads take the current into the chip where they create a magnetic field whose strength depends on the current. The loop has very low resistance so that at even at very high current there is almost no voltage drop. The Hall sensor is a solid state device whose electrical output is affected my a magnetic field. The chip processes this and produces a voltage output proportional to the magnetic field and hence the input current.

The output at zero amps is 2.5 volts. The output changes at 40 mV per amp up or down depending on the direction of the current.  At 10 amps, the sort of maximum running current of the window motors, the output will be 400 mV. Set the 500 ohm pot so that the output is 2.5v when the ACS756 has zero amps passing. Bear in mind the output of the LM324 can’t get less than about 1v or more than about 4v so the 6.8k resistor may need to be adjusted to get the output within these limits at the motor stall current which might be around 20 amps.

The main circuit comes next:

Current sensing