Robobass:
OK. Please be patient with me. I want to present the idea, and make sure it is feasible, before I invest more time in this approach. I see uses of the ACS712 to read current, but no mentions of making a threshold based switch with the Arduino board. I don't expect anyone to do the work for me. Hopefully a few hints though.
Hi,
ACS712 sensor converts current to volts (taht is the only magnitude that arduino can read and convert to digital information). It enters the arduino as a value between 0 (no amps) and 1023 (full range).
Once into the arduino, you can choose to convert it to true amperes (by multiplying it by the proper factor; it is recommended -converting the value, I mean) or not. Any case, if you want to know if it has reached a threshold, just compare the value with the number you want (normally something like "if (current > threshold) then ...")
Regards