I am trying to detect when there is a current in a given circuit using only a digital input and basic components (e.g. diodes, resisters, transistors); current is intermittent and power is small, 40-100mA @ 12V. I do not need to measure the current, simply know when it is present or not present.
I am trying to avoid additional chips e.g. INA219, optocouplers, etc. Is it possible to detect (again, not measure, only detect) current this way? Detection does not need to be rapid, within 100ms would be fine.
The load can have resistors, diodes, etc. in series or parallel. A voltage drop of up to 2 volts is fine. The power supply is stable around 12 volts and near the load. The digital input/output logic is 3.3V (Raspberry Pi); the presence of current would ideally be read as 3.3V (or at least 2) while the lack of current would ideally be read as 0V (or less than 2).
The important detail I forgot to mention is that the voltage source switches polarity. It is stable at 12 volts, but current can flow in either direction.