Pro Micro - Measuring voltage between two pins, without using GND

Question

Quite new to this but making pretty good ground. I have an issue I'm not sure how to solve. I want to add a Pro Micro to an RC Toy, I can easily power it from the toys GND and 5V power. But then I want to use the voltage going to an LED on the toy (uses a different ground) to trigger an event within the Pro Micro.

I can power the Pro Micro from USB and and accomplish what I need with AnalogRead(). But if I connect the GND from the toys main power input to the GND from the LED it no longer acts as it should. I'm thinking these GNDs are not common, or are a separate circuit.

What I'm wanting to know is.. I'm using PIN10 on the ProMicro for the AnalogRead(), can I make it so that maybe PIN11 is the GND for that reading?

Thanks.

Answer 1

No, you cannot - the ATmega's ADC is referenced to its ground, and there are also some strict limits on providing inputs more than a little bit outside of the range between its power and ground.

You may be able to read two analog inputs and subtract them for a sort of "software differential" analog input, but the signal on an LED is unlikely to be meaningfully "analog" anyway - it might be pulsed with a duty cycle, but it will probably not have a steady reduced voltage.

It's actually quite likely that the anode of the LED is connected to the toy's positive supply, and it is the negative path to the cathode which is being switched - this is the most traditional way of doing things, because historically semiconductor switches suitable for "low side switching" have had better performance than those suited for "high side switching". There will probably be a resistor on one side or the other of the LED.

With care and verification of suitable voltage ranges, you may be able to read the voltage on the LED cathode as a digital signal and invert it in software.