I understand how to sense 12 V and higher circuitry. Can someone walk me through how to apply to this sensing grounding of external circuit.
Can you put a pullup resistor on it (from 5V) and see if the grounded thing goes high?
Not use about pullup resistor. Basically I have access to a switch which complete the circuit by the providing ground to power up an external unit. How can I work with this to sense when the unit is on in the arduino.
What is the "external unit"?
It's possible you can detect the unit's power supply level when the switch is open and 0v when closed. But it depends on what the "unit" is.
This is essentially what Crossroads said, the "unit" is a pullup resistor, so now that I think about it why can't you just use a pullup resistor?
sorry I am newbie what is a pullup resister
A resistor from the Arduino pin to 5v.
But you haven't answered the question, what is the "unit"?
a mechanical lock power by 28v dc