I have a 0-30V DC signal and it can be physically disconnected from a circuit. I need to detect and generate a TTL output when this signal is disconnected.
The "detector" circuit and the signal will share a common ground.
The thing I can't figure out is 0V (GND) is also a valid "connected" state for that signal. Using standart approach of a pulldown for detecting unconnected state is not going to work.
Edit: Thanks for all your input. Additional information:
- Signal is not digital, varies between 0-30VDC. Sorry for misleading tag, removed that.
- Which side I need to detect: I need to detect at receiver side and also need report that to sender.
- Impedance of source: Low, but going higher than 50mA load will going to disturb other things that depends on this signal.
- Pulling down lower than 0, higher than 30: I actually thought that but it is not seems feasible since I don't have these.
- Pulling to middle: 15V is valid signal, can't use that.
- Injecting signal: Nice idea, but receiving side need to be "basic" as possible and I can't modify freely the sender side.
Is there any way to detect this electrically? Thanks