Detect "HIGH level" voltage without detecting own signal

Thread Starter


Joined May 24, 2011
Hi! I am not too familiar with circuits, in fact this would be my second mini project, but I am out of ideas.
[Sorry for the topic name, it is not high voltage, that was a mistake. Moderators could you please remove that misleading word?]

I have a device that measures if 12 VCC is present on a circuit. However this device sometimes also connects its own 12 VCC on the very same circuit.
What circuit do I need to 'filter this out', so I detect only 'foreign signals'?

Eg. the measured circuit has a lamp as a load. The device will check whether it is switched on or not. However, at the same time, the device regularly lits that lamp, but the device should not detect this, only when it is done by something else. So clearly I cannot just connect it parallel like this, because that would make the device also detect itself. I though about relays, but they may not be fast enough to detach the detection line before the device detects itself.

Any ideas are welcome :)
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Joined Nov 25, 2009
You could add a second relay in series with the first and take your measurement between the two relays. You also need to add a large pull down resistor in the testing point, leading to Ground.

There must be a more economical solution, though.

A XOR gate could also do it. Does someone know if there's a family of logical ICs working in 12V?


Joined Nov 20, 2008
Hi NeoXon,
  1. Can the device be modified to do what you require correctly, or must external circuits be added to do the job?
  2. What supposed to happen if the device is turning the lamp on and the switch is close?
  3. Can you post details of what is inside the device?
  4. You can use a half of a DPDT switch to disable the 12V sense input when the other half of the switch is turning the lamp on... assuming you are able to change the switch.


Joined Jul 17, 2007
Why does this sound to me like automotive running lights, brake lights and turn lights?

Thread Starter


Joined May 24, 2011
CMOS4070 should be everything I need. Oh my god, I could have think about an XOR gate myself :) Thank you all!

Thread Starter


Joined May 24, 2011
Well, I thought about it a little more, and actually I don't need a XOR, but instead I need an inverter and and AND gate, but still CMOS4000 family is the one I used use. Thanks again!