I am hoping to put together a simple 12V circuit whose load is a digital voltage meter. The circuit will have two 12V inputs and the goal is to have power to the meter when only one of the leads are hot, then turn off when both 12V leads are hot.

Seems like it would be fairly simple and might utilize a single transistor. Or maybe there is a small device that I can use to accomplish this?

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  • 1
    \$\begingroup\$ 12 V relay solenoid connected between the two inputs \$\endgroup\$
    – jsotola
    Commented Mar 9, 2021 at 6:19
  • \$\begingroup\$ Are you trying to achieve one but not both or an exclusive OR(either but not both or neither)? \$\endgroup\$
    – K H
    Commented Mar 9, 2021 at 6:38
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    \$\begingroup\$ Need more info… the other lead will be floating or grounded when not powered? also the lead which can be off is always the same or can be either? \$\endgroup\$ Commented Mar 9, 2021 at 7:10
  • \$\begingroup\$ I have added a pic to clarify the objective -- maybe a solenoid that opens instead of closes when the coil is energized .. \$\endgroup\$
    – gothicsurf
    Commented Mar 9, 2021 at 15:14

1 Answer 1


From the truth table provided I would say the Output equals NOT 12Vswitched. I would use a 12V relay with normally closed output to switch through your constant 12V source.


simulate this circuit – Schematic created using CircuitLab


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