brhans helpfully provided this circuit to test continuity of across a switch that may or may not be carrying mains voltage:


Unfortunately, my application requires panel-mounted indicators (I'm currently looking at Alpinetech PL12Ms) which, AFAICT, expect straight-up 12v, i.e. they (presumably) have an internal resistor and should not be hooked up with an additional resistor in series. However, simply removing R4+R5 breaks the function of the circuit through disrupting D4's ability to "short" current around Dred.

I fiddled with this a bit and the only thing I could get to work was to lop off R5, D4 and Dred and replace R4+Dgreen with a DPDT relay coil used to switch the indicators, but that requires a relay and a mosfet, and I'm a little worried about the "slow rise" (~100ms) on the relay coil voltage when the relay switches to "energized".

What is the "right" way to redesign this to use "packaged" / internal-resistor LEDs? For bonus points, what (if anything) needs to change to use 12V instead of 5V?

p.s. AFAICT there are currently no failure modes that would result in Dgreen being illuminated when the switch (RLY1) is open and would illuminate Dred when it is closed. (IIRC there is a mode in which only Dgreen is ever lit, but, while this could falsely indicate that RLY1 is open (not great), the problem would become obvious when closing the relay fails to illuminate Dred.) Retaining this property is strongly desired.

  • \$\begingroup\$ You can get panel-mount LEDs with no resistor fairly easily. Why not go that route? \$\endgroup\$
    – Hearth
    Dec 29, 2022 at 22:32
  • \$\begingroup\$ @Hearth, the answer to that should be evident. Namely, if I knew where to get such things, I wouldn't need to ask. (Although I'd also like an answer for its own sake, as I will hopefully learn something!) If you know where to get something equivalent to a Alpinetech PL12M, however, please share. Note, though, that I would prefer to have a similar indicator that can run off 120VAC, which I can get from Alpinetech. \$\endgroup\$
    – Matthew
    Dec 29, 2022 at 22:34
  • 1
    \$\begingroup\$ You should be able to find them from any reputable electronics components supplier, and plenty of non-reputable ones besides. I'd recommend Digikey or Mouser. \$\endgroup\$
    – Hearth
    Dec 29, 2022 at 22:35
  • \$\begingroup\$ @Matthew shopping questions are not allowed here \$\endgroup\$
    – jsotola
    Dec 30, 2022 at 1:29

1 Answer 1


easiest fix is to just add another MOSFET.


simulate this circuit – Schematic created using CircuitLab

if you're using 12V LED indicator modules you'll need to increase visolated to 12V


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