I am looking for a solution for some lighting that was installed recently. The lights are single LED downlights that are connected from a direct connection to 24Vdc through a constant current driver to the LED.

The problem is, there was supposed to be an emergency lighting supply connected to some of these lights as well, and there is a test button that will turn this supply on. This obviously would be problematic without some additional electronics that would allow the presence of the emergency supply to cut off the mains supply from an individual light (lights on a given circuit are not all fed with an emergency supply) and allow the emergency feed to take over, and of course, when the emergency feed is switched off, the mains would take over again.

I have been looking for a commercial product for such applications, but I have not yet come across any.

Does anyone know where I could look, or know how to do a DIY resolution with electronics?


  • 2
    \$\begingroup\$ A DPDT (or SPDT, but DPDT is more common) relay? \$\endgroup\$
    – Hearth
    Dec 16, 2020 at 4:52
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    \$\begingroup\$ you have it backward ... it should be switch the battery supply off when the main feed is switched on because emergency lights are controlled by the main feed ... the test button disconnects the main feed \$\endgroup\$
    – jsotola
    Dec 16, 2020 at 5:27
  • \$\begingroup\$ Hi jsotola, not in this system. This was a pre existing system where the emergency 24V fed separate lamps within a light fixture. Now the lights are single spot lights and another lamp cannot be accommodated. I understand your point though. \$\endgroup\$ Dec 16, 2020 at 7:22
  • \$\begingroup\$ So, to clarify further, when the test button is pressed the main lighting supply remains on. This is why I need to find a way to control the lights supply individuially. \$\endgroup\$ Dec 16, 2020 at 7:24
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    \$\begingroup\$ a 24V SPDT or DPDT relay seems like a good solution.but without a full description of the supplies and test procedure involved I can't say exactly how to connect it \$\endgroup\$ Dec 16, 2020 at 11:00

1 Answer 1


Maybe feed a 24vdc DPDT relay coil with the normal supply and put your test switch in series with the coil. Also connect your normal 24vdc to the normally open contact on the relay, and your emergency 24v feed to the normally closed. Connect the wire to the lights on the common terminal of those 2 connections. When you push the test button or have a power failure the relay drops out and the light is connected to the emergency 24v feed, release it and the relay energizes and it is fed from the normal 24v feed.


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