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  • That looks like a good plan, I'll give it a shot. I have actually been trying a few things and got it to work. First I removed the resistor, removed the 5v wire to the same relay, left as INPUT_PULLUP. Then I added a Diode from ground to the top side of the Module Relay for flyback. That seems to work, strobe and everything, but I may have a ticking timebomb!! Commented Jul 13, 2021 at 15:41
  • Added an updated circuit with my changes. Thoughts? Commented Jul 13, 2021 at 16:43
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    @MattD I would still recommend adding the smaller pullup resistor on the input pin. It will help it be more immune to noise. Snubbers are still a good idea to add, although the diode you added is doing most of the job of the snubber for the inductive load of the second relay. You should consider adding one to the second relay's contacts though, if only to increase their lifetime. Commented Jul 13, 2021 at 16:51
  • Thanks for your help, it's all working now, got 4 inputs and 8 outputs to do the same, that's a fair bit of wiring, I think I'll need to put together a strip board to tidy it all up. Commented Jul 13, 2021 at 22:01
  • The internal pullup was ok when the car wasn't running, but was not enough with the engine on. Putting in the 1K external pullup resistor worked a treat, even when running. Thanks! Commented Aug 6, 2021 at 11:51