I need to power up a make-or-break simple automotive relay (4pin, 40A 12V) via an OR gate.

OR gate as follows:

  • Source 1 is the cable that provides +12V to activate car dome light which I will tap into and
  • source 2 is engine-on port from the busbar, obviously +12V when engine is on.

So if the dome light is on OR if the engine is on, the relay will activate.

I understand I need to put one way diodes on both my sources and connect them to pin 85 of the relay. What type of diodes I need to purchase?

EDIT automotive relays have standardized pin numbers ie in this case a 4 pin relay has pins 30, 85, 86, 87

  • 1
    \$\begingroup\$ A four pin relay, and you want to connect to pin 85? Something is missing. \$\endgroup\$
    – JRE
    Dec 3, 2018 at 12:46
  • \$\begingroup\$ What is pin 85? \$\endgroup\$
    – Hearth
    Dec 3, 2018 at 13:56
  • 5
    \$\begingroup\$ @Hearth Automotive pin numbering is standardized. See (en.wikipedia.org/wiki/DIN_72552) \$\endgroup\$
    – mike65535
    Dec 3, 2018 at 14:16
  • 3
    \$\begingroup\$ @mike65535 That's information that should be included in the question, probably, since it seems both I and JRE are unfamiliar with it, at least. \$\endgroup\$
    – Hearth
    Dec 3, 2018 at 14:25
  • \$\begingroup\$ Only be carefull because in some automotive relays there is a flyback diode in parallel to the coil (between 85 and 86 pins). \$\endgroup\$ Mar 26, 2023 at 11:42

1 Answer 1


The problem you face is the dome light is usually switched on the ground side, via the door contacts. The engine-on is switched on the battery plus side in contrary. You cannot simply diode-or this circuit because you don't have a common potential.

You need a helper relay to overcome that problem. The diode must be rated for the coil current of the power relay. A 1N4001 should be sufficient.


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Thanks for the reply. Sorry I wasn't clear: The dome light is activated by a +12v +ve cable which I will tap into, so no negative. I will also tap into an engine on port in the busbar, so both my OR inputs will be positive. Plan to use a standard vw 4 port relay (ports 30, 85,86,87) Many thanks! \$\endgroup\$
    – George
    Jan 30, 2019 at 14:40

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