Scenario: On my old military Land Rover I have a three-way pole switch which turns the pilot and tail lights on (top position), or also turns on the headlights (bottom position).

  • Top: Pilot and rear lights ON
  • Middle: All lights OFF (unconnected terminal)
  • Bottom: Side and rear lights + Headlights ON

The challenge is:

  • to use as little relays as possible (ideally only one) due to space constraints. My fuse/relay-box is already too full.
  • headlights must be powered through a relay (currently they are wired through the switch, which is a known problem on older Land Rovers (burnt out switches, poor lighting)
  • pilot lights may be powered directly (i think -- they take 8 watts max alltogether)
  • I have a tried a Schottky diode as it seemed to provide a solution for having power flow in only one way, but when I'm measuring I detect that voltage does leak in the wrong direction (???). I use the SB550 5A 50V Schottky diode and placed it between the terminals on the switch itself (not as depicted below after the relay, but I don't think that matters).

So, this is what I got so far (the connector on the image leads to the lights):

two scenario's I am not that experienced with circuits, I hope someone here can review my schematics and provide some ideas or feedback on how to improve on it.

Soo many thanks!


1 Answer 1


You must reverse the diode and remove the wire from the cathode of the diode to relay (30), as shown below, else you'll be turning on the headlights through the diode with the switch in the "SIDE ONLY" position.

Also, since the current into cold incandescent lamps is about ten times the current when they're hot, take care that you use a Schottky which can handle that high current for a few hundred milliseconds. A vanilla silicon diode will work OK too, (and you'll get less leakage) since it'll only be dropping a couple of hundred millivolts more than the Schottky, which won't make much of an impact on the brightness of the side and tail lights, methinks.

enter image description here


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