I have bike with 35 watt HS1 bulb and I want to upgrade brightness of headlight so I decided to put H4 60/65 watt xenon bulb. An expert on this forum suggested me to use relays as the new bulb requires more power. I did some google to understand relays and how to wire new headlight with relays, but the confusion is that the links and videos for relay wiring that I found online was for cars and car has two head lights so they have used two SPDT(Bosch) relay for that but for my bike I need only one bulb so I guess single 30 AMP SPDT relay would be enough but I am confused what should be the circuit diagram for single SPDT relay to control headlight HI-LO beam. I know instead of making setup from scratch by myself many of you would suggest me to go for Eastern Beaver H4 kit but the problem is that I am in India and I dont have any paypal account to pay for kit and even not sure whether this kit would be economical for me or not thats why I decided to make this setup by myself.

I have 30amp SPDT relay, 30amp fuse holder and H4 xenon bulb and I need help of the experts on this forum to please guide me on how wire new headlight. A simple circuit diagram would be enough like this one at earlycuda.org: enter image description here

  • 1
    Why not just use the circuit given there? If you don't require the sounder, just leave that bit off - it is only there to remind you to turn off the lights.
    – Rory Alsop
    Commented Nov 17, 2012 at 20:27
  • @Rory Alsop: I was also thinking so but my confusion is that would single relay be enough or should I use two relay?
    – Sandy
    Commented Nov 18, 2012 at 3:46
  • The two relays would be for two headlights. You only have one.
    – Rory Alsop
    Commented Nov 18, 2012 at 11:21
  • @RoryAlsop lo and hi beam on separate circuits means you need two relays
    – mac
    Commented Nov 19, 2012 at 17:27
  • Mac- I'm a moron. You are absolutely right!
    – Rory Alsop
    Commented Nov 19, 2012 at 17:41

1 Answer 1


Your single bulb has two filaments, one for the low beam and one for the high beam. If you look at the back of the headlight bulb, there's three prongs. One is for the low beam, one is for the hi beam, and the last is the shared ground. The low beam and hi beam need to be turned on and off independently, so you need two relays.

relay wiring diagram

(image source http://www.rallylights.com/hella/HeadlampWiring.aspx)

Note two relays at left. This image shows two bulbs at right, you'll only use one. You still need two relays, and I hope this diagram helps show why that is the case.

Where the above diagram says "from OE low beam circuit" and "from OE high beam circuit," you'll be making connections to your existing headlight wiring.

Here's a typical socket for an H4 bulb. Your wire colors may be different, but the layout will be the same. This image is from the BACK of the connector, i.e. not the side the bulb plugs into.

H4 headlight connector
(source: easternbeaver.com)

So in this case, you'd connect the yellow lo beam wire to where the wiring diagram says "from OE low beam circuit", and the white hi beam wire to where the wiring diagram says "from oe hi beam circuit."

  • Upvoted! Good answer.
    – Rory Alsop
    Commented Nov 19, 2012 at 17:41
  • Hnmm.. I was thinking to use singe SPDT relay. LO beam of bulb would be wired to NC of relay so as soon as I switch on the headlight LO beam will start and HI beam will be connected to NO of relay so when I put HI/LO switch to HI relay will start and HI beam will be on. In sort if relay is off then LO beam will be on and when relay is on HI beam will start. I am not a technical person but hope I am making a sense.
    – Sandy
    Commented Nov 20, 2012 at 3:02
  • what do you think should I go for two SPST relay as you suggest or should I go with single SPDT relay as per my theory.
    – Sandy
    Commented Nov 20, 2012 at 3:07
  • my concern is why use two SPST relay when single SPDT relay can do the work.
    – Sandy
    Commented Nov 20, 2012 at 5:53
  • With the SPDT, your lo beam would stay on even when the key was out, as the relay is normally closed, so you'd have a direct path from the battery to the lo beam. This is why you need two SPST relays. You shouldn't use any of the NC terminals for this application.
    – mac
    Commented Nov 20, 2012 at 14:18

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