Author Topic: How To: LED indicators in mini speedo  (Read 2076 times)

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Offline yitzac1990

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How To: LED indicators in mini speedo
« on: August 04, 2016, 05:25:35 pm »
Hey all!

So I recently got a mini speedometer to throw on my 1978 CB750K.  The one I chose had built in LED indicators for high beam, neutral, and turn signal.  I wanted that so I didnt have to have a separate panel with LED indicators, I just had them all in one small place.

The problem is, the LEDs in the speedo module require a +12v signal to illuminate (they all share a common ground wire), and the neutral and oil pressure switches on the bike switch to ground, not +12v.  I know you could use a typical relay to make this work, with the neutral/OP switch activating the coil on the relay, but a regular relay is too large to fit in the headlight bucket.

So I did some research on this, and I learned about "reed relays."  They are basically miniature relays meant for lower voltages/currents.  Since they only needed to drive some very small LEDs, that require much less than 1 amp of current, I figured these would work fine.  This is how I made it work and I hope this helps some of you out!

I went to Radioshack and got 2 of these:



It is a 12v, 500mA (or 1A, i forget) reed relay.  They are about $4 each.  I did a search on DigiKey for similar relays, and they too cost about $4 or more, so might as well buy local.  I got 2, but basically you need 1 relay for each ground-switched indicator you want to run.

This is the schematic.  You can see the 2 side pins run the coil, and the 2 on the ends are switched.



Again, just like a regular relay except no numbers.  So if you are familiar with the Bosch type relays, this should be easily understandable.

This is how I wired each of my relays.  the left pin and one of the coil pins get 12v IGN.  the other side of the coil goes to your (whatever) switch (so it will activate the coil and thus switch the relay).  Then the right pin goes to whichever indicator needs to get 12v.



The example shown has a blue wire output, but I used a blue wire for the oil pressure switch, and a white wire for the neutral switch.  I wanted to keep it close to stock for bike colors.  The green I used on both relays as a "generic signal input" to the relay.  I just tell the 2 apart via the output wire (blue/white).

This is what it looks like after I soldered them, heat shrunk them, and bent the pins flat:



And this is both the relays ready to go after putting one large piece of heat shrink over the whole relay:



So basically, you switch on your motorcycle with the key, power goes to one side of the relay switch, and one side of the relay coil.  if the bike is in neutral, it will provide ground to the coil, which will switch it on and provide the +12v to the LED.  and if there is no oil pressure, it will do the same.

One problem - when my bike is in neutral, its fine...however when the bike is in gear, for some reason the neutral switch doesn't quite release the continuity to ground, so it keeps that LED on.  According to the wiring diagrams, the neutral switch should be a straight on/off switch to ground.  does anyone know if my switch on the bike could be going bad?

The oil pressure switch works fine though!  when the bike was not running, the LED was on.  as soon as I started the bike, the LED switched off.

Hope this helps some folks!  I couldnt find info on how to run a +12v LED with a switched ground signal anywhere really, so did the research myself and figure this could help some others.

P.S. - I know i said my speedo that I got doesn't have an oil pressure light...however I just used the turn signal indicator LED as the oil pressure warning light, because the bike doesn't have turn signals.
« Last Edit: August 04, 2016, 05:27:07 pm by yitzac1990 »
1978 CB750K

Online calj737

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Re: How To: LED indicators in mini speedo
« Reply #1 on: August 05, 2016, 03:54:20 am »
The stock OIL and NEU wires (Blu/Red and Grn/Red respectively) provide the ground side circuit you seek. You needed only to attach them to the ground side wire of the speedo, and provide switched 12v service from solid Black to your gauge for proper function.

As for the turn signal, if you did want to use the LEDs and your indicators, you'd either need 2 diodes inline from the bike's harness to the single input wire on the gauge to prevent bleed over. OR, you could have used 1 side of the harness Turn, and the stock Blu/Brn wire (turn signal buzzer circuit) as the other and eliminated the diode.
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Offline yitzac1990

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Re: How To: LED indicators in mini speedo
« Reply #2 on: August 05, 2016, 05:12:04 am »
Nope that wouldn't have worked.  Like I said, there is only a single ground wire feeding all 3 indictators on my speedo.  If I attached it to one of the switches, and the positive wires to 12v ign, then all 3 leds would illuminate.  And also since there is just 1 common ground, I could only use 1 indicator, Instead of all 3.

I know about diodes for turn signals but didn't put them in because I completely removed the entire turn signal circuit
1978 CB750K

Offline yitzac1990

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Re: How To: LED indicators in mini speedo
« Reply #3 on: August 05, 2016, 05:14:47 am »
This wiring was basically so I could turn that switched ground signal into a switched +12v signal.
1978 CB750K

Offline yitzac1990

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Re: How To: LED indicators in mini speedo
« Reply #4 on: August 15, 2016, 05:20:56 pm »
just a quick update:  I just installed a new neutral switch in the bike, and the neutral light works as it should now.  I guess the old switch was just bad, and not fully letting go of ground when the bike was in gear.
1978 CB750K