Make a DC Circuit for Your Honda CBF 125




The CBF 125 has a serious defect : An DC negative unsmoothed electrical system. So, kills bulbs with ease and prevents the use of xenon, LED's and other electronic components.

The solution? Turning things directly to the battery, but we must use a switch because otherwise things are always turned on. The solution is a relay.

The one i will be using has four contacts (pins), "30", "85", "86" and "87". The pins are connected as pairs, the "30" with "87" (where he have the switch) and "85" with "86", where there is the coil. When current passes in the coil the switch closes allowing current to pass. In other words, the aim was to find a "place" where the current pass only when we turn on the ignition (and remain with the engine running), and that obviously no longer provide power when turned off the bike. This "wire" goes to pin "86" (or "85", whatever) and "85" connects to ground (aka negative). Pins "87" and "30" (no matter the order) we will connect to the 12v from the battery and the other the electronics that we want to feed.
To summarize: When you turn the ignition key, current will pass trough the coil which pulls the switch and this allows the current coming from the battery to feed our gadgets.

The video below shows the result:

Sounds easy, but the problem is that all places where I would get the signal, the current was the AC, and the relay coil uses 12v, so the relay wil not work. Thus the relay does not help anything, but I did not quit, I ordered a relay and a 6v 5v to see if it worked, but also didn't work, the problem persisted. Solution (final)? A switch for xenon and no LED's.

However I got a month vacation and there I was back to see if it could resolve the matter. And I did. I was upset over the bike (ignition on but engine off) and started to press the brake, when my attention, a flashing light. It was the brake light. Now if this light was working with ignition on and the engine off, it could only be powered by the battery. I turned off the ignition and obviously the light no longer worked. I opened the bike, and found the plug on the brake pedal. There where 2 wires:

- Ignition off: 0v in both;
- Ignition switched on: 0v on one, 12v in the other, but under braking 12v on both;
- The motor is connected: a 0v, 14v another, but under braking on both 14v;

Note: with the engine running the voltage is 14v, not 12v because the battery is charging.

You found the ideal place to connect the relay coil! We will connect this pin to the wire that has 12/14v in the last two situations.

To make this modification you need beyond the usual tools (screwdrivers, pliers, etc. ...):

- Power Cord;
- Ties or connectors (or solder and soldering iron);
- insulating tape;
- 4-pin Relay with 12V coil;

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Step 1: Open the Bike

Step 2: Wire to Use

You will find the wires in the photo. The black has 12v with ignition on and the green only has 12v when you press the brake pedal. The wire we'll use is the black, the green can be used for other mod's, but will only pass current when you press the brake pedal.

Step 3: Wiring the Relay

Then just make the connections in the relay. 87 to the black wire, 30 to the ground. 85 to the + terminal of the battery and 86 to the things you want to feed. Normally these relay can take up to 40A, but remember the diameter of the wire will limit the current.

Don't forget to use a diode n the relay, my socket has an integrated diode.

Step 4: Connect the Load

Now you choose what you want to feed. I took advantage and cut the wires of the quadrant, parking light (and later the tail light) to have LED's on these places. It is advisable to use a fuse in the wire from the battery (depending on load).

If you want to simply install an HID kit you can pick up the sleeve that takes the wires to the left handlebar, cut the yellow wire, isolate the side which comes under the deposit and connect the other end to the wire from the relay. (Ill make a photo later).

I hope I was helpful, and any questions just ask. This is was my first instructable, i'm my writing English is not that good.
If you want this guide in Portuguese you can see it on my web page:

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    13 Discussions


    Question 1 year ago

    Hello. I know, it's been a while since you uploaded this Instractuble. I have a problem with my CBF125 '09. I have a new battery for the past 2 months. without your modification the motorcycle works well. With the mod, the battery does not charge as it would before, so i can't start the bike unless i jump start it. Battery just drains of energy, and when the bike is running voltage is arround 12.5 Volts, when revving at 3k. Can it be a alternator stator , or a generator problem?

    1 answer

    Reply 12 months ago


    I need to look at this instructable as I have upgraded it and improved it, but never updated here because I sold the bike 7 years ago.
    Please send me another message by friday so I can reply on the weekend.

    Thank you


    3 years ago


    I bought CBF few months ago. I'm interesting in electronic and I found solution. You can find it your self very easy. You need only a multimeter ( V ) and check voltage on each circuit. One is a green (-) and red/black (+). Second yellow (-) and green (+) .... yes that is correct. I already did new circuit and all LEDs, heated grips, 5 V charger works after start engine not after ignition key. If any one would like to now how to do that pls write. Also I upgraded indicators/hazard lights relay. I did a new transistor one (BUZ 11 A) and you can use a LED bulb or normal one ( no mater configuration, can be one LED and 3 bulbs frequency is the same 2Hz ).


    5 years ago on Introduction

    Please be aware, the Honda CBF125 light circuit IS NOT AC. It is negative unsmoothed DC.
    You can replace the dash lights with T10 LEDs but they will fail after awhile dur to excess voltage. A 12v relay can be used on the light circuit to operate when running lights are on (engine running).

    Transferring all the running lights on the battery side may overload parts of the circuit especially if you have other power hungry extras such as heated grips. etc.

    There is no mention of fitting a protection diode to the relay to protect the bike from the risk of back emf.

    There are errors & omissions in the article.

    All in all I wouldn't recommend this mod even if it does work.

    4 replies

    Reply 5 years ago on Introduction

    I know the circuit is not AC, is DC pulsar/unsmoothed, this instructable is pretty old, you should have notice the date...

    There is no overload, get your facts straight and analyse the motorcycle WD properly... The distance to the front light (higher power) to the battery or to the alternator is the same, so there's absolutely no problem!

    Regarding the diode it is included in the relay socket:


    Reply 4 years ago on Introduction

    Hi, I was wondering what rating diode did you use as there are many available?


    Reply 4 years ago

    You say you know it is not AC. Then why entitle you tutorial 'make a dc circuit for your honda cbf 125'?


    Reply 5 years ago on Introduction

    A 12v relay won't work, since the voltage is pulsar, it will open and close constantly.

    This instructable is pretty old, and i have sold the bike 2 years ago, that's why is outdated but fully functional.
    I have made a new circuit, using some filtering directly on the alternator, some op amps and capacitors... But since i have sold the bike, i never completed the circuit...


    8 years ago on Step 3

    Thanks so much for this. I used exactly the same technique, and it worked perfectly :-)

    1 reply

    Reply 4 years ago

    Hey dude. Can you explain to me how to do this please? I know nothing about this stuff. And would really like to install a HID kit on my cbf. Please reply man thanks


    4 years ago

    Hi jjmaia. I have a cbf125 and want to install a HID kit. I know nothing about making circuits, and this guide is very hard for me to understand. Can you please message me and explain to me how I can make this circuit work for a HID kit step by step please? Would really appreciate it mate


    4 years ago

    Were you afraid of somighint and deleted the comment? lol sad...


    6 years ago

    Please can you add more photos