Make your headlight bright at any RPM

I ride alot at night in the city and got tired of not being able to use my blinker, hold the brake with the brake light on, have a headlight on and honk all at the same time, while idling at some hilly SF intersection with insane SUV drivers on cellphones trying to kill me.

This circuit allows you to use a small battery as a buffer for times when you drop below minmum voltage needed to light your 'ped. Works just like a car circuit. When the 'ped is revving, it powers the light and charges the battery. When the 'ped is under-revving, it runs in reverse and the battery powers the light. No more flicker-flicker and no more dim-dim. The battery I used (1.3amp-hour) should give you about 20+ minutes of lighting backup at slow idle, which is far more than you need for a 2-3 minute long traffic light.

The rectifiers isolate each circuit and allow you to use a relay to automatically turn the light on and off when the 'ped is running or off. Otherwise, you'd have to use a switch to turn the light on and off. This might result in forgetting to turn on the light during the day, or forgetting to turn OFF the light during the day, which would drain your battery down to zero, and ultimately kill the cells.

The second rectifier circuit powers a SPST switch which basically closes the circuit between the battery and the light when voltage is sensed (i.e. the 'ped is running). When the 'ped is off, the switch opens, the light turns off and the battery stays charged. If you did this whole project with just one rectifier circuit, it would work, but the battery would remain connected to the coil of the switch when the 'ped is off, and might ultimately drain down if it's parked for a while. It's just a few extra wires and connectors and maybe $3 more, and I always like to avoid deep cycling of lead-acid batterys as much as possible.

I attached the circuit diagram for a super cheap and super quick fix. It works for my 2009 Tomos ST, which is 12V AC with chassis ground and is already regulated coming off the coil. Of course it might work for tons of other 'peds too. I am also sure there are 20 ways to do this mod and not everyone will think it's cool to want to use your horn/signals etc... Whatevs, for those who do, here it is.

Basically the idea is to avoid murking with the existing headlight/headlight switch circuit which could be frought with calamity if you clip the wrong wire or even look at one of the harnesses funny. Just ignore that circuit and go with a single beam (i.e. no high/low) lamp circuit.

You need:

2 Radio Shack 50v 25amp full wave bridge rectifiers (like $3 each)

Some 12/14 ga primary wire

Bunch of 1/4" male and female fasteners, and twist-top wire connectors

1.3 amp-hour lead-acid rechargable battery (weighs 1.5lbs, cost about $15 from

SPST switch from any auto place (maybe $5)

1) Tap into one of the leads coming out of the 12V AC regulator, which comes off the lighting coil. Run this to wherever you are going to store the guts of your assembly. I put mine under the seat, since I long ago chucked the oil injector.

2) Split this 12 V AC tap into each of your two rectifiers. Connect the other AC terminal on the rectifier to the chassis.

3) One rectifier will be the switch circuit. Wire the +/- out of the rectifier to the coil terminals on the SPST switch.

4) The other rectifier will be the light/battery circuit. Run the + from the rectifier to the battery and to the light (parallel), and run the - to the battery and to one of the two switched terminals on the SPST switch (parallel).

5) Connect the other light terminal to the other switched terminal on the SPST.

6) I ran the wire from the regulator and the two wires to the headlight through the oil level checker hole in the seat pan, then under the left side cover and up the conduit under the gas tank to the triple tree, and then into the headlight pod. Wire ties/tape etc... yadda yadda yadda.

7) Get a blindingly bright xenon light / halogen / tractor bulb and blind everyone while you finally can see the road in front of you.


Re: Make your headlight bright at any RPM

This is great! I've been wanting to do such a thing for a while as having a steady supply of 12v would also allow you to run sweet street effects and disco lights.

Do you happen to know how to go about asking for the SPST? Like, a part number or something?

Re: Make your headlight bright at any RPM

Disco lights. Cool.

It's easy, you can just google SPST, make sure you get one for 12 V DC and that can handle enough amperage, maybe 5-7 amps. Or go to an auto parts store.

One issue I have noticed is that if the revs get too low (like almost about to stall), the voltage might get so low that the switch will turn off (or buzz between on and off) and cut your circuit. Best way to fix this would be to make sure you have a good idle and you should be fine.

If your idle voltage is too low to trip the switch, you could maybe use a 6 V DC relay instead.

Re: Make your headlight bright at any RPM

nice writeup

Re: Make your headlight bright at any RPM

thread bookmarked.

Re: Make your headlight bright at any RPM

Hey, how about putting a small cap (and resistor to drain the cap) between the bridge and the relay to give a constant level.

Re: Make your headlight bright at any RPM

i thought about doing something similar but realized that my puch couldn't power the lights and charge the battery at the same time when riding

so the battery ran down eventually

Re: Make your headlight bright at any RPM

batteries don't get powered though, they store power, so if your battery ran out, then that means that your bike was putting out, on average, less power than your lights were using. Try again with lower wattage bulbs. You can downgrade your bulbs if you do this as you will not have to worry about high-rpm bulb blowout.

Re: Make your headlight bright at any RPM

also if you're having problems with relay oscillation use a zener diode to set the voltage cutoff point

Re: Make your headlight bright at any RPM

_"batteries don

Re: Make your headlight bright at any RPM

If it was putting out just enough power to run the lights, then what drained your battery?

Re: Make your headlight bright at any RPM

you should make this into a clean kit that is all inside a nice little box that bolts up to a ped.. then you can sell this to us riders who have little time on our hands..

Re: Make your headlight bright at any RPM

my lights drained my battery.

when i was stopped the battery assisted in keeping the lights bright, but when i was riding the coil didn't output enough power to charge the battery and run the lights at the same time

since it wasn't being topped up when the bike was running it eventually discharged completely

Re: Make your headlight bright at any RPM

Using a second rectifier and relay is an excellent idea. I have a similar system, but am using an on-off switch. I may try this instead.

I also find that the when the lights are on, the battery will eventually discharge. I just plug the battery into a tender to top it off after riding.

Here's a tip, a computer UPS makes a good battery tender for these small SLA batteries.

Re: Make your headlight bright at any RPM


i'm interested in why your moped wouldn't put out enough power to charge a battery. my readings indicate between 20 and 30 volts at full rpm. probably close to 20-30 watts with the right loading.

also curious to know how much power is getting slammed into the battery at full throttle. I've got a couple of these little sealed gel-cell batteries laying around that i'd like to do something like this with, but they can only take like .3 amps max at 12v to charge, from what i've measured the moped alternator would smash that.

Re: Make your headlight bright at any RPM

It didn't keep his batteries charged because, as I said, his bike was not putting out on average the same amount of power that his lights were using, thereby causing the lights to rely on the battery as a supplement.

When you're running a 35 watt 12 volt bulb on a moped, it's probably very rarely reaching it's full abilities, where when you connect it to a battery, it's going to draw the same amount of current the whole time; a full 2.9 amps.

I also would suspect that some mopeds would fry a battery if too much time was spent at full speed. It may be a good idea to put a 13.7 volt regulator in there.

Re: Make your headlight bright at any RPM

"Here's": a pretty simple 12V gel cell charging circuit. When a discharged cell is connected it goes into a fast charge mode, when the voltage starts to level off it goes into trickle charge, and when the volatage reaches 13.8V it turns off.

Re: Make your headlight bright at any RPM

But wouldn't that circuit expect a constant input voltage?

Re: Make your headlight bright at any RPM

i'm bookmarking that baby, nice. i could use that for several projects now that i think of it.

it shouldn't hurt to run it with less than 13.8v will it? I'd be running it straight off a regulator/rectifier.

hopefully if i get my shit together and figure out a TCI that will hook up to tomos a55 80w 12v ignition i'll be running that with plenty of power for everything.

Re: Make your headlight bright at any RPM

Mars - It would, but you could just run a voltage regulator before it, I would think. But you should also be able to use Maddie's circuit with just the addition of a voltage regulator, that's really all you need. That other circuit is nice because it has an IC that gives it different charging modes, but you don't really need it.

And about the SPST, that just means "single pole, single throw" which could refer to any kind of switch that has a single pole and a single throw, like a simple on/off toggle switch (there are SPDT, DPDP, TPDT, etc.). The switch Maddie is referring to is an SPST automotive relay. Cars use them for all sorts of thing, like when you turn your high beams on. There are two circuits, one that powers the relay, and the relay switch itself, and just like any momentary switch they come normally open (NO) and normally closed (NC) so if you're going to use one make sure you get the right one.

Re: Make your headlight bright at any RPM

Yeah, that circuit is nice, but SLA's are not nearly as finicky as NiMH or LiPO; just give them 13.5 volts and they're happy.

Re: Make your headlight bright at any RPM

The battery I got, a sealed - lead -acid (SLA) from Battery Mart (#SLA-12V1-3) has a cap in it that prevents it from exploding. So far the battery seems to be holding charge, I am running a 27 Watt xenon bulb, so I am assuming that I am putting out enough juice enough of the time to keep it charged.

The a55 (unless it's a sprint, which I think has a weaker light coil) should be able to maintain 25-30 Watts for a headlight. If you pull the 10W brake and 5W tail lights and replace them with LED's, you'd have even more juice. I just might do that. Prob only for brake, since you get no scatter with LED, and I'd like to still have my tail light seen from all angles.

Yeah, I like the cap and resistor idea. What size cap and resistor would I use?

Author: Mars (

Verified User: martian

* Date Posted: 10-11-10 13:45

Hey, how about putting a small cap (and resistor to drain the cap) between the bridge and the relay to give a constant level

Re: Make your headlight bright at any RPM


it was a combination of 12v lights and the crappy stock 6v coil on the puch

recently i put in a 12v coil from treats so maybe i'll attempt this project again with some sort of current-limiting circuit on the lights

Re: Make your headlight bright at any RPM

Bump - Help with this R/C circuit - I have not taken physics in over 15 years. Ideas on R and C values?

* Author: Mars (

Re: Make your headlight bright at any RPM

Oh, no not at all. I'm a metal head myself, but I'll try to remember to ask the EE people tomorrow.

Re: Make your headlight bright at any RPM

Thanks, yo.

Re: Make your headlight bright at any RPM

I went about the problem in a different way. I use a Dewalt 14.4 volt battery and strung up LED lights, including a turn signal system on my Foxi,which never had a battery. LED lights draw little power and I pop it in the charger when it seems to be getting low. You can see my bike a mile away now.

Re: Make your headlight bright at any RPM

Cool . Threw me off there for a sec till I saw the schematic - ya have a relay in there - you mentioned a switch and a relay - Anyways all is good. Kinda cheated and bought a $10 hand spotlight at BigLots . Had a battery and a halogen already . This one was a 12v model.

Re: Make your headlight bright at any RPM

Rob @t ATTN! Burrito /

the dewalt/ drill battery is a good idea- nice.

Re: Make your headlight bright at any RPM

drill battery might not be such a good idea depending on the battery chemistry (explosion/meltdown)

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