For anyone who has added LED bulbs to their vehicles turn signals or brake lights.
Since the LED bulbs use less Amps than normal bulbs, the flasher unit thinks there is a bulb burnt out and doubles the flash rate. This instructable will show you how to alter the flasher module in your car for variable flash speeds.

Note: This is for electronic relay type flashers only. This will not work on thermal flashers.

Step 1: The Flasher.

Here is the flasher I modified. Since I am using the flasher in my car, I just pulled this one out of another '97 Cougar at the junkyard.
There are two tabs holding the circuit board in, just pry them loose with a small screwdriver.

Step 2: Inside the Flasher

The inside of the flasher is pretty simple. it consists of an open frame relay, a couple resistors, a capacitor, a shunt, and the controller IC. I looked up the part number to find the schematic.

For this particular flasher, the 100k Ohm resistor connected to pins 4 and 5 will be removed and replaced with a 500k Ohm variable resistor.

Step 3: Remove the Resistor

Simply desolder the resistor fromt he board. Careful not to overheat and burn out the IC.

Step 4: Variable Resistor.

Here, the new variable resistor is connected. There are three pins on the variable resistor. The two on the ends connect to the 500k Ohm resistance. The center pin connects to the wiper. Connect two wires, one to the wiper and another to one of the side pins.

Once you set your flash rate, you can measure the resistance on the variable resistor and replace it with a fixed resistor if you want. I left it variable because I plan to continue to add LED bulbs to my vehicle.

Step 5: Connections.

If you wish to test the unit outside your vehicle, connect everything as follows:
B = Battery +
E = Ground
L = Lights (This is a positive output. Connect the other lamp wire to ground.)

*These connections may vary for different vehicles!*

Step 6: Schematics

Here is the original schematic for my flasher unit and the altered version with the variable resistor.
I have added the part values that are in my unit to this schematic.
<p>I think, the only right way to do this mod, is to change the resistance of the shunt resistor (the thick L shaped piece of wire) wich needs to have a higher resistance. I would proberbly work if the wire is replaced with a 0.5 - 1 Ohm 5W resistor. This would give enough voltage over the resistor so the chip would think that the load is higher :-)</p>
This is a great little modification and I was lucky to find it! The overall advantage is that you adjust the blink speed while utilising low power draw LED's, unlike other solutions which include adding ballast resistors to create the same amount of power load! <br> <br>Not all modern Flasher relays are like this as I discovered, I have a brand new one which is basically a relay, 2 transistors and a few resistors inside to make it work, nothing else! However as luck would have it the one on my vehicle (an old vw camper) I had replaced with a unit which pretty much matches the design of yours, same chip driving it too. <br> <br> I have found a couple of inherent problems though, not sure if you may have experienced this and found a solution.... <br> <br>1) while you can adjust the blink rate of your flashers, in the case of the 4 way hazard flashers it slows them down a fair amount too. I had to try and adjust the variable resistor (1M ohm as thats what I had to use at the time) so that hazards were just about flashing 60 blink/minute(maybe less) and that meant the indicators were about 120 blink/minute, which is just about on the limit of uk requirements, something worth noting and I am not sure if there is a way round this. <br> <br>2) One other problem I have been having which seems odd is power leakage. When the indicators go 'off' during the blink cycle they are actually illuminated really dim, not something that was happening before I added the POT and it seems to happen when you slow the speed down its more apparent, any idea on this?
Hello! michaelsteinbach<br><br>This is a nice instructable!..^^<br>I just wanna ask i have 3 pin flasher for LED which i bought from ebay I also replace my 4 halogen signal light bulb 10W each to LED and now my problem is when I signal to left or right the 4 LED'S lights up together. What's wrong?..Can you help me?..please..<br><br><br>Thank you very much...
Good one ! I drilled a couple of small holes in the top of the unit and installed a trimpot, and glued it to the top. Works a treat. Cheers.
Glad this worked for you.<br>Wish I could do it on my current car ('09 Mazda 6) but the flash rate is controlled by the BCM and doesn't seem to be programmable.
'Tis one of the bummers of the modern car. Would cost you a fortune to be able to programme these things. I am still driving a 1998 TD Isuzu twin cab, so don't have these problems. Will have to come into this century in a few years I suspect.....
Thanks..... It work perfect in an OPTRA chevrolet... I buyed the 500k knob potentiometer and added a 2' long cable and installed in my dash.. to give it a strobe effect once in a while...<br> I calculated that in my car it was needed about 250k resistance to make it flash &quot;normally&quot;<br>
I have a 2000 Dodge 3500 4x4 diesel and I replaced my tail lights with LED types and of course because of the lighter load on the circuit it would flash rapidly. I went in and found the resistor on pins 4&amp;5 and lifted one leg and then installed a PC mount mini 100k variable from Radio Shack p/n 271-284 cost $1.49 and tuned in the desired slower flash rate. In some cases you might need to replace the original resistor with a 200k and then add the 100k variable and then adjust, On my flasher the electrolytic cap is a 3.3uf as opposed the the 4.7uf in the schematics. Easy mod if you know how to solder, use a magnifier and have a steady hand.
really? car flasher &quot;relays&quot; (not sure if it called a relay) have IC's in them? i didn't know that...
OOPS!&nbsp; You've highlighted the <strong>CAPACITOR</strong>, but the note says <strong>RESISTOR</strong>.&nbsp; It's probably clear enough for most of us, but it <em>COULD</em>&nbsp;confuse the noobies!<br /> <br /> Just sayin'&hellip;<br /> <br /> Joe L<br /> <br /> P.S. - Great &quot;ible&quot;, I'll be doing this soon!<br /> <br />
Could you just use the equation for the frequency on page 4 of <a rel="nofollow" href="http://www.datasheetcatalog.org/datasheet/atmel/doc4727.pdf">the datasheet for the IC</a> and the value of the capacitor to determine what resistance you'll need for a given flash frequency? Or is that not going to work since the load of the indicator lamps is different?<br/>Or, could you at least use that formula to determine what the initial flashing frequency was?<br/><br/>Either way, I'd still do it your way, using a potentiometer, because you can set it exactly how you like it.<br/>
Yes, you can calculate it and set the flash speed to your liking but I like the ability to adjust the speed easily with the pot.
What would be needed to make the tail/turn signals sequential? I would like to add a 3 stage turn signal for each cycle.
It would be a completely different circuit I think. If your tails are separate from your turns, you could use some relay logic to take the signal from the blinker module and latch each light in turn then reset. You would have to add a timer to reset it after a couple second of non-blinking and if you do this with taillights that double at blinkers, the relays would cause the sequence to run on both sides if you tapped the brakes several times.
Excellent, I need to build an entire turn signal flashing unit for my atv. I can use this. Would you happen to have the schematics for such a unit? Good ible
are your re-building one from scratch or are you building a new one without a pre-existing signal? I'm just saying because if it's to replace the light-bulb with LEDs, but you have the signalling wiring in place, then this would work. If you're adding a turn signal to an ATV without one, then it would probably be better to use purpose-built components (I'm thinking a 555 based blinker would be simplest) than adapting car components to fit LEDs to fit IN an ATV
It would be one from scratch. If it gets really complilcated then I think I will shy away from building one. But if it straight forward I could probably do it. I just need a simple plan.
dude if there's no real worry about flash speed (FLED aren't far of anyway) use a set of flashing LEDs these are LEDs with a tiny flash circuit inside then you switch them on like normal so make a grid of flash LEDs for the light shape hook up to a DPDT switch or possibly a triple throw trpile pole... simple stuff basically same switch but no complicated blink mechanisms or circuits.
after a few seconds you may find that they turn on and off at different times!
Ah, I didn't think as far as the circuits getting out of line...
That is a good idea. I will keep it in mind. Thank you
I know this is an old thread, but I recently built a new flasher from scratch for my Accord to use with LED turn signals. It doesn't have any current sensing/hyperflash mode like normal flashers, just a 555 oscillator driving a relay. And a speed control dial so I can turn the blink speed back and forth to make other drivers think they're going crazy... I'm working on an instructable for it.
My suggestion would be to find something made for a bicycle or a scooter. They have &quot;urbanizing&quot; or &quot;commuterizing&quot; kits (don't quote me on those terms) but basically they're so that you can make your bicycle road-safe/ready. They're pretty light and rugged, and very simple, so they're usually cheap. <br/><br/>The hardest part of the(your) build IMO would be the user-interface, as in how you'd mount it and use it while riding. <br/><a rel="nofollow" href="http://www.bikeworldusa.us/Acclaim-Turn-Signal-Directional-Brake-Light/M/B000SMCY0E.htm">http://www.bikeworldusa.us/Acclaim-Turn-Signal-Directional-Brake-Light/M/B000SMCY0E.htm</a><br/>is a comercial fitting for a bike, so if you find something like that, you could use the circuitry for controls and just scale it up with relays to get brighter lights.<br/>
I have one very simular to that one that I was going to use just for that. However it is only for the rear of the ATV. I woul dhave to run some wires to the front also. I just don't know how much power it would require to do it. I would also want to wire it into the power system of the ATV instead of using AA batteries. I have the turn siganl switch that cam with it, it is thumb atctivated just like a motorcycle. It's perfect, like I said I was going to use it but I am unsure of the wireing and the power draw. Oh it also has LED lights. It has a brake light and turn flashers. I'm just LED ignorant. Any ideas?
Your ATV is probably running on 12V... anything with LEDs has a low power requirement, probably between 3-6 V (2 or 4 AA batteries at 1.5V each) Now, it IS possible to hook it up to the 12V, but you'd need some kind of regulator. One possibility is to find a car cell-phone charger with similar voltage and hook it up like that. easiest way to find the voltage, multiply the number of batteries by 1.5... HOWEVER, you should also look out for wattage. I'm not too knowledgeable on that, so we should wait until somebody else comes by. As for running the wire, I'm not really sure. You could just stuff it in the seams of the paneling I guess, under the seat, in the weatherstripping? I do advise that you hide it and attach it to something so it's not just hanging there.
My ATV is a 12 volt with a 500 watt magneto so power isn't a problem. The cell phone charger is a good idea, which I can do and most likely will do. I can run the wire, that isn’t an issue either. I think the hardest part is looking at someone else’s project and trying to figure out all the little electrodes and such and then add on to it. Isn’t that the most fun part though? I think so, because every time I do I learn something new. I just don’t have experience at circuit boards and smaller electrical parts. I have basic knowledge of LED’s and I know what a resistor is. But when it comes to capacitors and all the others that’s when I get in trouble. I wonder if this should be a collaboration project. I could post pictures of my LED turn signal disassembled and other people could weigh in with their thoughts.
Like I said, if you do a yahoo search on the IC part number U643B, the very first result is the datasheet for that chip.
Maybe a dumb question, but could you simply wire the variable resistor between the Light-out of the unit and the wire going to the bulb?
Not that one, there is way too much current.
this is a more correct schematic than the one you made.
Yes, you're correct. The symbol i used doesn't match the symbol set in use in that schematic. Thanks.
...could you set it up to blink in accord to the music you're playing on your stereo?
I don't think so. Even if you could, it probably wouldn't be legal. Blinkers need to flash at a constant rate.
So I guess my idea of a disco-car, with strobe-light headlights and disco-ball rims would be out too...
Nice bit of hardware hacking. You can, of course, replace the variable resistor with an equivalent fixed resistor once you have determined the correct flash rate. Cheers, Pat. Pending
I added this note to my instructable. Thanks.
Yes, this is true. I left it variable so that I can change the flash rate as I add more LEDs to my vehicle.
You mentioned having found the schematic to your flasher unit. Could you add that to the photos? Also one can easily find relay type flashers to replace older style thermal flashers. They probably have very similar circuitry.
I have added the schematics to the last page. Yes, there are replacement electronic flashers for most thermal flashers.

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