# Help Hax0ring my Honda, please

<insert applause/death-threats for alliteration in subject>

Hey everybody… now that I’ve finished my first major computer mod (a cool-looking hdd idle / activity led mod), I want to add some LED’s to my car.

I got this idea while trying to find a wiring diagram for my '98 Honda Accord to fix a minor electrical problem (not getting into that).

I found out the way the tachometer/speedometer work is by servo’s, and servo’s have control wires. So I was thinking… if the control wire for the tachometer is so easily accessible, shouldn’t it be possible to cut into it and do something with that information?

That brings me back to my original point: LED’s. Here’s my idea: use the tachometer servo control wire to control two LED’s. As the RPM rises from say 0 to 3x, the intensity of one LED (say blue) fades, and as the RPM rises from 2x to 5x+, the intensity of another LED (say red) increases. If these LEDs were put close to each other and perhaps a diffusion filter (ala glass frosting) was used, I would get the effect of a fade from blue to red depending on my engine’s RPM.

Anyways, the reason I’m posting this here is because:

1. I don’t know the specifics of the type of signal the tachometer/speedometer uses. It’s a servo, so some kind of PWM, I suppose?
2. I don’t know enough about electronics to create a frequency-to-voltage converter.

I did some searching, and I first found this: Idea for a Car Tachometer. The circuit here converts a car tachometer signal to an analogue voltage output. This is good. Unfortunately, this is beyond my (current) electrical comprehension.

A little more searching and I found this site. A guy at the bottom said that, "A simple RC circuit will convert a pulse train into an analogue voltage based on the frequency of the pulses. " Not knowing what a simple RC circuit was, I did some searching and came up with this guide: Understanding Signals. Capacitors and resistors, hoorj! Upon closer inspection of that PDF, I noticed it was produced by our good friends Parallax. Upon actual reading of the PDF, I realized they were using a BASIC stamp to generate and then read a frequency signal (using a Parallax computer oscilloscope device). So, that got me thinking… if I don’t understand the first circuit mentioned above, can I just use a BASIC stamp to do what I want to do?

Any help would be appreciated - I know there are a lot of automobile and electrical guru’s on ChiefDelphi.

As for the whole street-legal issue, psah! I don’t know enough about those rules to know for certain, but I never was a fan of plain things.

I think i can help you with this. I don’t know much about cars, but i know a bit about electronics. The first step would be to figure out the pulses. do you have an analog multimeter? Connect it between a ground and the signal wire to the tachometer. What happens as the RPMs go up and down? A digital meter may work, but im not sure. as long as the pulses have a duty cycle or in the case of PFM or PCM, a frequency that varies linerly with RPMs, a simple RC circuit can be used to filter them to a DC voltage. I think i can help you design a simple circuit to filter the pulses and drive the LEDs, using cheap radio shack parts. A basic Stamp is way overkill, and not needed. If you want to learn more about electronics for yourself, i would highly recomend a book called “pracital electronics for inventors” as a start.

Well the reason I suggested a basic stamp is because to get the effect I want, one LED’s voltage must DECREASE with an increase of RPM, and another LED’s voltage must INCREASE with an increase in RPM. Not sure how easy this is with just radio shack components, but if anyone knows, I’d like to learn.

If the tachometer uses a pwm signal, an RC circuit will filter it to an analog voltage. If you attach a small capacitor across the signal and ground wires, this should produce an analog voltage. Depending on the current draw, you may or may not need an amplifer circuit for this. If you do need an amplifier, look for an circuit that will amplify the current with an op-amp. Try starting with this site for the basics. You should be able to do this easily. You can also do interesting things with the op-amp such as add and subtract voltages. The amplifier is simple to build using only a couple dollars of parts (Maybe a little more expensive if you get them at radio-shack). I hope this helps you out.

I think circuit in the car tachometer link using the 555 chip turn’s frequency into an analog voltage. A PWM signal has the same frequency regardless of the input. What changes is how much time per cycle is at the high voltage and how much is at the low.

I understand that you want one to decrease as the other increases. You can use an op amp or comparator do this. I will explain this later. Right now i’m @ school and don’t have time.

Now that i have a little more time heres what i’m thinking. First you should probably familiarize yourself with op-amps. I found this on google: http://www.colomar.com/Shavano/intro_opamp.html
Once you have read that here is what im thinking. First you can filter the input using a simple RC filter. From there the signal is fed into 2 op amps, the inverting terminal of one and the non-inverting terminal of the other. The other terminal on the two amplifiers is tied to ground( or a potentiometer set up as a voltage divider if you want ot be able to mess with the maximum/minimum brightness). The voltage output of one of the op-amps will vary proportionally with RPMs, while the other will vary inversely. I left many components out in this description, but i will post a better explanation along with a schematic as soon as possible (prolly this weekend.) I really need to know what the voltages read.

I assume your car isn’t under warranty? (since it’s a '98) If, for some reason, it is, you may not want to do anything like that until the warranty wears off. If you modify an electrical system in a car under warranty, you will void it. I think you have a pretty cool idea. Why just red & blue? You think it’d be possible to get a bigger range of colors? I had an idea of giving each 1000 RPM a different color and taking out the other back lights, so at night, you would only see the RPM number and not the whole gauge. I can’t do much, though, since my car is still under warranty for 4 more years.

Using some LED’s, some zener diodes, some resistors, and an analog voltage source you can make an LED bar graph. You could have 5-8 LED’s in a line and assign each to 1,000 RPM’s. A little Knight Rider feel to your car I suppose…

Keep in mind this won’t be an easy project. Merely getting at tachometer behind the dash is a hassle in most cars. Then you have the whole issue of mounting the LED’s and building the circuitboard. Of course, firstly you have to figure out what kind of signaling technique is being used before you can figure out how to convert it.

**Using some LED’s, some zener diodes, some resistors, and an analog voltage source you can make an LED bar graph. You could have 5-8 LED’s in a line and assign each to 1,000 RPM

’s. A little Knight Rider feel to your car I suppose…

Keep in mind this won’t be an easy project. Merely getting at tachometer behind the dash is a hassle in most cars. Then you have the whole issue of mounting the LED’s and building the circuitboard. Of course, firstly you have to figure out what kind of signaling technique is being used before you can figure out how to convert it. **

I have been playign with the idea of building a VU meter for my computer with LED bar graphs like that. People say modding my computer is crazy, but i disagree.

one other thing you may want to think of, as i am not sure where you live and laws for your state, but any blue or red lights or led’s that can be seen from outside the car are illegal to have running while you are in your car, or it is running in New York State. A person i know had an led light bar on his dash, you really couldn’t see it from outside the car, but he drove by a NYS Trooper and they could of busted him for impersonation of a cop, luckily he was let go with a minor moving violation and a warning to remove the led’s. I would check into what color lights you can have in your car, before you do anything.
~Mike

From what I’ve heard, it’s perfectly legal as long as it’s in the car. Hell, my mom’s '96 Accord has read all over the dashboard, from an after market stereo. And, um, wow, they’re LEDs too. So I doubt that Dan will be in any trouble, as where he wants to mount them is even LOWER than the stereo goes in an Accord. Plus, a single blue LED and a single red LED won’t give off anywhere near enough light to have a cop bother to pull you over. Here on LI, they have more than two LEDs to worry about.

in most states it is legal unless you can see the actual light from outside the car. I put in neons and had to be careful where I put them so as not to be seen from outside my car. It is ok for the color to be seen, but not the actual bulb. This includes aftermarket modifications and not what the car already comes with.

Heh…yeah, here on Long Island I doubt the cops would bother to pull you over…But then again, Idunno where you live. As said before, check the laws before you do something that could be seen outside the car. Unfortunately, I don’t know enough about electrical stuff to help you, but it sounds like a cool idea. Hell, I’d definately go for more than two colors of LEDs to make that thing look ill…too bad I don’t got a car to experiment on lol

best of luck. Sounds like a really cool mod (when I read the title I thought you were going to ask us if your plywood three-teir spoiler was ‘fly’ or not. Thank god you didn’t) and I wish you the best of luck.

I think this would be great fun. Unfortunatly, I would have to use my spedometer if i were to try (my tachometer points straight down and wanders around aimlessly . . sometimes it says 8K RPM’s when I’m parked . . usually it just points straight down where there are no numbers and sorta moves about a bit).

One question: where are you planning to put this display? Inside or outside.

Hey!! an idea!! build four pairs of that, and put one in each headlight and taillight!! Imagine how cool it would look when you rev your engine!! THAT would be the coolest mod ev4r!! Or rig the color-fader thing up with red and blue underglo effects. It would be pretty sweet. I bet it would look awesome on my 13-year-old Sable. Don’t diss that car, I just got a new battery and replaced the communicator in my alternator! lets see it do another few 100k miles!

*Originally posted by Frank(Aflak) *
**best of luck. Sounds like a really cool mod (when I read the title I thought you were going to ask us if your plywood three-teir spoiler was ‘fly’ or not. Thank god you didn’t) and I wish you the best of luck. **

Hey, whatchoo dissing my dream Civic for, foo’?!

http://mmjax.com/images/fake-rice.jpg

As for the lights, that would be amazing
…as well as horribly illegal.

As I understand it, inside lights are cool as long as you don’t go crazy with them, but exterior lighting is illegal to have on unless you’re parked.

…WOW…

I think i see the 2004 game, to help place a door onto dans car, then to lift your robot off the ground using his 3 tier wooden spoiler for bonus points!

~Mike

*Originally posted by SuperDanman *
**Hey, whatchoo dissing my dream Civic for, foo’?!

http://mmjax.com/images/fake-rice.jpg

As for the lights, that would be amazing
…as well as horribly illegal.

As I understand it, inside lights are cool as long as you don’t go crazy with them, but exterior lighting is illegal to have on unless you’re parked. **

heh, I saw that on eBay. Thank god the guy who posted it was joking (he extolls the virtues of the timex tachometer (a wall clock bolted onto the dashboard) and his body kit from the well known makers of garden supply (the five inch flowerbed trim around the bottem) and the chrome ex-trashcan that is now a spoiler.

heh.

search for type-R civic on eBay and you can laugh and laugh . . whats even funnier is that some people took him seriously and added scalding replies . . . hehehe!

man

rice man rice. Dont get me started on it. My truck has a sticker that reads “anti-rice” across the windshield.

take a look.

http://www.cardomain.com/id/wurfelmann

yep

Neat idea.

My 2 cents:
-LEDs don’t dim very well by just lowering the voltage. The correct way to dim a LED is by pulsing it with a signal similar to the PWM used for servo and motor control. Normally, a constant frequency signal is used and the duty cycle of the signal is varied to change the brightness. (Duty cycle is the percentage that a signal is “on”. So a 50% duty cycle means that the PWM is “on” half of the time.) A good reference, but not too usable in your project is the datasheet for a LED dimming IC I’ve done some research over: http://www.semiconductors.philips.com/pip/PCA9532.html . The reason that this probably won’t fit well with your application is that it communicates via the I2C bus, which requires a microcontroller or other processor to master the I2C communications. I would be surprised if you couldn’t find some other LED dimmer out there somewhere… maybe even one that is voltage controlled.

-Based on my experience with alternators with tachometer outputs, I have a feeling that the tachometer works using a variable frequency, 50% duty cycle signal. A higher frequency correlates to a higher RPM. If you were to hook an LED directly up to the tachometer signal line, the LED would probably be at a constant brightness regardless of the RPM since the duty cycle of the signal never changes.

I can’t think of an easy way to take a frequency modulated signal and convert it to a duty-cycle modulated signal. The best way may be to take the frequency modulated tachometer signal and convert it to a voltage, then take the voltage and convert it to a duty-cycle modulated signal (possibly using a LED dimmer IC).

Best of luck.
-Craig Morin

Just to let you know i havn’t forgotten about this. I have just been busier than expected. I will make you a circuit, but i still need the signal reedings, or any other info on the signal would be good too.

I’ll take the led pulsing thing into account. I knew this, but kinda forgot. Wow that made alot of sense.