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NELS GT 07-10-2014 08:08 PM

Calling S197 Electrical Experts!
I do simple stereo AV work for friends for $ sometimes and I'm working on a project a buddy wanted to put into action on his S197

Apologies if this sounds strange but basically he wants ambient interior lighting that dims and brightens based on his RPMs - clearly this will be something he can turn off at will as it's kind of gimmicky, but it's a custom thing he wanted to do for fun at the track.

Here's my question. We have a lighting controller module that we're using for this that takes an input voltage that will control the brightness where 5V is the highest brightness, and 0V is no light at all.

Where would be the best place for us to tap power that will go with the RPMs, and if that power goes past 5V, what would be the best way to attenuate that power source down to 5V at maximum?

Ideally we want to deliver the full brightness (5V) at 7K RPM.

Thanks and hopefully some of you may have some pointers to put us in the right direction :)

jz78817 07-10-2014 09:30 PM


Where would be the best place for us to tap power that will go with the RPMs, and if that power goes past 5V, what would be the best way to attenuate that power source down to 5V at maximum?
nowhere. you'd have to build your own custom LED driver circuit that is powered by battery voltage, which takes an input signal from a speed sensor/ABS tone wheel and varies its output voltage based on the frequency of the input signal.

Cruzinaround 07-10-2014 11:19 PM

I rigged something similar to this on my fox body off the water injection pump motor. It would light up an LED meter based on how much power was passing through the injector motor.... Like a boost meter only it worked on the amount of vacuum that triggered the injector pump. The motor for the pump was triggered by the vacuum off the engine...the more vacuum the faster the pump pushed water though the injector and the more the meter lit up to show how fast the pump was working.


Chromeshadow 07-11-2014 07:54 AM


which takes an input signal from a speed sensor/ABS tone wheel
JZ, are you suggesting that the OP use an existing ABS sensor or purchase one that attaches to the engine and reads engine speed or get his own sensor and not tap into the cars braking system?
If the OP is interested in engine RPM I'd recommend against tapping into anything on the car that could adversely affect either the braking or the engine.
If this was my project, I would add a sensor that can measure engine speed from a additional crank sensor or the alternator speed. This link might be a starting place for you. I agree with JZ that you'll either need to build or purchase a RPM digital pulse to Voltage (or mA) convertor to get your 0-5 V for the LEDs. here is a sensor
here is a converter
These are just examples, I don't know that they work together or will work for your application.

NELS GT 07-11-2014 10:00 AM

Thanks for all the ideas so far guys.

In my naivety I figured you'd be able to tap the negative coil or something like you would with a tach, and then just do something to attenuate down to 5V maximum. I guess something like that is probably not an option?

If I used a hall sensor - where would you advise mounting it?

Chromeshadow 07-11-2014 10:19 AM

Depends on the year, if you have a single coil-old style distributor, that would work pretty well. What year is this engine?
The newer cars are harder to tap into without screwing something up. All of the newer cars are tested in EMC chambers to make sure they are immune to cell phone, cell phone towers, internally generated signals like wiper motors and much more. Making changes to any of the ignition systems in a new vehicle can make it susceptible to intermittent problems that would be nearly impossible to track down.
My 2012 does have a speed controlled sound level function in the radio, not sure how it works or if you could tap into it. Lots of the newer signals are digital pulses, tapping into them wouldn't get you something you could use.

NELS GT 07-11-2014 10:34 AM

Thanks for the info. It's a 2014

Chromeshadow 07-11-2014 11:05 AM

A simple way to fake it would be to hook a slide potentiometer to the gas pedal and run the tap to the LEDs.

It will brighten the lights based on the gas pedal position, not the RPM.

NELS GT 07-11-2014 03:21 PM

Hmmm.... I like the way you're thinking but that wouldn't quite work when you're coasting etc and the mid to top end of every gear would look too similar.

Before anyone suggests it I've already looked into an OBD solution and it's SO SLOW / laggy haha.

Let's say I get a hall effect sensor. Where would you mount this sucker without having to tear apart the engine to get to the flywheel or something?

Ford-a-tude 07-11-2014 03:26 PM

Get an Arduino board. With some fairly basic code you could handle all of this in software. Grab an engine pulse off one of the injectors, just tap a wire. Run that to an input pin on the arduino. The code would then count the pulses to calculate engine speed. Hook up an output pin of the Arduino to a transistor or something to drive your bank of ambient LEDs. The code you wright would PWM (Pulse Width Modulate) the LEDs. Longer pulses = brighter light, shorter pulses make it appear dimmer. You wouldn't need to regulate the voltage to 5V max either. You could learn all of this by watching some basic Ardunio tutorials on YouTube. Most of the code is probably already out there in examples, it would just require some cut and paste to make it work for you.

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