After only 17 years of irritating buzzing from the MSD tach adapter running my stock AMX tach, I decided to come up with a solution. A replacement MSD Tach Adapter is about $75, and would probably start making noise in a month or so anyway. Evidently the MSD Tach Adapter is basically just a coil like you'd find in a relay, but seeing as so many people on the internet have trouble with them making noise, I figured it would be best to eliminate the thing completely. After some research I found that the tach used in 68-70 AMX & Javelin was built by Faria and there was some good info online in the patent docs. The tach uses a "taut-band" gauge mechanism, which is externally identified by the need for a stop pin at the zero point. The sample 68 AMX tach I tore down measured 725 ohms across the meter, and I could drive it to full scale with only 0.65 mA.

I also figured now was a good time to add a pair of shift lights, since I was going to build something from scratch anyway. I first tried to design a circuit using the LM2917 frequency to voltage chip, but it's a real struggle as frequency to current isn't documented very well. Using it and an op amp I could implement the tach and shift light functions, but to add nighttime dimming to the shift lights took quite a bit of extra parts, so I decided to move the project to Arduino. That way I could make it more flexible and add some fancy features.

So here are the features in a nutshell:

1) It makes the stock tach work with an MSD ignition but with no irritating noise

2) It calibrates the tach for accuracy at all rpm points (some tach boards only calibrate 4000 rpm and assume everything else will be close enough)

3) It has two adjustable independent shift lights

4) The shift lights have different day and night brightness level adjustments

5) There are no external control knobs or switches - calibration and setting is via Bluetooth

The MSD box outputs a 0V-12V square wave with approximately 20% duty cycle. The Arduino measures the frequency of the 12V square wave from the MSD box extremely accurately, then it sends a PWM output directly to the tach gauge itself, with 100% duty cycle being pretty much full sweep of the gauge. All of the factory electronics are replaced with the Arduino.

The calibration and settings are made via and Android app using a Bluetooth connection to the Arduino tach electronics. The full sweep is set manually by driving the tach with the app and adjusting a 20 turn trim potentiometer to get full gauge travel, but after that is set everything can be reassembled any other settings are made just with the app and the tach.

Calibration of the tach is done at every 1000 rpm increment from 1000 to 8000 rpm, and at 500 rpm as well. The app steps through each calibration point so you can see how far off the tach is at that point, and then you can enter a correction into the correction table. If it reads 4225 rpm at the 4000 rpm stop point for example, you enter -225 in the box for 4000 rpm. The program interpolates between the corrected values so the end result is extremely accurate, probably to within less than 25 rpm.

Another menu in the app allows setting the RPM for each of the shift lights, and also sets the daytime and nighttime brightness levels. Nighttime brightness level is triggered when the headlights are turned on, as the circuit is monitoring the voltage in the brown harness wire going to the headlight buzzer.

So far everything has been done on an Arduino UNO, but that's a little too large to fit inside a tach housing. I'm working on a small custom PCB that would carry the few external components needed plus an Arduino Pro Mini and the HC-05 Bluetooth module and still be small enough to fit inside the stock tach housings.

As far as the shift lights themselves, really any leds will work. I was using an RGB led for a while, but now am working on a ring of leds that I can put around the tach face on my '70 era tach. The brighter the better for the leds, as they can always be dimmed down with the software.

Here's a video of the Android App and how it works with the tach:

My first version of this relied on the Arduino's on board voltage regulator and that just wasn't capable of reliably running the number of LEDs I want to use for the dash/shift lights. So I added an external switching mode regulator module to the board. That's going to be much more efficient than the linear regulator that normally used, so far less energy being burned off into heat. 

I also added a polyfuse on the input power to protect the car from the circuit if something goes wrong, and a diode in the input as well to prevent damage from accidentally hooking up the power backwards. Which I tend to do. A lot. 

Another "feature" which is kind of boring is that I added shunt resistors on the gate of each of the MOSFETs to stop the brief flash from the shift lights / dash lights when you turn on the car and the Arduino first starts up. It's a little thing, but still was irritating me. So the board ended up a little bigger in order to hold the additional components, but is still pretty small overall. I get the boards done at OSH Park (who is actually in the U.S.A.!) - the purple solder mask color is kind of their thing.

Here's where it mounts to the tach:

Since this isn't Facebook I can tell you guys when things don't go perfectly.

I was able to get everything hooked up in the car last night and learned a few things:

1) Tapping the +12V and ground from the instrument cluster is a bad idea. The pulsing 5V voltage reg for the gauges causes the LED dash lights to pulse as well. Not cool.

2) Using a keyed 12V source to run the Arduino means that it will not be active if the key is off and I try to turn on the dash lights. So no dash lights with the key off. Also not cool.

3) Even with 20 or so LEDs installed, I still think the shift light feature won't be as bright as I'd like it for daytime use. The LEDs are very directional, and I'm currently just bouncing the light around inside the instrument cluster. It makes for very nice dash lights though. 

Taking +12V straight from the fuse panel fixed item 1. Now it's as steady as the old crunchy dimmer mechanism permits it to be. 

For item 2 the fix would be to drive the circuit from the dimmer voltage if the key is off. It'll take a pair of diodes to get that to happen and prevent one source from backfeeding the other. The dimmer voltage ranges from 9.3V to 11.0V, so the dash lights wouldn't be as bright as normal with the key off since they would typically get 12V nominal car battery voltage. It would work, but will need to use a diode with low forward voltage (loss). For future revs of the circuit it might be a good idea to use 5V LEDs for the dash lights and power them from the RECOM 5V voltage regulator, then there'd be no difference in brightness between Key-On and Key-Off operation of the dash lights. 

I checked and with my Mazda the dash lights don't come on with the headlights without the key on, but in my 2011 Dodge they do. Both have a buzzer though. 

Anyway, the dash lights are very nice. Bright, even, and very adjustable. And any color in the rainbow can be set via the Bluetooth app on the phone.

And here's the access to the dimmer voltage - the orange wire:

This gives a pretty good idea of the brightness of the LED dash lights. You can even see the green glow reflecting off the '69 instrument shade: