Designing a Better Tach Amp - PeachParts Mercedes-Benz Forum

techguy512 · #1 12-12-2007, 10:43 AM

Well, thanks to some excellent reverse engineering work by OldPokey, I finally decided to get off my dead end and start the project I've been thinking about for two years, designing a replacement for the Tach Amp we all love to hate.

I've decided to use an 8-bit RISC microcontroller as the heart of the project. I have the breadboard complete, and have written the code to get the microcontroller talking to the tach gauge. I can sweep the needle to any value, and dwell at any point for any length of time.

Photos are below

1) Rough breadboard complete
2) Sensor interface built on a separate board.

Next step is to optimize the magnetic sensor interface circuit, then write the code to bridge between the sensor and the tach.

As an adjunct to this project, I'm thinking of designing a tachometer diagnostic tool. The tool would plug into the tach amp socket, and perform the following diagnostic tests:

1) Sweep the needle around the gauge to test the gauge and wiring
2) Indicate supply voltage within range with a green LED
3) Check magnetic sensor and wiring for open circuit - indicate with a yellow LED
4) Visually indicate pulses received from the pickup with a red LED that flashes at an engine speed dependent rate.

Using this tool, it would be possible to diagnose about 90% of the possible problems with the tach circuit. Would there be any interest in this as part of the tool rental program?

More updates as they develop.

R Leo · #2 12-12-2007, 11:24 AM

Great idea!!

I'd be interested in this and, whatever you come up with tach amp-wise. And, I'm in Austin with an '83 if you need another test article.

rrgrassi · #3 12-12-2007, 11:29 AM

I'm east of Dallas with an '82. No tach problems, yet anyway.

jkoebel · #4 12-12-2007, 11:32 AM

Wow, that's amazing. You could probably make some damn good money selling those on the Internet after you're through with the prototyping, too.

vstech · #5 12-12-2007, 12:31 PM

contact funola
he did a bunch of stuff with the tach amp earlier this year.

techguy512 · #6 12-12-2007, 06:59 PM

Quote:

Originally Posted by R Leo

And, I'm in Austin with an 83 if you need another test article.

Thanks. I'm going to try to build up 2-3 more of these for testing, and I'd like to try them in a variety of cars prior to moving to a PCB.

Assuming of course I can get the first one working.

fdanielson · #7 12-12-2007, 08:13 PM

I'm really impressed with the engineering you've done so far but it seems that you may be making this more complicated than needed. How about using a 555 timer in a monostable circuit as a Schmidt trigger? Something like this -

http://www.kpsec.freeuk.com/555timer.htm#monostable

Cervan · #8 12-12-2007, 08:43 PM

Quote:

Originally Posted by techguy512

Well, thanks to some excellent reverse engineering work by OldPokey, I finally decided to get off my dead end and start the project I've been thinking about for two years, designing a replacement for the Tach Amp we all love to hate.

I've decided to use an 8-bit RISC microcontroller as the heart of the project. I have the breadboard complete, and have written the code to get the microcontroller talking to the tach gauge. I can sweep the needle to any value, and dwell at any point for any length of time.

Photos are below

1) Rough breadboard complete
2) Sensor interface built on a separate board.

Next step is to optimize the magnetic sensor interface circuit, then write the code to bridge between the sensor and the tach.

As an adjunct to this project, I'm thinking of designing a tachometer diagnostic tool. The tool would plug into the tach amp socket, and perform the following diagnostic tests:

1) Sweep the needle around the gauge to test the gauge and wiring
2) Indicate supply voltage within range with a green LED
3) Check magnetic sensor and wiring for open circuit - indicate with a yellow LED
4) Visually indicate pulses received from the pickup with a red LED that flashes at an engine speed dependent rate.

Using this tool, it would be possible to diagnose about 90% of the possible problems with the tach circuit. Would there be any interest in this as part of the tool rental program?

More updates as they develop.

talking about electronics... how about cruise control amps...

techguy512 · #9 12-13-2007, 01:29 PM

Quote:

Originally Posted by fdanielson

it seems that you may be making this more complicated than needed. How about using a 555 timer in a monostable circuit as a Schmidt trigger?

A several reasons....

1) The magnetic pickup produces a TINY signal. About 500mV p-p at idle, dropping down to something under 250mV at higher revs. I don't have enough space to build the amplifier I need to boost the signal to trigger something like a 555.

2) I hate RC timing components. I never have the right values around, and they drift all over the place with age and temperature. I'd much rather just change some software code to adjust timing.

3) I also get the flexibility to do some interesting things. I believe I'll have enough code space to build in a self-test. When you switch the key to the glow position, the amp will execute a self test routine that sweeps the tach needle to 5k RPM and holds it there for 1 second. It then drops to 2.5K RPM and holds for 1 second, finally sweeping to 0, at which point normal operation commences. I like the visual indication that everything is working. Makes me sleep better.

Plus, my microcontroller is cheaper than an industrial temperature range 555.....

winmutt · #10 12-13-2007, 01:36 PM

Help me control the VNT please

techguy512 · #11 12-23-2007, 07:50 PM

Hi everyone....

As a follow-up to my last discussion, my tach amp redesign project
has progressed nicely, and I am now at the point where I would like to solicit
some volunteers to help me test my Protype Rev2 (Photo 1 below).

This is a completely modern tach amp, based on an 8-bit RISC microcontroller.

Rev2 has been redesigned to now fit inside the tach amp canister. I have
the electrical components all placed on a PCB (Photo 2 below), but I'm holding
off on finishing the tracking until I get some "real world" testing completed.

Code is written and is working well in my car (300D, Model Year 1982, build
date 5/82). I would love to get a few volunteers to ride with this tach amp for
a week or so and feedback any observations. I'm looking for a variety of
cars (Ds and SDs) and I'm particularly interested if someone in a COLD climate
could lend some help.

If you're interested in helping out, please PM me with your year and model.
I have two of the Rev2 prototypes which should be ready to ship out by Thursday
or Friday of this week.

#12 12-23-2007, 08:25 PM

Seems like overkill to me. The tachometer is a glorified voltage gauge. Most of the tach amp problems are with poor contacts (Thats why most place a cigarette butt under the cap) or bad pickup sensor sending it a poor signal rather than a failure of the amp itself.

techguy512 · #13 12-23-2007, 08:36 PM

Quote:

Originally Posted by ForcedInduction

Thats why most place a cigarette butt under the cap

The cigarette butt trick worked once for me. For about 15 minutes. I've driven for two years with no tach. Until this week.

airbus · #14 12-23-2007, 08:38 PM

On my search for a tach I came across another good idea: A guy in Germany is selling brand new tachs which he modified to take the signal off the alternator. No amplifier needed at all. For those who speak German, here's the link:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180198007241&ru=http%3A%2F%2Fsearch.ebay.com%3A80%2Fsearch%2Fsearch.dll%3Ffrom%3DR40%26_trksid%3Dm37%26satitle%3D 180198007241%2B%2B%26category0%3D%26fvi%3D1

Slightly overpriced, though, but this is only for the idea.

techguy512: The 'cigarette butt fix' is most likely the cause of my destroyed tach.

#15 12-23-2007, 08:44 PM

Quote:

Originally Posted by airbus

On my search for a tach I came across another good idea: A guy in Germany is selling brand new tachs which he modified to take the signal off the alternator. No amplifier needed at all. For those who speak German, here's the link:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180198007241&ru=http%3A%2F%2Fsearch.ebay.com%3A80%2Fsearch%2Fsearch.dll%3Ffrom%3DR40%26_trksid%3Dm37%26satitle%3D 180198007241%2B%2B%26category0%3D%26fvi%3D1

Slightly overpriced, though, but this is only for the idea.

Read this thread about the alternator source: http://www.pointedthree.com/disc/forums/showthread.php?tid=5047&posts=25&start=1