9 replies to this topic

[Jammy]

Could someone please explain how a rev counter works, since I need to know the electrics/mechanics behind it to see whether an idea for my custom dash will work. I guessing the ones that connect to the coil simply use the difference in amps throughout the rev range to ascertain the rev?! Or am I completely wrong?!

[mini998]

hope this helps..

http://members.tripo...tal_speedo.html

[Dan]

Digital stepper motor tacho's actually count the pulses, whereas normal analogue tacho's rely on magnetism. The more frequent the pulses the greater the magnetic field generated.

[dklawson]

While there are stepper motor tachometers like Dan mentioned, the lion's share are not.

The traditional "electric" tachometer is based on some form of digital to analog (D/A) conversion circuit whose output is hooked to a d'Asonaval movement. In this type of meter movement, there is a fixed magnetic ring, slit at one point and supported on little stands. (Think of a horseshoe magnet bent into an almost closed circle). The gauge pointer is supported on a needle bearing spindle that passes through the center of the magnet. Attached to this spindle are bias springs to control the needle's rest (zero) position and a moving magnetic coil which is looped around the ring magnet. Highly flexible wires carry current from the D/A circuit to the moving magnetic coil. As you increase the input pulses from the car's ignition, the analog output of the D/A circuit rises, thus passing more current through the moving magnetic coil which subsequently moves the gauge's needle or pointer.

Stepper motor type gauges are used more for speedometers than tachometers because their electronics lends itself well to scale factors necessary to match tires sizes and final drives to the desired display. However, the expense of a stepper motor is not typically warranted for a tachometer. Stepper motor gauges typically have a slower response time also.

I assume you're not interested in how mechanical tachometers/speedometers work.

That's probably not the information you're looking for but that's the Reader's Digest answer. If you're looking to make a truly "digital" gauge then you can use the link provided in the earlier post (or others found by googling) to get what you need. Keep in mind that while "digital" dashes were in vogue years ago, the prevailing thought about them is they aren't as user friendly because you have to actually "look" at the gauges to read them. A glance is not generally sufficient. Analog (pointer) gauges can be almost as accurate and are easier to read.

EDIT: I neglected to answer one direct question you asked. The circuits don't really work by monitoring current flow. Even the old Smiths RVI tachs (which are current sensing) work by "counting" the current pulses. You could try making a circuit that would average the current flow over a second or two and use that to display some indication of speed, but it wouldn't be consistent. If you're using a traditional ignition with points, every time you changed the dwell/gap you'd have to recalibrate. Your calibration would change as the points wore. There would also be changes as the output of the alternator changed and as the coil heated up.

Edited by dklawson, 13 May 2005 - 12:17 PM.

[jimbo_edmondo]

My standard tacho is C**p if i change gear from around 4krpm 2nd-3rd around 3k the needle swings around all over the place from 2k up to 4k for ages before settling anyway i can solve this?

Sorry for jumpin in here jammy

[dklawson]

Jimbo, I'm afraid the only way I know to dampen tach needle swings is by replacing the gauge. It's a trade-off between stability and fast response time. One of my Australian friends says he's had the best luck outfitting Veglia Borletti tachs out of Fiat/Lancias to his Minis. I personally prefer VDO. You can do some google searching for capacitor filters you might add to the circuit but I'm afraid most of what you find will be about filtering out alternator noise, not improving stability.

I doubt your car is old enough to benefit from this, but.... If your car is over 25 years old it may pay to remove the tach and replace all the capacitors on its circuit board. The electrolytic caps dry up over the years and stop doing their job. I've done this on much older instruments but it isn't fun.

[Jammy]

An immense thanks to Dan and especially dklawson for that huge description on how they work. The link you gave in your first post was exactly what I was looking for, thank you!

[dklawson]

Mini998 provided the link.

Good luck, let us know what your project is.

[Jammy]

Ah yea, sorry mini998, great link you found there!! As I said, exactly what I need!

[jimbo_edmondo]

Yea cheers dk some more valuble info.