History and Story of the Tachometer

The German word for "story" and "history" are the same: Geshichte.  This is appropriate since humankind lives from storytelling.  Want to explain to your band of hunter-gatherers which berry not to eat? Use a good story.  Want to teach your children what to do (or avoid)?  Use a fairy tale.  Want to sell a new invention to a Venture Capital firm?  Better have a good story!

This then is the brief history of the tachometer and story of bringing BB's 55 year old unit back to life!

A tachometer is any device for measuring rotational speed.  Actually, the root words go to the Greek "tacho" which is speed, so it's actually a speedometer but language is what us humans make it into, so Tachometer is rotational speed and Speedometer is linear speed.

Early tachometers worked on centrifugal force, for example, showing shaft speed by the angle of a stick that was dragging in a pot of rotating Mercury!  I hope no one touched the stick later!

BB's tachometer is elegant: the simplest construct that will do the job.  Technically, it's a mono-stable amplifier.  Electrical engineer's may immediately recognize that from the schematic:

I feel like I felt while getting my EE tests back with C's on the front page!

You understand in technical terms, I'm basically trying to get my great-grandfather to talk to my grand-daughter and understand each other.  This is a 55 year old analog (although not strictly, as the tach has some of the first Germanium transistors in production!) talking to a 5 year old digital engine controller from Australia.

Here's how that looks at one of the frustrating moments:

There's a USB oscilloscope on the ground, a laptop reading the engine controller on the seat (out of sight) and a rat's nest of alligator clips that I would later figure out.  This went in waves.  From a previous post, you may remember that I had installed a power resistor and transistor to allow me to flare the tach on key-on, like Knight Rider!  That was still there and my ECU features a tach output signal so I first thought to connect that right to my transistor.  That worked great until smoke started coming out from under the dash!  I guess it was a bit too much power!

Then I decided to pursue the low power approach of connecting the ECU output (a 50% square wave proportional to engine speed) directly to the main transistor in the above schematic.  This filled me with trepidation as this tachometer is no longer available.  Blow this one up and whistle for your engine speed!  However, with advice on resistors from my brother, who did well in EE college, I successfully connected the two worlds.  Sadly, the ECU puts out a square wave which is 50% on, 50% off.  This is too much for the tach, which is expecting a very brief pulse from the primary ignition coil (which BB no longer has with her new electronic ignition).  So the needle went to redline at idle speed.  Darn.

However, purloining my memory (which takes a lot of purloining these days!), I remembered that the connection to the old ignition coil had sort of fallen apart as I removed the tach from the dash.  I decided to try to reassemble it but the finger nut that held it in place had disappeared.  After some searching, I discovered that this is the only metric fastener on the car!  It was M3x0,5mm.  That held it and using an additional power resistor (plus eliminating some short circuits!) lowered the temperature level, and now I have a tach!

Hopefully, that helps the autocross times!

Thanks for reading and keep having #funwithcars !