EDIT: please skim over this whole thread. It's gone through a couple stages of failures so the first posts are almost always useless.This is still a work in progress so please bare with me!
Tachometers read a frequency emitted by means of an electric field (signal or what ever) from a spark plug firing or from a magnet passing a detector on a spinning gear (as i believe jimbote? is doing).
I always wanted a tach but i didn't have a VDO cluster with a tach on it so i couldn't follow the guide on converting one. I figured, there must be a way! I also didn't like the idea of reading a digital number for RPM...I knew there must be a better, inexpensive DIY VW way!
I am currently, with the help of my electronics wizz friend, making a "flip flop" out of d-latches to divide the frequency signal from the stock alternator W terminal by a factor of 8. You can buy these components at any electronics store but a method of testing your boards is a must. Check your solders with a multimeter and have a suction gun to de-solder if need be.
Courtesy of Vince,
Square wave:
8 volts AC:
Since it's a square wave we can easily divide the signal and get a rough idea as to what the RPM is. This is not a concrete measure of what you're reading and without a micro controller it will not be perfect but for those wanting to keep expenses down this will cost you less than 10$.
Vince reckoned that you would need to divide it by a value of 7.456. Slightly less than a value of 8, therefore, the reading will not be perfect by any means let alone the chance of the belt slipping and so on. But, in theory, it should be spinning at exactly the same rate as the crank.
the D-latches can take up to 15 volts so we don't need to protect them with resistors or any of that stuff. It's a very straight forward three d-latch in a series divide by 8 circuit.
I can post pictures of the parts and the idea at a later time.
I may add a micro controller later if this works out.
Essentially if i do that i am making a dakota diesel digital tach (
http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=128/category_id=-1/home_id=-1/mode=prod/prd128.htm) read out that would out put to any tach you want. It wouldn't have to be a digital screen. And would likely cost way less than $65.00 if you made it your self.
But that's another thread for another day
We'll have to see how it goes.
Note: I'm keeping this separate from the other tach thread because this may not turn out to be quite as successful as i hope. it's an experiment and i thought it would be nice to get some feed back from you all.
I plan to use this idea in addition to a micro controller to make the equivalent of a tach read out, with no engine mods, that you can use an aftermarket tach on.
Vince if you read this can you post about how many hertz you were reading at 900rpm (idle) or 1000 or what ever. I don't have a running engine right now to test it. I have a working circuit on a proto-board right now
I'll post pics maybe tonight.