Very very electrical question., Tachometer waveform Options

[sgomes]

Ok, I don't expect to get an answer to this question but here goes nothing:

I've got this fancy tacometer drive thingy that is supposed (IMG:style_emoticons/default/unsure.gif)

Assumptions about the tach:

It expects at least +5V pulses more is ok.
It assumes a four cylinder engine.

My assumption on how tachs work:

You get one pulse for every spark. So.....

assume 6000rpm for the sake of argument
6000rpm= 100 revs per second * 60 seconds
4 sparks per revolution.
4 * 100 = 400 pulses per second.

I've got something close to 200 pulses per second comming out of my sender at about +10V.
The tach isn't moving.

[lapuwali]

There aren't four sparks per crank revolution. There are four sparks per distributor revolution, which runs at half the crank speed. So, 200 sparks per second is correct for 6000rpm.

I'd expect 13-14v pulses from the low-tension wire on the coil, since that's what the voltage regulator really regulates the system voltage to, not 12v, and certainly not 10v. Dunno why you'd assume the tach would work with 5v.

[sgomes]

Thanks! There is always a factor of two in there somewhere!! (IMG:style_emoticons/default/lol2.gif)

In answer to your second question about the 5V:

http://www.914world.com/bbs2/index.php?act...0&hl=tachometer

...and since the tach isn't moving I guess I gotta tear into THAT thing now! <_<

Gotta search for "tachometer rebuild"....

[mightyohm]

5V square wave will NOT work with anything but the 911 CDI type tachs.

The tach wants a square-ish type waveform, but with voltage peaks at the edges of at least several tens of volts. This is due to the inductive kickback a stock coil will cause - the tach triggers off of the voltage peaks. I think the tach is pretty picky. I was planning to reverse engineer a tach and convert it for 5V use but I haven't gotten around to it yet. The early tachs (pre 74) are full of germanium transistors and stuff. (IMG:style_emoticons/default/icon8.gif)

[sgomes]

Looks like I'll have to look into a 74 or later tach. In my previous thread Otmar says he's driving his with a 12V square wave with no spikes on the leading edges. I have a 73 and maybe it expects a more nasty signal.

I know this device will work with some 914 tach since I've seen it! Unfortuanately I can't get a hold of the person who had it.

[BIGKAT_83]

After I converted my 74 914 tach for a V8 operation I calibrated it with a function generator that produces a square wave siganal and it worked fine.
I tried using the same square wave generator on a early tach and it did not work. I just assumed that the early tach was bad.

SGOMES........If you need a later tach PM me I've got them laying around everywhere.


Bob

[JeffBowlsby]

So I have a dumb question...

Why does an electric car need a tach anyway? Couldnt the shift points just be read off the speedo?

(IMG:style_emoticons/default/confused24.gif)

[lapuwali]

Why does an electric car even need a gearbox? I understood the torque curve was literally flat from zero revs on up, so you put it fourth and leave it there. Maybe use second if you wanted to smoke the tires. No need for a clutch, either.

[sgomes]

Bowlsby (Jeff? isn't it?) :

Yes, I could just figure out what the RPMs are and shift on speed only. The engineer in me just wants more info! Also, and more importantly, this device is also a rev limiter. Series DC electric motors have this neat feature called "runaway" where it takes less and less current to go faster and faster. (IMG:style_emoticons/default/ohmy.gif) You need to be able to cut off the "fuel" before it detonates.

James:

You're right it doesn't need a gearbox. I'm not a mechanical genius like you guys so using the stock gearbox solves the problem nicely. Otmar's electric 914 is proof you don't need anything more than third. He has a 915 gearbox and the shift linkage isn't even in the car! (IMG:style_emoticons/default/blink.gif) Hell, you don't even need reverse! Just run the motor backward!!! (IMG:style_emoticons/default/aktion035.gif) I however like to have two gears. 2nd for around town and third for winding it up on the freeway. It's also a bit easier on things if you still use the clutch for staring out from a stop. You don't have to but it feels less like someone just rearended you! (IMG:style_emoticons/default/rolleyes.gif) The thing that is really hard to get used to is not using the clutch when you come to a stop! I swear you think you've stalled the car every time! No sound!

BIGKAT_83:

PM is on its way! I signed on this morning to ask that very question! I love this club!