How does a Tach do what it does?

I know a tach counts the coil recharge/discharge rate, right? How do those coils that can be used on a 4-6-8 cylinder cars work by just mearly switching a lever? The reason I'm asking is that I need to have a tachometer for the thrust engine and lift engine on the hovercraft, but I don't want to use the lame digital gagues, if its not too much trouble/expensive to modify a mechanical tach. Does anybody know how a tach works? If you do, POST HERE! I could probably figure it out if I knew how they worked. Thanks!

Oh yeah, the hovercraft design is coming together nicely. The frame is done, run thu FEA and we have a minimum factor of saftey of 2: very nice. The lift duct and supporting members are done. The thrust duct is done, just awaing FEA on that part. I'm working on the cockpit right now, so figuring out if I should go with digital or mechanical gagues is kida cruitial. Also, we had a tour of aerojet with one of the classes I'm in, and I ran into the head design engineer. He was very interested in our project still, so I'm going to inivite him and the other two head engineers to our design presentation on the 8th of next month. Man, I really want to work there!
[addsig]

I'm not sure how the guage type tachs work, but this is how I would build one:

Pulses from the coil are recieved by a digital counter circuit based on the 555 Timer IC. The counter circuit determines how many pulses per how much time, and converts that into a binary number (which is the RPMs).

If you want a digital display, you just send that binary number representing RPMs to a display driver IC which converts it to decimal on the LCD or LED display.

If you want an analog display (conventional tach with needle) then you send the binary number which represents RPM to a DAC (Digital to Analog Converter). The signal is converted to analog (low voltage = low RPM, high voltage = high RPM). The analog signal is sent to a standard gauge which is nothing more than a coil of wire, a magnet core, and a needle.


Oh, and one more thing:

WE WANT PICTURES OF YOUR PROJECT!!!


[addsig]

<TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font size=-1>Quote:</font><HR></TD></TR><TR><TD><FONT SIZE=-1><BLOCKQUOTE>
On 2003-11-18 17:31, CWToyota wrote:
Pulses from the coil are recieved by a digital counter circuit based on the 555 Timer IC. The counter circuit determines how many pulses per how much time, and converts that into a binary number (which is the RPMs).

If you want a digital display, you just send that binary number representing RPMs to a display driver IC which converts it to decimal on the LCD or LED display.

If you want an analog display (conventional tach with needle) then you send the binary number which represents RPM to a DAC (Digital to Analog Converter). The signal is converted to analog (low voltage = low RPM, high voltage = high RPM). The analog signal is sent to a standard gauge which is nothing more than a coil of wire, a magnet core, and a needle.
</BLOCKQUOTE></FONT></TD></TR><TR><TD><HR></TD></TR></TABLE>

yeah, that's more or less what I was gonna say . . .

[img]/phpBB/images/smiles/icon_wink.gif[/img]

Okay guys, thanks, I guess I will not be modifying a tach then, I dont know enough. I can't believe an old tach out of say a mustang was this complex. Oh well. DAMN! Pic's of the design will start showing up after the 8th of next month. At this point, its all just bits and pieces.
[addsig]

I doubt the old tachs were that complex... I'm just thinking for the best accuracy, that's how I would do it. I could be WAY off from how real tachs work.

I'm sure the old tachs were fully analog. Meaning that they just take the signal from the coil, and modify it slightly, then dump it into a coil/magnet style meter. you could probably do it this way.

If you want to modify a tach, just hook up a 4 cyl tach, then modify the numbers on it so that they are multiplied by four. You could just print a new overlay on the computer. replace 500 RPM with 2000, replace 1000 RPM with 4000, etc.

The other way is to develop a digital box that goes between a normal 4 cyl tach and the coil. The box would only send every fourth pulse. It would basically dump the first three pulses and allow the fourth to go through. I think this would be the best easy way.
[addsig]

<TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font size=-1>Quote:</font><HR></TD></TR><TR><TD><FONT SIZE=-1><BLOCKQUOTE>
On 2003-11-18 18:09, Cyclone Joe wrote:
Okay guys, thanks, </BLOCKQUOTE></FONT></TD></TR><TR><TD><HR></TD></TR></TABLE>

For the record cyclone joe - I didn't help, believe me! I was only kidding . . . I had absolutely no idea how they worked! (feeling guilty so early in the morning . . .)

Can you tell us how you would build this black box???
[addsig]

This one is severe overkill, but I think it would do the trick [img]/phpBB/images/smiles/icon_smile.gif[/img]

http://www.nuflotech.com/Data%20Shee...%20Circuit.pdf


_________________
1976 Capri II 13.80@103.12 MPH
http://www.burly4x4.com/capri
1989 Mustang 'Notch' (460 swap in the works)
1984 Mustang 302/C4
1978 F250 460/C6

<font size=-1>[ This Message was edited by: cwtoyota on 11/20/03 4:11am ]</font>

I found this divide by two circuit on the net, it could be modified for divide by four duty, or just put two of these together:





[addsig]

Its amazing how much pictures help. I think I could do it given this diagram is correct... I think though I am going to stick with re printing the rpm range for a 4 cylinder tach. I'll have to say you're bad ass. Thanks for the help man.

Joe
[addsig]