Digital tachometer with logging etc.

Hi all,

I would like to build a digital tach that does a bit more than just
display the speed of an automotive engine.

I'd like to have the following features:
-engine speed display
-2 or 3 analog inputs aside from the engine firing signal
-ability to control say 3 or 4 outputs based on inputs
-limited logging that the unit could analyze to control the outputs

I've dabbled in electronics for a few years but haven't done much since
the "advent" of the PIC. It seems like a PIC would be the place to
start for this project. Any suggestions on units and development kits?
Something easy to program would be a big plus. I have an engineering
background but not in EE... ;-)
Thanks in advance for any input!

Regards,
Matt

 

mattbeaubien@yahoo.com wrote:

I'd like to have the following features:
-engine speed display

If your application is for a car newer than about 1995, you might look
into using the OBD-II diagnostics port. This is normally used to
diagnose engine performance, and there is lots of engine status info
(including RPM) available on it.

There are commercially available tachs that just plug into the OBD-II
port and work. (The port has +12 V @ 4 A as well as the data line.)

The problem with OBD-II is that it has its very own bus and protocol.
It's not RS-232, RS-485, Ethernet, or anything common. You can tie
up an entire small microprocessor just talking to the bus. There are
"scan tools" available that contain a small uP that translates between
OBD-II and RS-232.

I bought one such tool from Harrison R&D
http://www.ghg.net/dharrison/obdscan.html for about $100 and used it
to add a tachometer and engine load meter to my Toyota. For the output
display, I used a chip (the FT-639; I'm pretty sure it's a PIC with
custom programming) from FerretTronics http://www.elabinc.com/ferrettronics.htm
that takes RS-232 input and drives servos, like the ones used in radio
controlled aircraft. In the middle I had an old PC, but it could
easily be replaced with a microprocessor like a PIC. The code on the PC
sent the RPM request to the scan tool, got the result, scaled it (the
cheap servos I used weren't very linear), and sent it to the
FerretTronics chip.

If you want to just use the PIC to sense the signal at the ignition
coil, you might see if you can find the June 1987 issue of
Radio-Electronics magazine. There is a digital tach project in it - all
of the counting and display would be done by your PIC, but you can use
the input circuit to turn the 300+ V pulses coming from the ignition
into a nice 9 V square wave.

-2 or 3 analog inputs aside from the engine firing signal
-ability to control say 3 or 4 outputs based on inputs
-limited logging that the unit could analyze to control the outputs

You might look into the Basic Stamp or the Picaxe microcontrollers.
These are PICs that have a Basic interpeter built in. The Picaxe chips
are much cheaper than the Basic Stamps. Both of them come with a Windows
program that lets you write programs, run them on the PC, and download
them to the chips.

Both the Picaxe and the Basic Stamp have analog inputs, but you might
have to go to an external A/D chip if you want many inputs or high
precision.

If the outputs are digital, no problem. If the outputs are analog, you
might need to add an external D/A converter.

Both the Picaxe and the Basic Stamp have limited amounts of on-board
memory. You could add a serial EEPROM to store more data.

Matt Roberds