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!
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
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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
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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.
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.
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