A view of the setup from the rear seat.
You know, car manufacturers are funny these days. They sell you an automatic with a tachometer as a standard feature, but make it an option in a manual. It seems they are oblivious of the fact that a tachometer is of little use with an automatic tranmission, but very helpful when driving a manual.
Having recently purchased a manual '92 Toyota Corolla, I decided to build a tachometer for the vehicle. It consists of a Noritake GU128x64-311 VFD module, a Hitachi SH-1 evaluation board, and some miscellanous power convertion and signal conditioning circuitry.
A view of the setup from the rear seat.
The Noritake GU128x64-311 is a nifty device which has a fairly simple microprocessor interface. It has a bright, blue-green display which makes it excellent for night viewing, and it has a wide viewing angle. The only flaw is that in direct sunlight, the display tends to wash out, which prevents me from using it as a projector for a heads-up style display. The display is quadruple-buffered for smooth graphic animation.
A view of the setup from the driver's seat.
The Hitachi SH-1 EVB is a cheap little tigger that you can get through most of Hitachi's US distributors. It is a really convenient and easy way to build a quick project around an SH-1. The board comes with a ROM monitor with source code; I simply modified the monitor to drive the VFD and read the engine sensor. The source code can be found in vfd.c and in vfd.h.
I took advantage of the integrated timing unit (ITU) feature of the SH-1 to determine the duration between ignition pulses in the engine of my corolla. From this number, it is easy to derive the number of RPMs.
A closeup of the tachometer display.
The only other ingredients required are an opto-isolator, which protects the SH-1 from the 400V spikes which appear on the iginition unit output, a clock prescaler, and some voltage regulators.
The signal from the engine is taken off the ignition diagnostic port, found near the driver's side shock absorber housing in the engine compartment. This signal is a 12V p-p nominal square wave with ignition ocurring on the rising edges. The two cautions when using this signal are that there are 400V spikes on the rising edge, and that grounding this signal is potentially very damaging to the ignitor. I use an opto-isolator to help protect against the spikes, and carefully routed the wire to prevent anything from biting through the insulation and shorting the ignition to ground. On the back end of the opto-isolator, I use a TTL buffer to provide a little extra protection in case the opto-isolator breaks down. Note that the engine makes one revolution every two ignition pulses.
Since the power busses on a car are extremely noisy, one has to exercise a certain degree of care when building the power converters. Adequate decoupling and filtering capacitors, along with a couple of ferrite beads are helpful in keeping the noise level down. I tapped power from a connector that would be plugged into a clock in the dash, but was an option the original owner decided not to go for. This power line is part of the accessory power circuit, and is on the same fuse as the cigarette lighter.
Open the glovebox and look--full of wires.
bunnie@nospam.mit.edu (delete nospam to make address work)
Last modified by bunnie@mit.edu Sun Sep 6 22:01:10 1998