The existing design uses a simple transformer to detect the current pulse that occurs every time the coil fires. The resulting signal is converted to a constant-width, constant-voltage pulse using a two-transistor circuit. The constant-width pulses then drive the meter movement; the closer together the pulses, the higher is the average voltage, and the greater the meter movement deflection. The downside of this very simple circuit lies mostly in the component choices and the fact that there is little compensation for component aging or temperature. Since I was planning to install an MSD ignition amplifier, I needed to modify the tach anyway, and I decided I might as well update the internals to at least the '70s. Interestingly enough, that didn't really simplify the internals of the tach...
The power supply works from the Tiger's 12V electrical system and provides a filtered 8 volts to the rest of the tach. The resistor and Transzorb on the input are intended to protect the regulator from voltage transients that exceed the regulator's capabilities. The calibration oscillator provides a 30% duty cycle output signal at a fixed frequency that corresponds to approximately 3500 RPM. The idea is that I will measure the frequency (or period) of this oscillator using the scope, determine what RPM that corresponds to, then tweak the tach driver adjustment until the tach reads the correct RPM. The ignition input basically consists of some filtering and protection so that the 555's trigger input is protected from the ignition circuit. The tach driver uses an adjustable resistor that gives a fairly wide adjustment range of the output pulse width to allow for unit to unit variability as well as tuning for 4, 6, or 8 cylinder engines. The 100 ohm resistor in series with the meter movement, combined with the pulse width set by the resistors and capacitor, together determine the meter calibration. I wanted the pulse duty cycle to be about 50% at 6000 RPM, so I juggled the output resistor and the 3.3K resistor value until I got there.
If you compare this to the Smiths design, you'll see that things definitely
didn't get simpler...
Another view of the tach electronics, showing the meter movement. The two spiral springs transfer the coil current from the stationary contacts at the top to the coil which is mounted on the rotating shaft.
I measured the mounting hole locations and cutouts necessary to clear the meter movement, and made up a drawing on the computer. I measured the components that I was going to fit and spent a while juggling placements and wire routing options. After I got to a reasonable answer I printed the result and then cut out the pattern on a piece of prototyping board (one-sided copperclad and tin-plated). Since I don't like drilling lots of small holes I decided to use a poor-man's surface mount method, where you basically clip the leads off of anything that you mount to the board. The traces were cut using an X-acto knife, after marking all the component lead locations.
For this prototype I used primarily leaded parts (with the leads cut off as required) but a few surface mount parts did find their way onto the board. Going to full surface mount would reduce the clutter quite a bit but some surface mount parts are a little more fragile than the leaded components and I didn't want to be chasing a cracked resistor at some inopportune time. The aerial wiring is part of the calibration connection, which I removed once I'd reassembled everything and tweaked the calibration adjustment.
I needed to
make some adjustments to the mounting hole locations but things went together
pretty well. Note that the meter movement is a balanced assembly and until
the needle is back on the shaft, the shaft position will be dependent on
how you've got things tilted. This is especially important to keep in mind
when you're reassembling the needle on the shaft. I left the current sensing
transformer on the backplate PCB, and just soldered its wires (just visible
on the lower right) to the main PCB so they wouldn't get in the way.
I drilled a
1/4" hole for a grommet to lead the tach input wire out. I did not make
the calibration controls externally accessible but I will probably add
Update: Nov 15, 2002