Tachometers are a "must have" item for driving enthusiasts. If
you prefer a manual car, a tacho lets you know when to change gear and can help
you keep engine rpm within the best operating range. An accurate tachometer is
also a vital tuning aid if you have an old car and you prefer to do the engine
tune-ups yourself.
Traditionally, analog tachometers have been circular in shape
with a needle (or pointer) which sweeps in a clockwise direction as the engine
speed (rpm) rises. The scale behind the needle is usually marked in 100s of rpm
and there’s also often a colour scale to indicate the normal rpm range (green),
a high rpm range (orange) and an "over-the-limit" range (red).
In recent years, digital tachometers have also become quite
popular with car enthusiasts. These directly show the engine speed on 7-segment
LED displays or on an LCD but they do have one disadvantage – the forbidden red
zone, where you can do serious engine damage due to over-revving, isn’t
indicated on the display. Instead, it’s up to the driver to remember the where
the redline is and drive accordingly.
This design overcomes that problem by including a bargraph
display. This display operates in conjunction with the digital display and has
10 LEDs – seven green and three red. As the engine speed rises, the seven green
LEDs progressively light and then the three red LEDs all light together.
Main Features
- 4-digit LED display showing up to 9900 rpm; 10-LED bargraph with redline indication.
- 100 rpm display resolution.
- Works with 4-stroke engines with up to 12 cylinders and 2-stroke engines with up to 6 cylinders.
- LEDs 8-10 (red) in bargraph display light up together for redline indication.
- LED rpm indication thresholds in bargraph can be individually set (eg, to allow the unit to be used as a gearchange indicator).
- Automatic calculation and setting of the LEDs 1-7 rpm thresholds when the LEDs 8-10 rpm threshold is set.
- Optional dot or bargraph display.
- Rev limiter output signal (can drive the SILICON CHIP Rev Limiter switcher board described April 1999).
- Adjustable rpm hysteresis for limiter output and bargraph display.
- Three switches for setting calibration, bargraph and hysteresis values (Mode, Up and Down).
- Automatic display dimming during low light conditions.
- Rpm sensing directly from ignition coil or via low voltage signal from engine management computer.
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In effect, the bargraph has eight steps – seven for the green
(normal) range and one for the redline. These eight steps can be programmed to
operate at any value within a 0-9900 rpm range, so the new Digital Tachometer
can be used with virtually any engine (provided its redline is less than 9900
rpm).
By the way, a reading of 9900 is also the limit for the digital
readout but that should be more than enough for any normal engine. Beyond
9900rpm, the 7-segment LED displays show a value of "-00" to indicate the
overrange.
