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.

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.