Motor Circuit Diagrams

# Tachometer Pulse Divider Schematic Circuit Diagram

The author is a motorbike racer in the Classics class of a Dutch Motorcyclists Association. He recently replaced the contact points on the engine of his motorbike (a 500-cc BSA Goldstar with a single-cylinder four-stroke motor) by an electronic ignition. The new ignition system produces a spark for every rotation of the motor, compared with a spark for every two rotations with the contact points, so there are twice as many spark pulses. As a result, the tachometer indication was no longer correct.

A new tachometer suitable for use with an electronic ignition (such as a Krober unit) is rather pricey at around \$250. Accordingly, the author first looked through past Elektor July & August issues for a suitable divider circuit — after all, it should be possible to solve this problem with few electronics. It didn’t take long to find something suitable in the form of a monostable multivibrator. The circuit is shown here required only a couple of changes to the original design, and now the original tachometer again shows the right motor speed. Final tally: problem solved for \$7; \$243 saved, and the priceless pleasure of setting the bike right yourself.

A tachometer is an instrument that measures the working speed of an engine, typically in revolutions per minute (RPM). It is commonly used in cars, boats, planes, and other vehicles. Most tachometer gauges have either an analog (dial) or digital (LCD or LED screen) display. tachometer, a device for indicating the angular (rotary) speed of a rotating shaft.
The tachometer measures your engine speed in terms of revolutions per minute (rpm). The tachometer is a semicircle, with the numbers 1 through 8 on it, and a needle that moves as the engine speed changes. For example, when the tachometer needle is pointing at the 2, that means that the engine is operating at 2,000 rpm.

The word tachometer comes from the Greek tachos, “speed,” and metria, “measure.” Because an unexpected loss of engine speed often leads to trouble, observing the instrument dial allows an operator to anticipate engine failure.
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