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 con-tact points, so there are twice as many spark pulses. As a result, the tachometer indication was no longer correct.
Circuit Diagram :
Tachometer Pulse Divider Circuit Diagram
A new tachometer suitable for use with an electronic ignition (such as a Krober unit) is rather pricey at around € 175. Accord- ingly, 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 a bit of electronics. It didn’t take long to find something suitable in the form of a mon-ostable multivibrator. The circuit 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 € 5; € 170 saved, and the priceless pleasure of setting the bike right yourself.
Author : Sjabbo van Timmeren – Copyright :Elektor
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