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.