Rev Counter for Mopeds

Older mopeds are not usually fitted with a rev counter, which is a bit of a shortcoming. The making or finding of a suitable indicator instrument or display is often the greatest obstacle for the hobbyist. The author of this circuit has devised a practical solution to this problem in the shape of a (cheap) bicycle computer. Such bicycle computer is easily attached to the handle-bars and it usually has a large and very readable display.

The moped engine’s generator is used to detect the rev speed. The generator is connected directly to the engine drive shaft and generates an AC voltage for the on-board electrical system. The frequency of this voltage corresponds with the rev speed of the engine. This frequency, however, is too high to be used directly by the bicycle computer. The solution for this is to divide the frequency of the signal by 16, using a binary counter of the type 7493, before connecting it to the cycle computer.


Rev Counter for Mopeds Schematic


The generator signal is first rectified by D1, R1 and D2 and then limited to 2.5 V. Transistor T1 turns it into a usable logic signal. Counter IC1 contains four flip-flips, one after the other, which divides the signal by 16. This signal drives, via T2, the white LED D3. LDR R6 reacts to the blinking LED and is connected to the cycle computer in place of the supplied wheel sensor.  The generator signal also sup-plies the power for the circuit. D4/C1 provide rectification and filtering, after which the voltage is regulated to 5 V byT3 and D4.

For a correct read-out (calibrated rev counter), the bicycle computer needs to be adjusted for a wheel circumference of 889 mm or 89 cm (wheel diameter 28 inch).  Make sure that when building the circuit it is suitably protected against vibration and moisture. Mount the LED and LDR directly opposite each other and keep in mind that they need to be well shielded from ambient light.

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