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A speed sensor on the wheel of a mini motorcycle sends pulses as the wheel rotates. It may send 1, 2 or 4 pulses per tire revolution, so let's say that at top speed, it may have 10, 20 or as many as 40 pulses per second.

It would seem possible to program a small digital device (i.e. arduino) that would sample the rate and then generate pulses at a rate that is reduced from the input pulse rate. So for instance, say that at 45 km per hour, the pulse rate is 20, the device could be programmed to generate a pulse rate that's 80% of the input rate, so output a rate of 16 pulses per second.

Having a full digital computer to do this task seems like overkill to me. I guess it's possible that there is such a device "out there" that reduced pulse frequency that could be purchased off the shelf. Presuming not, my question is whether or not there is a chip that could manage this task, and if so, is there a sample circuit, fed by 5VDC or 12VDC that could be used as a starting point for building a "pulse rate reduction device" such as this.

Additional Information to Address the Comments:

The problem I'm trying to solve is that the motorcycle (Skyteam ST50-8A) has an electronic ignition system that limits the speed of the bike, even though the RPM's are still pretty low. The ignition system has a second limiter (engine rev limiter) which would be unaffected since slowing the pulse rate from the wheel rotation would not have an effect on the rev limiter. But if I could make the bike's controls "think" the bike was going slower than it was, that would allow the bike to break through the 45 km/hr limitation that's coded into the bike's electronic control. Of course the speedometer would still read "45", but in the 80% example, that would mean the true speed would be about 10 km/hr faster.

The problems I saw with a microcontroller solution is cost, larger size, power, less able to withstand harsh environment (likely crammed into the headlamp assembly).

I don't know enough about how the typical electronic controls on the mini-bike work, but I doubt that dropping every 5th pulse would be interpreted as going 20% slower, but it could. I am attempting to get the hub (the device that sends wheel rotation pulses).

Dale
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  • Say, if you have digitized the pulse rate, you can multiply it with any factor that you want. So, what is the reason you want a "pulse rate reduction device" ? – Long Pham Aug 06 '18 at 16:14
  • You'll have much better luck if you tell what you're trying to do. You need to specify the output you want in more detail. Your example could be satisfied by a counter circuit that blocks every 5th pulse, for example, but I doubt that's what you want. –  Aug 06 '18 at 16:24
  • Does "seems like overkill" mean cost is the issue? Or is it the added complexity of using a microcontroller? Because to me a microcontroller seems like the simplest and most robust tool to solve the problem, given the uncertainty about the tachometer pulse ratio needed. – MarkU Aug 06 '18 at 16:37
  • Dale, if this is merely to get a speedometer to "read correctly," for different diameter tires/inflation/whatever, it may be a lot easier to just modify the speedometer readout mechanism so that it is adjustable in the way you ask. You need to write a LOT MORE about what is feeding what to what and why. – jonk Aug 06 '18 at 18:00
  • If the anonymous down-voter would please advise me as to what the problem is, I would revise or delete the question. – Dale Aug 11 '18 at 01:02

3 Answers3

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A CD4017 decade counter may suffice.

enter image description here

Figure 1. Division ratio is selectable from 1 to 10. Source: electroSome.

All sorts of clever tricks are possible with this chip and there are many tutorials on the web.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Whatever scheme you use you need to consider how the receiving device will react to the missing pulses.

The pattern shown in Figure 2 could be generated by feeding Q0, Q2, Q4 and Q6 via diodes to the receiving device and using CO (carry out) to reset the chip.

Transistor
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Perhaps you need an analog solution for an analog tach. to convert 80% of voltage. or a fractional counter for a digital tach. By mult x8 then divide by 10

Tony Stewart EE75
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What you are looking for is called a Counter. There are many different chips and you can combine them to divide pulses by any number. There are even programmable counters where single chip has configurable divisor, like CD74HC4059.

Note, that using counter will reduce pulse rate and increase pulse duration at the same time. I suspect your original speed sensor has same dependency. However if you want to keep pulse duration fixed you would need to add monostable multivibrator (timer) at the output.

Maple
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  • If there is something wrong with an answer I'd really appreciate a comment to go with -1, so that I wouldn't give bad advice next time. – Maple Aug 08 '18 at 21:58
  • Also got a uncommented downvote on the question itself, and so I don't know why. Upvoting. There is obvious value in your answer. – Dale Aug 11 '18 at 00:59