Hi, newbie question about pulse frequency modulation

Well, at least i think its pulse width modulation that im after and i hope im in the right place, apologies if im not. Anyway, to the matter at hand.

Basically i am doing an engine conversion on my car and as a result the rev counter and speed sensor signals will not read correctly on my dials. What i wish to do is modify these signals without having to spend a ridiculous amount of money on an off the shelf item.

The sensors are Hall Effect and send a voltage pulse. What i want to do is come up with my own little divider and/or multiplier circuit that will simply sit in series with this pulse signal wire and hopefully give me the correct output signal.

Is this possible and if so any idea on how i can do it or can you point me in another direction? All help would be greatly apreciated and if someone can help me come up with something im sure a thank you can be worked out.

Thanks very much for any help/advice

Maccy

The most versatile way is probably a microcontroller but it depends on what skills you have. Do you know the amount of error?

Keith

i dont have a great deal of skill, but my dad is an electronics technician so i'd hope he would have the necessary skills. When you say the amount of error, error in what? the input, or the allowed error in the output?

If it is a variable output (which is what i want ideally) then the error shouldnt matter too much as long as its consistent I dont imagine there would be error in the input.

For some extra information there are, i think, 10 triggers per revolution so that would be a max of 70000 pulses per minute (at 7000rpm) giving a signal range of approx 170Hz (at idle) to 1600Hz (i hope Hz is correct, its a while since ive learnt electronics).

I was meaning how much do you need to shift the pulse rate? E.g. 70% or 130% for example.

One other thought - I don't know what is in the existing tacho/speedo, but can they be tweeked? It might be worth researching for hacks on the internet. Manufacturers are likely to base them on a circuit or assembly they can set for different models rather than having a different version for every model.

Keith

Added after 2 minutes:

Another option is converting the pulse rate to a voltage then using a voltage to frequency converter to change it back to a different rate.

Keith

approximately 60% is the necessary adjustment. Can this be achieved?

I will do some research into the circuit at the clock end, thanks for the idea

MaccyD said:

approximately 60% is the necessary adjustment. Can this be achieved?

Click to expand...

You should be able to produce any ratio you like, within reason.

Keith.

Ah right, thats excellent to hear. Im not sure until i get the speedo connected up, but if required would i be able to scale up, so 120% for example? Or would that not be possible.

So, where can i buy one of these microcontrollers from and what other components do i need to make one of these little units that will adjust my signal?

Thanks

Increasing or decreasing the pulse rate is no problem - it is simply a case of how you write the software. There may be a simpler way than a microcontroller - maybe someone else would chip in with an idea. For someone used to writing microcontroller software it would be their first choice. Others may prefer a different solution which doesn't require software. I would suggest you look at the Microchip PIC devices https://www.microchip.com

You don't say where you are but there are plenty of places that sell them - Mouser, Digikey, Newark in the USA for example. There are low cost development and prototyping boards available. The development software environment (MPLAB) is free.

There are so many suitable ways of doing it that it is difficult to pick a micro for it. The pulse rates are fairly slow so it could be done all in software or you could use hardware timers in the micro to simplify the software. Something like the PIC16F616 is an option. It has some timers if you want to write the software that way.
https://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en026028

It may be easier to base it on one of the devices in a starter kit such as **broken link removed**

They are pretty cheap and some of the boards may be small enough to use in your final product. Some have a prototyping area where you can put any voltage level shifters required.

Keith.

ah right, that seems quite a bit more complex than i was expecting. I didnt realise programming would be involved, probably foolish on my part. I was hoping there might be something purely component based that in the right orientation would have the desired result.

Also im based in the UK

In which case I think you need to look at frequency to voltage conversion followed by voltage to frequency conversion.

**broken link removed**

focus.ti.com/lit/ds/symlink/vfc32.pdf

http://www.analog.com/en/analog-to-...e-to-frequency-converters/products/index.html

**broken link removed**

Still not a trivial design I am afraid.

Keith.

ah right, bum. I thought there would probably be some kind of off the shelf pulse frequency modulator.

One other solution is the CD4046 (plus some counters). It is a phase locked loop chip. www.nxp.com/documents/data_sheet/74HC_HCT4046A_CNV.pdf

You would need some counters/logic to scale the frequency. One snag is that the VCO is free running so when the car is stationary it would still have an output. It may be that it is low enough to not be a problem, otherwise some logic would need to be added to disable the output below a certain pulse rate.

Keith.

Ah right ok, thanks for the advice keith you have been a great help, its really apreciated.