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Electronic Speedometer using stepper motor

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Dear All

Perhaps this is where a little knowledge is a dangerous thing but I hope you guys can help. I want to build an electronic speedometer for a kit car. I was thinking that I could detect the rotation of the drive shaft, put some electronics in the middle and then drive a stepper motor connected to the input of a mechanical speedometer to do the job. It’s the electronic bit in the middle that’s beyond me!

The little knowledge I have gets me to the point of thinking that if I was to detect say the four bolt heads on the drive shaft flange as they passed say a hall type detector this would give me the speed related input signal. I would then want to process this signal to give the output to drive the stepper motor at the required speed. I would need to be able to vary the relationship between the input and out put signal to achieve the required calibration. For example one 360 deg rotation of the drive shaft may need to translate to only 120 deg rotation of the stepper motor etc.

Can anyone please point me in the right direction on this one.

Regards, and thanks in advance.

If you are thinking of that then why not go all the way and fit a digital display instead of a mechanically driven speedo head?

Given that the gearbox you use will - probably - have a speedo cable fitting you will have to fit a cable or a blanking plate.

If you fit a short cable and put the other end into a box containing an interface - convert the cable rotation to a number of pulses per 360 degrees - send the pulses up a wire (or two) to a digital display driven ( no pun intended) by a PIC. The PIC counts the pulses, does a bit of math then displays the current speed. You would have to ascertain the pulses/mph ratio for the convertion.

I think that this would be an easier method than trying to count the flange bolts.

The digital display would not be as instantaneous as the mechanical type but in a kit car that would not be a problem. You could even convert the rest of the displays to digital.

Just a thought...



I've actually been contemplating the same project as Rory, and came to a somewhat nearer solution to yours Pilot. My additional query would be...To avoid the complexities in using a PIC, wouldn't a simple Frequency-to-Voltage converter suffice? Then the frequency created by converting the speedo cable rotation into pulses, can be fed into the F-V converter and displayed using a simple Digital Panel Meter.

At least thats the way I see it...does this sound viable at all? My only block is the method of converting the actual voltage to a readout corresponding to the actual speed. I presume that's where the use of a PIC would be handy. Is there any method otherwise that you know of?
For accuracy and to minimise interference from other systems I think it would be better sending a datastream up the cable rather than a variable voltage.

If you used an 8 bit parallel connection between gearbox and speedo head then you can have a range of 0 to 127 mph with an accuracy of 0.5 mph.
an alternative

I discovered that the speedometer on my 95 Chevy Beretta is actually electronic - the indicator or meter deflects based on voltage applied. My Beretta is a pretty basic auto. If your plan was to obtain parts from a scrap yard you might find this to be simpler to interface than the stepper motor and mechanical drive.

Not sure if ABS (antilock braking system) sensor hardware, from a wreck, can be adapted for your situation. The sensor appears to read the passing of what look like gear teeth on the periphery of a plastic wheel that near the constant velocity joint on the front driveshafts.
Thanks for your replies.

The reason I've gone down this route is firstly that the cable drive on the gearbox is now obstructed by the chassis and I was wanting to retain the "classic" look of analogue instruments.

Stevez's comments about using and ABS sensors is a good one and I think Jaguars had electronic speedos quite early. Its ths bit in the middle to get the calibration right thats going to be the clever bit. Is this where a PIC comes in?


Rory - it sounds like the PIC would be programmed to behave as a Freq to Voltage convertor so whether you use a PIC or F/V convertor the function is similar. You have pulses that accurately represent a shaft speed proportional to road speed - and you need them in a form that will be useful to you.

I am taking some time in my hobby to learn enough about and equip my bench so that I can use PICs. I am one of the newbies to PICs but my observations suggest that once I learn a little bit I'll be able to accomplish many of the tasks that are/were previously left to hardwired or semi-hard wired electronics. The apparent beauty of the PIC is the programmability and relatively low cost. There are also many experts out there who will lend a hand when I get stuck.

In the speedometer application the PIC might facilitate some other things - possibly logging speed/time so that you can calculate rate of acceleration. On a fixed course that might help you understand the impact of a tune up or timing change.

FYI- I mention the ABS sensor because the hardware is built to endure the rigors of automotive applications. The sensor itself might trigger well with the passing of a bolt head or something else - you'd have to experiment. The point is that you might need only mount the sensor close enough to something that already exists. That keeps things simpler in terms of hardware.
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