Analogue speedometer circuits?

On our kit car, we have a speedo using an old mechanical setup (spinny magnetic disk) but as we are going to be changing gearboxes, diffs and so on, I want to turn it into an electronic one.

I basically envisage a reed switch operated by a magnet on the propshaft that feeds a signal to a circuit that will then drive a stepper motor/servo to turn the dial. Obviously it will need some form of calibration, but this can be as simple as a tweakable variable resistor combined with trial and error comparing to a GPS, I don't need a fancy device where you set diameters of wheels and it works it all out (although if its simple then great!)

So, I assume the pulses will feed a freq-voltage convertor and this will then operate the motor to turn a set amount from rest for a rising voltage, presumably some sort of feedback required there? I think i have the theory down OK, but I could do with some circuits- any on the net that people have found that I can work from?

Also, replacing our mechanical bits will involve losing our odometer... I imagine this is more complex to make so not worried immediately, but any examples of these will be handy also.

Free ride in the car to all who help! (If you live nearby....) :o)

TIA, James

Reply to
James
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You already have an encoder that puts out a pulse every D feet and you want to scrap it and go analog?

Will wear out too soon.

You can calibrate D, measure the time between pulses, calculate velocity from them, and read out the result on a 7-segment or bar-graph display and _you_ would prefer a fragile meter movement? Digital seems like a slam dunk here.

You're going to need digital parts to store this anyway.

Reply to
JeffM

I have used a Honeywell GT1 Hall effect switch to pick up the signal from the bolts connecting the gearbox shaft (on a 1935 Mercedes). Sensor costs about £20, power supply between 5 and 25 V open collector output..

Wim

Reply to
Wim Ton

There are magnetic speedo sensors available that screw into the transmission where the speedo cable used to. Both Hall Effect and variable reluctance types are available, either would be considerably more reliable than a reed switch.

There are also several companies that specialize in aftermarket speedos and other automotive meters for people building hot rods and such. Some make analog as well as digital panel meters.

Dennis

motor/servo

this

voltage,

have

Reply to
Barbarian

On Fri, 14 Nov 2003 4:25:22 -0800, James wrote (in message ):

Seems that driving the unaltered speedo input with a variable-speed motor would be simplest. You would translate the pickup (via hall sensor or other means) rpm and generate a calibrated voltage that would drive the speedo's motor at appropriate rpm. No alteration of the original speedo needed. Keep odometer function, too.

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DaveC
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Reply to
DaveC

other

Keep

Hadn't thought of doing it that way, does sound a lot simpler; thanks for the brain splurge!

Although we know we can buy electronic analog speedos as Barbarian suggested, but they are a bit costly, hence the fudging about! Cheers, James.

Reply to
James

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