Analogue speedometer circuits?

[James]

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....) )

TIA,
James

 

[JeffM]

...we have a speedo using an old mechanical setup(spinny magnetic disk)

...turn it into an electronic one.

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

...reed switch

Will wear out too soon.

freq-voltage convertor

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.

losing our odometer

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

 

[Wim Ton]

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 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

 

[Barbarian]

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

 

[DaveC]

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.

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.

 

[James]

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.

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.