Hall Effect For Electronic Speedo

[Graham B]

I have a Telemetrix 0-140 speedo installed that relies on 4 magnets attached to the prop shaft working a v. small reed switch. All is fine to about 50 mph and then I might as well have wind speed indicator. The needle can sit at 60 maybe 70 from 3000 to 4500 revs, then jumps to 90 or back to 30. My mate with a locost has almost the same symptoms, just different speeds the thing thinks is right.

 

Does anyone know the answer . --- Can someone design a Hall effect system ( cheaper than the manufacturers) - that will give pulses from a single pickup.. Or is this a pipe dream..

 

 

thanx in advance

 

Graham

[baj25]

Graham,

Has this set-up worked ok in the past and suddenly started doing this, or is it a new set-up? Any idea what the maximum input frequency of the speedo is? Or the reed switch- I can imagine at higher frequencies it would struggle to keep up. Or maybe the prop shaft is moving further from the magnets as speed increases? 4 magnets would give 2x the frequency of 2. It may be simple to try removing 2 magnets and recalibrating speedo to see if there is any improvement. A solid state sensor would certainly be better, but they cost a few bob. I was lucky with mine because I use them in work so got a, er, significant discount.

HTH, Brian

[Guest elwe]

On the megasquirt site (somewhere) is their circuit for turning a VR sensor input into a nice square wave (as per a hall sensor). You could use this and a cheap VR sensor from a scrap yard.

 

Alternativly your guage may be able to use a VR sensor directly. My Revotec one can use either. Revotec sell VR sensors to go with it rather than hall effect ones.

[Guest madmax]

as brian said kev in liverpool removed two magnets and cured the same problem. He also found that the cheap magnetic unit from a window security alarm switch worked ok. Colin

[Graham B]

Tim

 

If I knew what a VR sensor was I might progress.

 

Daves unit used to work OK until we did some rejigging of the rear axle. Mine has never been stable, even after chatting to CA and Merlin motorsport, all they could suggest was a very expensive sensor upgrade.

 

 

 

Graham

 

Colin

 

I looked at the switch setting matrix supplied with the guage - newver got into the maths to work out what half calculated figure would be or if the switches could be so configured.

 

Graham

[Guest elwe]

Tim

 

If I knew what a VR sensor was I might progress.

 

A VR sensor is a variable reluctance sensor. I has 2 wires and is fitted a couple of mm from a ferrous (steel) wheel with 1 or more teeth. As the wheel spins a voltage is induced in the sensor as each tooth passes it. The voltage produced depends on the speed of the wheel. The output is typically a sine wave with very variable amplitude. Whereas a hall sensor produces a nice square wave output of 0V and 12V. Hence the megasquirt circuit to covert from VR to hall.

 

VR sensors are used at the wheels for ABS, on 36-1 EDIS trigger wheels etc. THe easiest one I have seen to get from a scrp yard was on an early 90s Astra. On the offside end of the engine the crank pulley has teeth and a VR sensor is fitted to read it.

[Graham B]

What a stroke of luck. My DOHC has one to pickup the crank position for the ECU...and I have a spare.

 

Thanks Tim

[e666]

Hey

 

this is one of my fail points from my recent SVA - same speedo, stuck at 30 and then at 40 and jumped all over the place from 25 - 55

 

e

[Graham B]

Resolved:--

 

I have knocked 2 of the magnets off the prop, made sure the 2 now affixed are in the same polarity ( with another magnet ). Squared up the sensor so it is directly above and square to the magnets - spaced about 1/16 " away.

Speedo with only 2 was more accurate -ok to 70 and then bashed about.

After squaring -- reads smooth as you like to well over 100, up and down.

 

 

The table supplied with the guage does not have a setting listed for every calculated figure. I worked out that a value of 13 was for every binary switch -so got it very accurate.

Used the Sppedcalc program to get the wheel diameter figure ---very useful.

The guage now agrees with the figures supplied by Speedcalc.

 

A happy bunny

 

thanks all

 

Graham

[Snapperpaul]

Not tested mine yet but used shielded cable, i heard that non shielded can some times cause problems.

[Guest bytesize]

I have a SuperSpec with an MT75 gearbox and was constantly breaking Speedo Cables. The cable drive was fine untill you really put your foot down then the cable gave way under hard acceleration.

 

I ended up buying an Acewell Digital Speedo from the 16vMini Guys £65, and an MT75 electronic output unit from a Scorpio £5, god bless the local scrappy. (This way you dont end up with magnets and switch's)

 

The gearbox sender was a doddle to fit, remove the existing blanking plate and fit the sender - 10 minute job.

 

We ballanced the signal, dropped the output level from the gearbox sender from 12v down to 6v for the speedo input (simple resistor bridge - about 30p worth) and connected it to the Speedo, hey presto, perfect stable reading up to 70MPH.

 

If anyone is interested let me know and I will send you the details of what you connect and how you ballance the inputs.

 

Acewell 3100, nice little unit and fully road legal, Speedo, Tacho, Shift Indicator, Fuel Guage, Trip meter's, a lot of function for not a lot of money... Oh and it has a really cool blue backlit screen..