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Home made speedo


wood-gee

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While I'm posting threads on my current daydreams, my other plan is to make a home made speedo.

It would consist of a hall effect sensor attached to an arduino (mega), which would perform the processing, and this then to feed into either a 7seg display or LCD depending which appeals most at the time. The end result would be a far more accurate speedo, (mine just gives up completely occasionally), and one easily configurable for changes in drivetrain or wheel.

Part of my reason for airing this is that I can't decide how to trigger the sensor. Does anyone on here have any experience with this kind of thing? Will it do bad things to my wheel's balance if I were to attach a couple of magnets 180 degrees opposed to one another on it?

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Did not know this! Just the kind of speedos you'd find on ebay or the like? Could still be tempted to splice a wire off it in that case as I've dreamed for a long time of making an mpg meter for the it. With that and a flow meter on the fuel it should be very straight forward to calculate it!

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Long ago, when I had a series, and was running 750x16 tyres - so my speedo was way-off, then one day managed to break the internal spring. So I made a DIY speedo.

Mine used an RC Servo inside the original speedo housing plus the usual magnet & reed switch on the prop shaft connected to a Z8 Microcontroller. Back in them days, a Z8 was state of the art (essentially a Z80 processor with an EEPROM and a tiny amount of RAM, 512 Bytes IIRC) and just about the only microcontroller as we know them now on the market - but fine for this.

I calibrated it using road signs - which was about as accurate as was easy back then as GPS didn't exist (at least for civilians).

It did some cool things. The original speedo only read up to 70mph so I made it return to zero at 70 and start climbing again from there. OK it required a bit of adding up - but it meant I knew accurately up to about 90, it's top speed.

It was also a volt meter which read volts / 2 x 10, a tacho (rpm /100) and mpg.

I liked it because it looked standard (and displays of most kinds were hard to drive at the time).

Si

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Bowie - I'll get looking into it in that case!

Simonr - that sounds a fantastic idea! I think mine may have gone a similar direction, recently replaced the cable but it packed up again after being pretty off and on, so as you say that'd be a great way to keep things looking stock. Is the magnet sufficiently heavy to affect the balance of the prop//can you remember how you attached it?

Good effort incidentally getting up to 90, I can get to about 65 before it sounds like an explosion seems imminent! Was also considering putting a small LCD in place of the charge light on other side, which similarly wouldn't change the look of the dash much whilst offering potential for some exciting features; maybe a thermometer, the possibilities are endless.

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.....Is the magnet sufficiently heavy to affect the balance of the prop.....

You could just stick a bit of metal weighing the same as the magnet on the prop 180 degrees from the magnet and then not have to worry about it. Or even fit a second magnet to balance it out and just remember you will have double the number of pulses.

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I used a bicycle one with a big jubilee clip around the brake drum to hold a magnet. Balance hasn't been an issue on a slow S1. I upgraded to a stainless steel sensor for longevity. I fit can't take a big enough wheel fit 2 magnets.

Hall effect doesn't need a magnet, just a passing bit of iron :)

Servo driven speedo is cool. I was thinking of a steam punk speedo using a 1930's style clock. Three hands for speed, revs and whatever else you fancy?

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I used two magnets (so it balanced) with two jubilee clips (so they balanced).

My getting up to 90 was in part due to gravity and the tail wind - but also because I connected a stepper motor to the mixture screw of my (Zenith) Carb and connected it to a CB channel knob. This allowed me to re-tune the engine while I was driving. It's one of the reasons for building an mpg meter too. I just had to select the 'CB Channel' for what mixture I wanted. IIRC Ch 31 gave the best top speed, but low mpg and 21 gave best mpg. The numbers didn't relate to anything specific, just the rotational position of the screw.

Kind of a stone-age equivalent of Mega-Squirt I guess!

Si

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An arduino would be great for such a project. Have a look at an arduino mini pro as well. Does have USB connector for patches and updates later on and is much smaller an with pins instead of sockets it's easier to solder to a permanent board.

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This thread has got me excited. One of my challenges is speed indication. I got the Toyota gbox/tfer case and was wondering how to get the speedo to work. the xfer box has a cable drive attached to it and I would like to keep the standard series speedo. So easiest is to get a cable made up from the Toy box to the series speedo and see what happens. I calculated my ratios to give me close to 120km/h @ 3000 rpm in 5th gear. With a full mechanical setup I'll have to take it to a instrument shop for calibration with gear changes (to the speedo unit).

Advantage: mileage and speed corrected all in one.

Disadvantage: Cannot manipulate it myself and what if I want to fit larger wheels.

Is there an aftermarket adjustable electronic sender (rotary type) that can be made to fit the standard mechanical output of a gearbox mated with a Speedo/Odometer?

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if someone can create a simple programmable speedo system which uses a series gauge i would definately be interested in borrowing the idea, maybe paying for someone to bewitch the microcontroller system for me! haha

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just had a thought... the trouble with mine is it for some reason keeps snapping capbles, no its not trapped anywhere or anything lol.
how well would it work to create a simple speed sensing system which uses a motor, which just attaches to the back of a standard gauge, with a variable resistor in the circuitry to calibrate road speed depending on tyre size, gearing etc?
i would imagine you can get away with little amounts of electronic wizardry, the idea that comes to mind would be to use a "generator" type maboby driven off of something, (current speedo drive hole) which puts out a variable voltage which derectly relates to its RPM, a few resistors here and there and a magic wand should see that idea working, although the "generator" would be fairly susceptible to off road contaminants when wading and the likes which is a downside as i do like to wade deep!

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So if someone with some electronic engineering knowledge could get on this it would be great.......

Well if you could somehow fit the td5 defender sender (which should be rugged enough) you could then use that to feed pulses to a microcontroller and drive a stepper motor from that, simplez

How does the TD5 sender work though? Does it pick up pulses from rotating magnets or a disc. If you want to use an existing gbox/tfer case speedo drive it needs to be a self contained unit that does it all outside the gbox/tfer case. The rotating disc/magnets and pickup need to be in a compact sealed unit that can be made to fit the speedo drive.

I have a weather station at home that measures wind speed and with the shipping the reed switch broke. I replaced it and it seems a rather simple design. (As far as electronics go) The outside diameter of the assembly only measures 32mm, which is fairly compact. So rotating magnet and a reed switch. Seems simple, but how reliable? The way I see it that is all you need at the gbox. All the electronics can reside somewhere else, like in the gauge. One wire (power) into the pickup and one wire out, since you get a pulse out as the reed switch opens and closes.

Then the electronic magic, as that pulse equates to a speed signal, that must become an output signal to a stepper motor or torque motor (help me out here). Somewhere in there should be a potentiometer to adjust the output and this can be used to calibrate the speedo gauge. I'm sure this info can then me used to drive a lcd odometer and resettable trip meter.( eg. every 1000 pulses = 100m or something) effectively a pulse counter.

Why is this not available on the market already? So many kit cars, gearbox/drivetrain conversions, diff ratio changes, wheel size changes, and I have not come across a replacement speedo gauge/ sender kit.

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The TD5 speedo sender is self contained and on a Defender connects to the transfer box where the speedo cable normally goes, would it be possible get one to fit the toyota transfer case?

YBE100530-TRANSDUCER-SPEEDO-DRIVE.jpg


You would need to do a little interfacing with a transistor and possibly a schmitt trigger chip to condition the signal, then you could pipe it into an arduino. There is a driver board and code for driving gauges from an arduino https://www.tindie.com/products/TheRengineer/analog-gauge-stepper-breakout-board/ that would probably do the job for you but you will still have to write some code.

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So I've done some calculations. (I'm terrible at Math.) Here's what I got:

I put my gbox in 4th gear high range (1:1 input to output ratio) (Toyota box)

3.125 rotations for 1 rotation of the speedo output drive.

So 1rev of engine = 0.32 rev of speedo drive.

So @ 1000rpm you would get 320 pulses/min as per previous reed switch magnet setup.

That is 5.333333333 pulses/sec or a 5.33333333Hz signal.

33" tires circumference © = 2.63m So 1 rev =2.63m traveled.

At 1000rpm in 4th with 33" wheels I'll do 33.57 km/h (4.7 diff ratios).

So 5.3333333Hz = 33.57km/h.

1Hz = 6.3 km/h.

For distance calculation 1 pulse = 2.63/4.7 = 0.5596m.

So every 179 pulses = 100m traveled.

Did I miss anything?

Can anybody do the speedo drive calculation with a standard series gbox for me? (How many speedo drive rotations per engine revolution?)

If I'm very lucky (like winning the lotto lucky) my series speedo can be driven directly from the Toyota box.

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The TD5 speedo sender is self contained and on a Defender connects to the transfer box where the speedo cable normally goes, would it be possible get one to fit the toyota transfer case?

YBE100530-TRANSDUCER-SPEEDO-DRIVE.jpg

You would need to do a little interfacing with a transistor and possibly a schmitt trigger chip to condition the signal, then you could pipe it into an arduino. There is a driver board and code for driving gauges from an arduino https://www.tindie.com/products/TheRengineer/analog-gauge-stepper-breakout-board/ that would probably do the job for you but you will still have to write some code.

I'm sure it could be made to fit. Let me do some pricing. Is a td5 Speedo gauge and odometer adjustable? Can it be calibrated?

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Erm it's adjustable but not simply, the link in my signature shows some research we did into how the TD5 speedo works. Basically it can be set to any number of pulses per mile (or km on the km version) by reprogramming a serial eeprom on the board. You may be able to get a VDO speedometer from their vision range which is 'user programable'

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Erm it's adjustable but not simply, the link in my signature shows some research we did into how the TD5 speedo works. Basically it can be set to any number of pulses per mile (or km on the km version) by reprogramming a serial eeprom on the board. You may be able to get a VDO speedometer from their vision range which is 'user programable'

Reprogramming the EEPROM would involve what exactly? Interface plug, computer program, dip switches, potentiometers?

Does or can the arduino perform the function of modifying the signal between the sender and gauge? Like programmable correction factor?

Let me check that link......

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If you know the revs-per-mile of a Series gauge you can read the pulses from the TD5 sender (or a magnet on the prop, or whatever) through a microcontroller and then just drive a small stepper motor bolted to the back of the Series speedo at the corrected RPM. That's the minimal modification / reversible version.

Edit: Gareth, if you read CwazyWabbit's thread on TD5 speedos it's all explained already.

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If you know the revs-per-mile of a Series gauge you can read the pulses from the TD5 sender (or a magnet on the prop, or whatever) through a microcontroller and then just drive a small stepper motor bolted to the back of the Series speedo at the corrected RPM. That's the minimal modification / reversible version.

Edit: Gareth, if you read CwazyWabbit's thread on TD5 speedos it's all explained already.

I've looked into the thread now and I must admit that it's beyond my level of electronic knowledge, availability of equipment and resources. For your solution above I would need part numbers or specs. If I have to start experimenting with combinations it will cost me big bucks and a lot of time before getting it right. I do have some avionic friends at my disposal though and could rope them in.

Just to confirm that I have understood what I've read:

Only way to change mileage is reprogramming the 16bit word (hexadecimal). Even to zero the gauge? No jumper or shortcut?

I like the look of the td5 speedo.

I will not be able to get access to any equipment for EEPROM reprogramming. I might be able to get a td5 gauge and sender though. Is there a way to modify the ratio of the signal between the sender and gauge eg. for every 5 input pulses it puts 3 out. This could be a great unit for any speedo corrections without changing the speedo or sender.

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Yep only way to reduce the mileage on the TD5 speedo is to reprogram that eeprom (even zeroing it). You reprogram a different part of the same eeprom to change the number of pulses per mile as well. There is a commercial device out that allows you to alter the number of pulses as you suggest, google 'speedo healer'

Do you use km/h or mph speedos in South Africa?

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