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4" V8 Tachometer


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I went on an interesting little journey adding a tacho to my V8 converted S3. Having made a custom panel to house Smiths fuel and temp gauges in the centre to make room for a 4" tacho in the right hand space in the instrument binnacle, I duly went to hunt one down. I wanted a Smiths/Jaeger look, but couldn't find one the right size/number of cylinders, so I did a bit of poking about and bought a few. I found a Jaeger mechanical one the right size, and a Smiths RVC the wrong size, but with the right type of electrical movement...

The Smiths RVC electronic tacho:


The Jaeger mechanical tacho:


Getting the bolt pattern for the electronic movement to drill the 4" housing:




Face re-drilled, fitted to movement and fitted to housing:



Here's what it looks like installed:


But of course, it doesn't work yet. You may see the little "4-cylinder" mark... So, a bit of research revealed that it works in the very simple way: The original coil on the movement is connected to 10v from the voltage reg, and the other side via a little circuit to ground. The circuit grounds one side of the coil for a fixed time every time it detects a pulse from the ignition coil - the rest of the time, it's left to float up to 10V. This varies the amount of time the coil is grounded, producing an average voltage which drives the coil to a given position against the spring and fixed magnet for that given voltage, carrying the needle with it. Simple tech! The technical term for this type of circuit is an "monostable": It's only stable in on state. The linked circuit is made with a few transistors, plus a little extra to drive the coil and handle ignition pulses. I shamelessly uses the tach input from the Megasquirt, given that's exactly what it's designed to do! Output is an common-emitter buffer.

The original:


Some photos of the breadboarded prototype and testing:



The more evenly spaced trace is my simulation of the coil with a signal generator. The other trace is the output from the circuit:


Having draw up the circuit and PCB artwork, then laser printed to acetate, I UV exposed a copper clad board with photo-resist pre-applied:


Developed, the etched with ferric chloride (devil stuff):


The finished product against my breadboard prototype:


Drilled ready for components:


Populated and ready for testing:


Packaged up and ready for be installed in the case:


I'd calibrated it on the bench (it's easy to work out what frequency 8 cylinders at, say, 3000rpm gives you), but a final twiddle of the trim pot while on the vehicle got it perfect before finally installing.

Hope it makes some sense, and you enjoyed the read. I haven't seen too much electronic stuff on the forum yet, so something a bit different :) I know I could probably have bought a VDO or similar, but where's the fun in that?? I had loads of fun setting this up, and now have an electronic tacho design I can calibrate to any number of cylinders. I'm sure it'll come in useful in the future...

I'll post the circuit diagram and PCB artwork at some point too. And as the copper clad boards are standard size, and it's a waste not make a whole batch, I have 10 or so spare PCB's, so if anyone wants one feel free to shout! It's only a couple of quids worth of components to complete.

All the best


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Nice work, looking at your bench it is obvously your bag, there are others on here so you not alone (not me).

I have an electronic rev counter but needed to change it for use with Megajolt / EDIS ignition so I bought a converter kit from Spider something. This also replaces the existing electronics but can be configured for any number of cylinders up to 8 by playing a sound clip into the unit and adjusting a variable pot to get the needle to the right revs. My tacho was an early Jag type but it works better with later mechanics so MGB but not Triumph. Looking at you photos mine is the same type as yours. So mine is OK now but some times it completely misses the low revs as it does not go lower than 2,000 RPM occasionaly but since everyone sayes you can't use the output from the EDIS or Megajolt I am happy that it does work well enough! I am looking for the better type to match a digital speedo as the new tacho that matches a new speedo are an added cost that I don't want! My aim is to modify the Defender digital speedo transducer to a Series transfer box then fit the aftermarket speedo.

Just another project.

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Cheers guys. I never got to do this kind of stuff at school, but picked it up when I was messing with audio amps. A lovely skill to have IMO.

Yeah, I saw those little units on ebay. They looked good, but this cost me about a fiver, so went my own way. Are you running it spliced into one of the connections between the EDIS module and coil packs? It ought to give a signal just as solid as any electronic coil trigger.

If you wanted to mess about a little, it would be easy to put the movement of an electronic tacho into a speedo and adapt my little circuit to trigger off the transducer. The calibration range is (deliberately) huge! Thinking about it, I have an LT230 with electronic speedo, so I might look into that. Hmmmm... Thanks for the idea!

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Hmm, let me/us know how that goes as it could be a really good idea?

So if you adjust the start offset for the pulses to get one speed correct (say 30mph) does the rest of the range work? If the dial marking is linear and the pulses also linear I guess it should? But I think it depends upon the marking spacing won't it? Ah I just realised, the pot adjusts the range to suit the marking and the angular position of the mech in the case sets the first speed position to the dial? Or is it that the 0mph alignment sets that, which is adjustable with a stop?

Another addition I want to add is a switch to turn off the transducer or speedo when I'm driving at an offroad centre or on private land as I have a limited mileage insurance so currently I lose road miles when driving offroad.

For my tacho I just use the IDM output from the EDIS directly which everyone says does not work. I also tried the Tacho outputfrom the MegaJolt which also worked. The only issue I have is something is not quite right, maybe a volt regulator needed as the tacho stops working if I turn the heater fan on!



Did not mention I liked the quality of your etchings (ooh err) I used to work in R&D and that was the most difficult bit.

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Hehe. Cheers! I always use pre-sensitized boards from RS - usually 100 x 160mm - laser printer acetate sheets, and software from http://www.freepcb.com/ just because it's simple. Most simple stuff can be made single layer with a little thought to the layout, so I tend to stick to that. The cheap fluorescent blacklight works a treat for exposure with a piece of glass on top to keep it all flat, then it's just a case of developing with whatever is recommended by the board manufacturer and etching. I usually keep the tray with the ferric chloride warm, and agitate frequently as it speeds up the etching time and gives sharper traces with less undercut. A proper etching tank would be nice, but they're pricey!

I went through all those questions and more when I first started messing with it! What I found is that the movement I was using is calibrated for a linear response to pulsewidth change, so getting it right at 3000rpm means it's correct (or very close) across the range. The pot in my circuit adjusts the pulsewidth/frequency response of the monostable. There may be a way of adding a constant (to make it over-read by a fixed percentage across the range), but I haven't looked into that yet.


Those little tabs you can see in that pic are for fine calibration if needs be. I believe you bend them a little to change the response at that point on the dial. You can see that a few are tweaked a little bit from the factory calibration, and the ones near the top end are actually slightly longer. Very clever IMO!

I hadn't actually thought about the odometer when musing about the electronic speedo. I'd have to put some thought into that, but it shouldn't be difficult to get something up and running, and cutting it off when off road will be trivial. I'll keep you posted!

EDIT: Sounds like your IDM connection is marginal for that tacho. I'd definitely try splicing into one of the connections between EDIS module and coil pack. As I recall, there are three wires, one of which is +ve, the other is switched ground for each coil, and it's one of those you'll want to splice into. Pretty sure that'll work!

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