Tacho options for a petrol dash?

[bigbadbob76]

Good to hear you got the sig gen.
1-i'd expect it to have a common power and signal ground but you can check that with a meter.
If it does then it will common the grounds of two supplies itself. If it doesn't then keep the supplies seperate and connect signal ground to tacho ground.

2- neither are calibrated so take your pick. Probably the sig gen is more accurate but that's just a hunch.
50Hz mains isn't calibrated either so don't rely on that as a test.




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[CJH]

1-.... you can check that with a meter.
If it does then it will common the grounds of two supplies itself. If it doesn't then keep the supplies seperate and connect signal ground to tacho ground.

Didn't think of that - yes, the signal generator's supply ground and signal ground are common. But when I power the signal generator via a USB adapter (e.g. a USB battery) and the tacho board via my bench supply, their earths will be different I think, unless I connect the signal generator's signal earth to the tacho board (/bench supply) earth. Is it OK to do that?

[bigbadbob76]

Yes, fine to do that.

[bigbadbob76]

you'd need at least .... 78V to trigger the tacho input using Chris's late circuit.
The LT side of the coil will reach this value by back EMF as the circuit is opened by the ignition module.
It's not just 12V as you might think.

Don't touch the tacho input, I did tonight and it tingles.

[CJH]

Haha - I know, already did that myself!

[CJH]

I did a quick test of my tacho using the signal generator. With the 200 Ohm resistor in R4 I tweaked the variable resistor RV1 until I got it virtually spot-on. There was some slight misalignment of the needle at some test points, but too small to be interpreted on the rpm scale, so I've noted the approx distance that I estimated the needle tip to be offset.

1000rpm = 33Hz, indicated 1000rpm minus ~1mm
2000rpm = 67Hz, indicated 2000rpm minus ~0.25mm
3000rpm = 100Hz, indicated 3000rpm exactly
4000rpm = 133Hz, indicated 4000rpm minus ~.25mm
5000rpm = 167Hz, indicated 5000rpm plus ~.5mm
6000rpm = 200Hz, indicated 6000rpm plus ~1mm

So there's just a hint of some remaining scale error (-1mm at 1000rpm, +1mm at 6000), but there's also noise of about 0.25 to 0.5 mmm, so that's probably not significant. Nevertheless, I'll swap the 200 Ohm R4 for a 191 Ohm and redo the test to see if anything changes.

For the record, the 3000rpm reading was spot-on at 100Hz, and at 101Hz the needle moved about 0.5mm to the right, so some of the noise in the needle positions could easily be due to error in the signal generator.

[bigbadbob76]

Good stuff Chris.
If you're doing this in a building rather than out in the van you'll be picking up some 50hz hum which will affect things.
Also your sig gen might well be noisy, it was £20 after all but good enough for tests.

[CJH]

I swapped the 200 Ohm R4 for a 191 Ohm - didn't make any difference to the ability to tune the needle. It needed to be adjusted fractionally, because of the difference between 200 and 191, but once adjusted the overall accuracy across the range wasn't changed.

I built up a 'final' version for myself, with the big C2 capacitor laid flat, and with the IC soldered to the board rather than sitting in sockets, so as to make the board as low profile as possible. C1 also had to be laid flat, as that then became the tallest component.

I used extra long header pins for the connection to power and the tacho signal, so that they stick out of the back of the tacho mounting frame just like the original board.





I calibrated it on the bench, then installed it all in the dash. Seems to work fine.

Now I miss my clock.

[bigbadbob76]

Now I miss my clock.

I need to find a clock option now too.
Small digital one stuck on somewhere probably.

Good work on the tacho though.

[937carrera]

See that little LCD on the bottom, surely that can be made to fit somewhere beneath the warning lights, you would need to have the time adjustment buttons.

Next step will be the full trip computer, but I won't mention that for the moment

oops

[bigbadbob76]

Is that a challenge?

[937carrera]

Not for men of your abilities

[bigbadbob76]

You're the PCB man Chris....
do you fancy reverse engineering this pcb for the VW MFA display?

[bigbadbob76]

Damm you Carrera!!!
You've got me thinking, Raspberry pi or arduino driving an old nokia 5110 display?
I have a pi, in one of these cases, sitting doing nothing.



Too modern maybe, the vw display is more in keeping with the '80s van.

[CJH]

Is that a challenge?

That would certainly be an interesting project. The topic of doing this with Arduino has come up once or twice on this forum, and I think it would be doable. The Raspberry Pi would also be a possible platform I think, and the Pi Zero is small and cheap.

I found a site where this has been done already, but using a dedicated computation board I think, rather than a generic Arduino or RPi. The link is here (the site is in German, but the auto translate feature in Google Chrome does a good job of converting it to English). I've been in touch with the person who developed that, but unfortunately he no longer sells his device. His display shows "Display of speed, speed, distance and travel time, tank contents and much more". I think that one of those 'speeds' must be engine rpm, and must be a poor translation by Google.

I think it could be possible to develop something similar. We now have the means to convert coil pulses to current, and could presumably measure the current digitally. I didn't check, but presumably the voltage on the SAK215 also changes with current, so maybe that's easier to measure digitally. A key to some of the 'trip computer' functions would be to have a digital measure of road speed. I am sure there are devices already available to convert speedo cable revolutions to digital speed, but maybe the same approach as used for the tacho could be used, provided we could generate a pulse for each turn of the cable.

E D I T - Oops, pressed 'Submit' instead of 'Preview'.

Distance would be the integral of speed over time, so the device needs a real time clock, but then that would also double as the time display. Temperatures and pressures would come from sensor voltages. Fuel level would be rather unreliable because of 'sloshing', but measuring fuel flow (and subtracting return flow) would be impractical I think.

Could be an interesting project.