Getting Old Analog gauges to work...

[DVD3500]

I did a search and read about some people getting their old gauges to work.
Please remember I am a not well versed in all things electric (yet) but from what I understood most were relying on CAN signals powering a potentiometer or a some form of pulse width modulation...?
Please let me know is that is incorrect...

My car is an old Triumph Spitfire. The gauges are almost all 5-volt resistance based. I can get the values fairly easily and I am savvy enough to determine them myself.

The idea would be the get the fuel gauge to read the state of charge, the temp gauge to read the controller temp (a friend who converted his Spitfire had problems with the controller overheating, hence why I figure to put it there).
I also have an after market oil pressure gauge and because I had two Spitfires (long, sad story) I have a spare temp and fuel gauges.
Anything I might want to use those for...?

The sytsem I plan on using form a conversion company comes with a small panel with all the vital information. The gauges are a nice-to-have and more to make the car a bit of a "q-ship": https://en.wikipedia.org/wiki/Q-ship i.e. look as standard as possible ... but hide the POWER inside.

I of course have a tachometer but I am thinking tracking the RPMs of the electric motor is kind of silly.... It is also purely mechanical so likely more trouble than it is worth... I saw someone use it to see regen but I can't see how a mechanical dial can be converted. If I am wrong let me know!

Speedometer is direct from the standard transmission/gearbox so that is fine and will remain...

Many thanks in advance and feel free to tell me I am barking up the wrong tree!

PS: One thing a I CAN do is a bit of graphic design and I plan on recreating all the emissions information on the sticker under the hood/bonnet to reflect the new powertrain.

[MattsAwesomeStuff]

To state the obvious, the point of a gauge is to display useful information to the driver. Not just to display things, there's a reason we display things.

If you consider the speedometer to be the #1 gauge for information the driver can use, the tach was probably #2. The Tach tells the driver the RPM, but that's not exactly what a driver cares about, RPM mostly tells the driver how hard they're pushing their engine, the actual rotation speed isn't relevant. So to stay thematically consistent, I think the best use of the tach gauge is to be a current meter. And, I'd probably put this on the battery line, not the motor line, to tell the complete picture.

Also, I don't want it to be linear. Useful information means that the whole gauge range needs to be useful. Since nothing even remotely compares to acceleration from a stop, (at least 500-700% of the steady power draw at highway speed), if you make that the linear ceiling, then, the place you spend 99% of your driving time will have such minor changes to it that it won't convey useful information to the driver. You'll just sit at the bottom 20% of the gauge with no change in behavior impacting the dial. So, probably logarithmic is what I'm thinking, with much of the middle of the gauge being somewhere around ordinary driving. At this scale, you'd still have enough resolution to visually see the impact of turning on a heater, A/C, seat warmers, maybe even your brights. And at a stop, perhaps the bottom 25% of the scale would certainly highlight the little things you're doing.

Oh, I lied, I'd put regen as probably the bottom 20% because that's interesting to see. It would be logarithmic too but, if I was mapping the gauge, I'd make it more steeply logarithmic than the acceleration since you're not spending that much time braking but can still spike it as high as acceleration.

I'm kind of tempted to have my tach be motor current only and convert a smaller gauge to be the 12v current. I have a 12v current meter in my dash (more common before voltmeters became the norm), but, it's on the battery and I don't care about the 12v battery, I care about the DC/DC load (basically, your accessory load). Or maybe tach could still be battery and the 12v would be DC/DC. Regardless, the old 12v meter is centered on 0, and, if I have it on DC/DC I would want to convert it to zero being the start of the gauge. I don't care about power flow back into the 12v battery, EVs are not as finicky as alternators, I'll presume it's fine. I can slap a voltmeter on the 12v if I really care.

...

Years ago someone gave me a little device that does 2 or 3 gauge conversions. It takes pack voltage and runs your fuel gauge, I think it might do tach based on a current sensor, and one other thing. I've never heard this product mentioned anywhere else so I doubt it was made for very long, early low-run EV business from 10 years ago is my guess.

A modern version would probably be some kind of cheap arduino or ESP thing. I wouldn't know where to start. I can't be the first person to want to do this outside of a fancy VCU, but I also wouldn't know where to look. I don't think it's a challenging task, there's gotta be some open source solution laying about.

[DVD3500]

I have read this several times and while I understand every word (English is my native language) I am not entirely sure what you are trying to say...
I get the comment after the "..." everything before that seems like some sort of "thinking out loud"?
I am not trying to be rude I am just trying to decipher what your intent is...
Thank you.

[MattsAwesomeStuff]

everything before that seems like some sort of "thinking out loud"?

Everything I say is just me thinking out loud

I am not trying to be rude I am just trying to decipher what your intent is...

You talked about what you might want to do with various analog gauges. You talked about how the Tach was kind of useless. So I started talking about what purpose I saw in having a tach, and what I would do with the gauge that used to be the tach. This in the context of my own conversion.

There's no intention, I'm just having a discussion.

[royhen99]

Do you know which type of fuel/temp gauges you have? There are two types assumimg 60s/70s vehicle, either moving magnet or bimetalic strip. As far as I know neither are 5V, they are either 12V or 10V.

[DVD3500]

I will double check but am pretty sure all bi-metalic.
I have a Smith gauge repair manual and I don't remember seeing any magnets.
There is a voltage stabilizer on the back of the speedo so I will double check what it is for.

[muehlpower]

Matt describes what should be displayed and how. On the other hand, the task is to solve how the existing displays can be controlled. Your car seems to have electrical instruments except for the RPM and speedometer. If you keep the original gearbox, the speedometer will continue to work. The tachometer is either driven by a shaft or receives pulses from the ignition coil. The temperature, oil pressure and fuel gauge are supplied by variable resistors in the respective sensors. Clock for the tachometer and analog values as PWM signal can be easily realized with an Arduino. For tachometers with shaft drives, there are adapters that convert electronic signals into rotary motion. Primarily for speedometers, but should also be suitable for tachometers.
https://www.dakotadigital.com/index.cfm ... prd886.htm

[DVD3500]

Ah... Arduino... I will look into that! Thanks!