Anything not specifically related to the Speeduino hardware. Eg sensors, bluetooth, displays etc
Hello All,

TLDR: Can I control an old style CLT gauge with an unused PWM output from Speedy?

Me again, back with some more musings on how to do something I should already know!

The Stag has it's original CLT gauge, which is effectively an ammeter, where the current that flows through the gauge is dictated by the resistance of the sender.
Now I've replaced the sender (original 1 wire to new 2 wire bosch type), I need to interface the gauge. Ideally I'd rather not have 2 senders on the engine.

My first thought was to use a digital potentiometer on a separate microcontroller to replicate the resistance of the old sender (which is similar to bosch, ~2k5 cold and ~150r hot), however this would mean around 100mA to cope with, which is more than I could find a dig. potentiometer to handle.

So my next thought is to utilise an unused mosfet (e.g. idle output) to PWM ground the gauge sender, through a current limiting resistor, to drive the gauge. This can be linked to the coolant temperature the Speedy is measuring, and so would be calibratable through TS to get the gauge to read correctly.
The gauge is fed through a 10V voltage regulator (which I've replaced with a modern chip rather than the original switching type).

My actual question: Would this/Can this work?

I think max average current through the mosfet would be (10V / 60r) =~ 160mA, if I had a 60r inline resistor and had the DC at 100%

Cheers in advance for anyone's thoughts!
Hi there. This might work but I being me would make it smoother ;-) Years back I designed a control circuit for guys from polish Fiat Coupe forum that controlled all 6 gauges to add the "opening ceremony" or "staging" or whatever it's called when the needle goes to the end of the scale and back to zero on start-up. Yep, there was a hype for that back then :D
Anyway, 4 out of those gauges are pretty much what you've described so in order to nicely emulate variable resistance with PWM I've used a voltage controlled op-amp+MOSFET based current source that had simple RC filter at the input taking PWM from a micro. In your case it might be possible to skip the micro if the characteristics are similar in shape, you could calibrate that in analogue domain maybe.
I attach this part of the schematic. I should have a video of it in action somewhere ;-)
amp meter gauge control.PNG
amp meter gauge control.PNG (51.27 KiB) Viewed 2916 times
I think your basic hypothesis is valid, and it's how you intend to implement it is the question. Trying to stay within TS and Speeduino code, adapting an existing PWM function with an interpolated table might be a path. A hair-brained example idea is to convert VVT to use CLT as load, and scale the PWM output to your gauge ground, through your minimum (max-temp) resistance of 150 ohms. 100% duty cycle = max temp reading.

Of course if you're going to do the work, you could use alternatives, such as converting the gauge to stepper as in some threads here. Or, tapping the sensor output to a standalone Nano or ATTiny behind your gauge. No extra engine compartment wires, if that's a big concern, and the setup travels with your gauge no matter the engine management. Finally (my morning coffee is empty :lol:) you could go the other way, running the gauge as stock, and reading the sensor output to Speeduino a different way, e.g., with a current-reading circuit feeding the CLT input to Speeduino (4mA→67mA = 0V→5V). Lots of options!

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