It really depends on your ambition with the project.

As for the sensor itself, there are highly standardized resistive types (PT100, thermistors, etc) that are very easy to work with, and are available in countless shapes and sizes. Start by reading up on them 😀

I'd DIY something around a 5V Arduino Pro Mini or some kind of ATTiny, that would do sensor readings and drive some kind of display.

The easiest way would probably be to use one of those cheap digital outdoor thermometers with the sensor in the end of a wire, and simply replace the sensor with something physically smaller. If they've used a standard resistive sensor (they probably have) it's easy enough to determine the type. Measure the resistance through the sensor at a couple of known temperatures and compare to resistance tables for common sensor types until you find a match. Replace with a smaller sensor of the same type.

As for references, a glass of water with ice cubes will always end up very close to 0 deg C. Boiling water could be used as a second reference, it'll be right below 100 deg C. Or just use the room temperature in your house.

You can do something like this - there are very cheap ready made boards that add a tempature control with fan curve: https://www.aliexpress.com/i/32883724008.html
They cost ~£3 and usually are called "DC 12V Temperature Controller Denoised Speed Controller ON/OFF for PC Fan/Alarm Board Module With wired cable"

I've used them on a couple of systems and they're great for adding fan control to old systems that otherwise just run that fan full blast. The setup is a hassle since the most instructions for it are picture #6 on that Aliexpress page, but it is possible to set up a temperature range and fan speed with just the one button interface.

I did try it out on a Voodoo 3 I think and took it off because the fan just ramps up to 100% anyway, the Voodoo Banshee / Voodoo 3 cards get quite hot and stay that way so a smart fan controller doesn't do much since thermal load doesn't go up and down too much with load.

kaputnik wrote on 2023-02-27, 11:33:

The easiest way would probably be to use one of those cheap digital outdoor thermometers with the sensor in the end of a wire, and simply replace the sensor with something physically smaller. If they've used a standard resistive sensor (they probably have) it's easy enough to determine the type. Measure the resistance through the sensor at a couple of known temperatures and compare to resistance tables for common sensor types until you find a match. Replace with a smaller sensor of the same type.

I was using one of those around the turn of the millennium. What I found is that by the time you dug the thermistor out of it's "probe" housing, it was a tiny half grain of rice sized thing.

Thermalwrong wrote on 2023-02-27, 16:07:

You can do something like this - there are very cheap ready made boards that add a tempature control with fan curve: https://www […]
Show full quote

You can do something like this - there are very cheap ready made boards that add a tempature control with fan curve: https://www.aliexpress.com/i/32883724008.html
They cost ~£3 and usually are called "DC 12V Temperature Controller Denoised Speed Controller ON/OFF for PC Fan/Alarm Board Module With wired cable"

I've used them on a couple of systems and they're great for adding fan control to old systems that otherwise just run that fan full blast. The setup is a hassle since the most instructions for it are picture #6 on that Aliexpress page, but it is possible to set up a temperature range and fan speed with just the one button interface.

I did try it out on a Voodoo 3 I think and took it off because the fan just ramps up to 100% anyway, the Voodoo Banshee / Voodoo 3 cards get quite hot and stay that way so a smart fan controller doesn't do much since thermal load doesn't go up and down too much with load.

My end goal is adding just the probes to Maxi Gamer Voodoo 1 and Voodoo 5500. The models I have will run only with passive cooling, so fan control is optional to me. But thank you nonetheless!

Very similar to what Thermalwrong is suggesting, I purchased a fan controller and simply used its temp probs on my P3 without any monitoring. 1 on the CPU, 2 on each of the V2 SLI, final one on the GF4 (just because I can)
Benefit is you can see the results in real time (mine are mounted onto the top of chips so not 100% accurate but close enough) 2 of the fan connectors are unpopulated, so you could probably have all 4 empty if you wanted a uniform look.