Built a cheap DIY fan controller because my motherboard never had working PWM

Summary

MSI Forgot to Wire My Fans, So I Built My Own Fan ControllerMarch 22, 2026Links How it all started Nostalgia hit me like a ton of bricks recently. The early 2010s were calling out to me. I really wanted to build a retro computer that could run old games, but at the same time, I wanted something that could still be somewhat useful for modern things. So I've started going to flea markets, asking friends for parts and looking out for good deals. I got a really nice collection going, including a socket 754 motherboard, a Sempron Mobile 3400+ CPU, and some DDR1 ram, but when building the actual PC, this is what I finally settled on: CPU: AMD Phenom II X6 1075T RAM: 2x4GB Corsair 1600MHz + 2x4GB Corsair 1866MHz GPU: NVIDIA GTX 960 Motherboard: MSI 970 Gaming (AM3+) Peak older brother core. Well, except that GTX 960, but it's the newest card that has official drivers for Windows XP, and I wanted to go all out. To be fair, the GTX960 is something that somone would've bought back in 2015 to upgrade their AM3/AM3+ rig, so it still fits the theme. If you disagree, just know that I'm always up for a fiery debate. The motherboard The heart of the build is the MSI 970 Gaming (MS-7693), and on paper, it's pretty solid. The black PCB, the red accents, full solid capacitors, super ferrite chokes. And MSI's marketing team went hard on it - Military Class 4 components, Killer E2205 NIC for "prioritized gaming traffic", Audio Boost 2 with the Realtek ALC1150, and support for SLI and CrossFire through two PCIe 2.0 x16 slots. The VRM can handle overclocking without catching fire, it's got 6 SATA III ports, and it even came with USB 3 at a time when it was only a "nice to have". For 2014, I think it's pretty impressive, and even today it still feels somewhat modern. One Small Problem When I first fired it up and went into the BIOS, I immediately noticed something was off. CPU Temperature: 255°C. Fan Speed: 0 RPM. Foreshadowing: 255 is 0xFF, the value you get when you read a register that ...