This is a Atmel AVR ATtiny13 based project which I use to regulate the fan of my Korad KA3005P power supply, taking into account the temperature and current load.

It interfaces with the KA3005P control circuit and takes the fan control of the original KA3005P as input on PB3, measures the fan's tachometer signal (open-drain, standard 3-pin Molex fan connector) on PB2, has a temperature measuring Epcos 100kOhm NTC (Vishay 2381 640 63104, brown black yellow gold, B = 4190 K) connected to PB4 in series to a 47kOhm resistor. Also, debugging data is output as RS232 with 9600 8N1 on PB1 every sampling cycle (approximately every 800ms) in the form of five bytes (all binary):

1. Temperature, measurement in °C
2. Temperature that is accounted on top by the sampled input PWM (full PWM adds another 16°C)
3. Current fan rotations per sampling period
4. Target fan rotations per sampling period (for Arctic F8, that's 50 for full throttle at 12 VDC)
5. PWM register value OCR0A

I've created a build video of this circuit with further explanations, which you can find on YouTube. You can see where I wired up the cables at timestamp 4:08.

The schematic has been drawn with Altium CircuitMaker and is also available at Altium Vault. Fuses of the ATtiny13 need to be programmed to 9.6 MHz internal clock:

-U lfuse:w:0x7a:m -U hfuse:w:0xff:m

GNU GPL-3.