• https://www.raspberrypi.org/products/raspberry-pi-4-model-b/

• https://www.thingiverse.com/thing:3774560

• https://www.instructables.com/id/PWM-Regulated-Fan-Based-on-CPU-Temperature-for-Ras/

• Noctua NF-A4x10 5V
• NPN transistor 2N2222A
• diode 1N4001
• resistor 1K

Install sysbench

shell# apt-get install sysbench

CPU stress test

shell# sysbench --test=cpu --cpu-max-prime=1000000 --num-threads=4 run

Without Fan

shell# vcgencmd measure_temp
shell# temp=78.0'C

With Fan

shell# vcgencmd measure_temp
shell# temp=54.0'C