Very often analog gauges are used to measure some rate parameters. In most cases they are based on voltmeters where needle position is driven with PWM signal. Pat found a tachometer from old car lying around and decided to use it for displaying something useful – CPU usage. He starter building this project on Raspberry where tachometer would display its CPU usage.
First of all he had to figure out the signals needed to drive tachometer from 0 to full scale. He found a table with frequencies and PWM values on Tekronix 3252 information site. Then another issue was the signal level. Tachometer needs 5V to drive it while Raspberry Pi signal level is 3.3V, he used 7404 buffer to bring that level. The rest is Python code running on Raspberry Pi, which reads CPU value and then sends driving signal on pin 11.