How many times have we heard "hey you could have just replaced that microcontroller with a 555"? Maybe 555 times! But we always find wiring up a timer chip to be a bit of a pain, you need quite a few components - especially if you want a buffered output. That's why we designed the Adafruit 555 PWM Output STEMMA - a fully-assembled 555 timer board with adjustable PWM square-wave output. We originally came up with this idea when someone requested we make a 'PC Fan Dummy' board, that can generate a 100Hz signal for faking a PC fan's tachometer signal to a motherboard. But then we thought that sometimes we want a square wave for driving a piezo, or as an audio input, or to modulate an IR signal. So we made it a little more general-purpose.