In another life (it seems) I breadboarded and soldered my way through many electronics projects, a time I still recall with great fondness. One of the very fundamental building blocks of a lot of projects was the 555 timer chip, providing the heartbeat to many things that flashed, squawked and counted.
Ken Shirrif has a fascinating teardown of this mini rockstar of the integrated circuit world - what it does, how it does it, and how it's used. Talk about a blast from the past.
Ken Shirrifs BlogThe 555 timer has hundreds of applications, operating as anything from a timer or latch to a voltage-controlled oscillator or modulator. The diagram below illustrates how the 555 timer operates as a simple oscillator. Inside the 555 chip, three resistors form a divider generating references voltages of 1/3 and 2/3 of the supply voltage. The external capacitor will charge and discharge between these limits, producing an oscillation.