How it works
The 555 timer is one of the most widely produced integrated circuits ever made. Internally it contains two comparators, an SR flip-flop, and a discharge transistor, wired around a resistor-divider reference. Externally, a resistor-capacitor (RC) network sets the timing.
In astable mode, the 555 free-runs, continuously charging and discharging the timing capacitor to produce a square wave — used for blinking LEDs, tone generation, and clock signals. In monostable mode, it produces a single, fixed-length pulse when triggered — used for debounce circuits and one-shot delays.
The output frequency (astable) or delay (monostable) is set entirely by the external RC values, which is why the 555 is taught as a foundational timing IC before students move to microcontroller-based timing.