Astable vs. Monostable: Two Different 555 Timer Jobs
The 555 timer IC can be wired in two fundamentally different configurations that solve different problems. Astable mode has no stable state at all — it free-runs, continuously oscillating between high and low output, making it a self-contained square-wave oscillator useful for anything needing a repeating clock or blink signal. Monostable mode has exactly one stable state; triggering it produces a single output pulse of a fixed, predictable duration, then it returns to waiting — useful for debouncing, timing a single delay, or generating a fixed-width pulse from an arbitrary trigger event.
In astable mode, the two resistors (R1, R2) and capacitor C don't just set frequency — they also determine duty cycle, and because of how the charge/discharge paths work through R1 and R2 differently, a standard 555 astable circuit can't produce a symmetric 50% duty cycle without an additional diode to separate the charge and discharge paths — a common gotcha for anyone expecting a clean square wave from the basic three-component configuration.
The 555 remains one of the most-produced ICs in history precisely because these two modes cover an enormous range of basic timing needs with just a couple of external passive components.