The 555 operates in two primary modes: Astable (oscillates forever) and Monostable (one-shot pulse).

Supply Voltage: The standard NE555 works from 4.5V to 16V. If you are using a 3.3V system, you must use the CMOS version (LMC555).

The 50% Duty Cycle Trick: In astable mode, the duty cycle is always > 50%. To get a perfect 50% or less, you can place a diode (1N4148) in parallel with R2​ (cathode facing pin 7). This bypasses R2​ during the charging cycle.

Stability: Always place a 10nF (0.01µF) capacitor from pin 5 (Control) to ground. This stabilizes the internal voltage divider and prevents noise from triggering the timer early.