The NE555 is connected in the astable mode and uses only three timing components (RA, RB, and Ct. A 0.01-/aF bypass
capacitor is used on pin 5 for noise immunity. The operating restrictions of the a-stable mode are few. The upper frequency limit is about 100 kHz for reliable operation, as a result of internal storage times. Theoretically, it has no lower frequency limit, only that which is imposed by Rt and Ct limitations.