555 Timer Calculator (Astable & Monostable)

Works out the timing of the classic 555 timer. In astable mode, from RA, RB and the timing capacitor C, it gives the frequency, the period, the time the output spends high and low, and the duty cycle. In monostable (one-shot) mode it gives the pulse width from R and C. The design mode works backwards: give a frequency, a duty cycle and a capacitor, and it returns the exact RA and RB, the nearest E24 standard resistors, and the frequency and duty those standard parts really give. The timing comes straight from the chip's comparators, which switch at one third and two thirds of the supply: charging from 1/3 to 2/3 takes RC·ln 2, and a one-shot charging from zero to 2/3 takes RC·ln 3. Datasheets round these to 0.693 and 1.1; the tool uses the exact values, which the verification reproduces by simulating the capacitor voltage. Values can be typed with SI prefixes — 4.7k, 4k7, 100n, 10u, 2M2. Limits worth knowing: the standard circuit cannot give a duty cycle of 50 % or less, because the high time always includes RB; a diode across RB, or taking the inverted output, is the usual fix. Real timers also add propagation delays of a few hundred nanoseconds and the capacitor's tolerance, often ±10 % or worse for electrolytics, so above roughly 100 kHz or when precision matters, measure rather than trust the formula. Keep RA at 1 kΩ or more, as datasheets advise, so the discharge transistor is not overloaded.