Astable oscillator and monostable one-shot calculations for the NE555
The NE555 contains two comparators, an SR latch, and a resistor divider that sets thresholds at 1/3 Vcc and 2/3 Vcc. These thresholds drive both astable and monostable operation.
Frequency: f = 1.44/((R1+2×R2)×C). Duty cycle: D = (R1+R2)/(R1+2×R2) — always > 50% without a diode bypass on R2. High time: tH = 0.693×(R1+R2)×C. Low time: tL = 0.693×R2×C.
Pulse width: tw = 1.1×R×C. Triggered by a negative edge on pin 2 (TRIG). Output stays HIGH for tw regardless of further trigger pulses.