Calculate frequency and duty cycle for a 555 timer in astable mode.
The capacitor charges through R1 and R2 but discharges only through R2, so the high time always exceeds the low time. Duty cycle can therefore never reach 50 per cent in the basic astable configuration. Achieving a true 50 per cent duty cycle requires a diode across R2 so charge and discharge paths have equal resistance.
555 Timer
f = 1.44 ÷ ((R1 + 2R2) C); duty cycle = (R1 + R2) ÷ (R1 + 2R2)
f = 1.44 ÷ ((R1 + 2R2) C); duty cycle = (R1 + R2) ÷ (R1 + 2R2) The capacitor charges through R1 and R2 but discharges only through R2, so the high time always exceeds the low time. Duty cycle can therefore never reach 50 per cent in the basic astable configuration.
Achieving a true 50 per cent duty cycle requires a diode across R2 so charge and discharge paths have equal resistance.
This calculator takes 3 inputs: Resistor R1, Resistor R2, Timing capacitor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.