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555 timer astable calculator

Frequency and duty cycle of a 555 oscillator.

Try the circuit in the labBuild it on the simulator's bench and measure it.

How it is worked out

In astable mode the capacitor charges through R1 + R2 and discharges through R2 alone, between ⅓ and ⅔ of the supply: thigh = 0.693 (R1 + R2) C, tlow = 0.693 R2 C, f = 1.44 ÷ ((R1 + 2R2) C).

Example: R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF blinks at 6.9 Hz with a 52 % duty cycle. The duty cycle is always above 50 % in this circuit; a diode across R2 lets it go lower.

Keep R1 at 1 kΩ or more: it limits the current into the discharge pin.

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