555 Timer Calculator

This works the 555 timer both ways round. Enter R1, R2 and C for the astable and it returns the frequency, period, duty cycle and the high and low times; enter R and C for the monostable and it returns the pulse width. Give it a target frequency and duty cycle instead and it solves for the two resistors, then gives the nearest E12 and E24 values and the frequency and duty cycle those real parts would actually produce. It uses the exact ln(2) and ln(3) constants rather than the 1.44 and 1.1 printed in datasheets. Timing is independent of supply voltage, so no voltage is asked for.

Calculator Type

The free-running oscillator. R1 from Vcc to pin 7, R2 from pin 7 to pins 6 and 2, C from there to ground.

From Vcc to the discharge pin. Charging runs through R1 and R2 together; discharging does not touch it.
From the discharge pin to threshold and trigger. It is in both the charge path and the discharge path, which is why it counts twice in the period.
1 uF is 1000 nF and 1 pF is 0.001 nF. Use the marked value; its tolerance is almost always the largest error in the whole circuit.
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The one-shot. R from Vcc to pins 7 and 6, C from there to ground, a falling edge on pin 2 to start it.

From Vcc to the threshold and discharge pins.
1 uF is 1000 nF and 1 pF is 0.001 nF. Use the marked value; its tolerance is almost always the largest error in the whole circuit.
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Pick the frequency, the duty cycle and a capacitor you have, and this gives the two resistors, then the nearest E12 and E24 values and what those real parts would actually produce.

Must be above 50. The basic astable has no way to reach half or below, because the charge path always contains one more resistor than the discharge path.
1 uF is 1000 nF and 1 pF is 0.001 nF. Use the marked value; its tolerance is almost always the largest error in the whole circuit.
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Pick the pulse width and a capacitor, and this gives the timing resistor plus the nearest E12 and E24 values and the width they would really produce.

1 second is 1000 ms. The 555 is comfortable from a few microseconds to a few minutes; past that the capacitor leakage is larger than the charging current and the pulse ends whenever it feels like it.
1 uF is 1000 nF and 1 pF is 0.001 nF. Use the marked value; its tolerance is almost always the largest error in the whole circuit.
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