# #445 555Timer/VariableDutyCycle/ControlAdjust

Using the control pin to adjust duty cycle of a 555 Timer astable oscillator.

## Notes

The standard 555 astable configuration uses a built-in 2/3 x VCC upper threshold reference.

Given R1 = 4.7kΩ, R2 = 47kΩ and C1 = 1µF this produces an oscillation at 14.590 Hz with a 52.38% duty cycle.

The control pin (5) provides the opportunity to adjust the upper threshold, allowing the duty cycle to be vary above and below 50%.

While this provides access to duty cycles below 50% (unlike the standard astable configuration), the disadvantage of this approach is that both duty cycle and frequency are affected.

## Construction

The circuit is essentially the standard astable, but with the addition of adjustable control reference voltage.

## Test Results

When adjusted to 52.3% duty cycle, the actual frequency is 14.4 Hz - very close to the predicted value. This is essentially the same as having the control pin bypassed/disconnected in the standard astable configuration.

The minimum duty cycle possible with this astable configuration is 34.4%, at a frequency of 19.92 Hz:

The minimum duty cycle possible with this astable configuration is 62.1%, at a frequency of 11.42 Hz:

Duty Frequency Note
34.4% 19.92 Hz Min duty cycle
52.3% 14.4 Hz
62.1% 11.42 Hz Max duty cycle

## Credits and References

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