#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:

Credits and References

• LM555 Datasheet
• Designing 555 Astables - learnabout-electronics