Fork me on GitHub

Project Notes

#444 555Timer/VariableDutyCycle/DiodeSteering

Using diode steering to achieve adjustable duty cycle at fixed frequency in a 555 Timer astable oscillator.



The classic 555 Timer astable circuit can be made adjustable by using variable resistors. However it is very difficult to make duty cycle adjustments independent of frequency. One approach is covered in LEAP#145, but it requires some careful calculation to adjust R1 and R2 in correct proportions to adjust duty cycle without affecting frequency.

Another approach demonstrated here is to use “steering diodes” across a variable resistor so that charge and discharge resistance can be adjusted without affecting the mean resistance (and thus frequency) of the circuit.


In this circuit, charge and discharge paths are steered by diodes. Given a duty cycle proportion of D:

  • charge: R1 through D1, portion of R2 = D x R2, to C1
  • discharge: C1 through portion of R2 = (1 - D) x R2, D2 and to ground via pin 7

When duty cycle is 50% (i.e. R2 pot set half-way), the circuit is essentially identical to the standard astable but with R2 at half its value if one ignores the diodes. So the standard astable frequency calculation should be a reasonable approximation of frequency.

R1 should be kept small compared to R2 to ensure the widest duty cycle range, but not so small that excessive current is drawn during discharge.

With R1 = 4.7kΩ, R2 = 100kΩ pot, and C1 = 1µF, I would expect frequency to be around 13.75Hz with duty cycle ranging from around 4.5% to almost 100%.




In these traces, CH1 (yellow) is tracing the output, and CH2 (blue) to voltage at the 555 trigger/threshold:




Some measuresments, showing that actual frequency is a little off the predicted 13.75Hz but remains quite stable over the full duty cycle range:

Duty Actual Frequency
5.1% 10.82Hz
50.0% 10.52Hz
95.0% 10.70Hz

Testing some other capacitor values:

C1 Actual Frequency Predicted Frequency
100nF 164Hz 137Hz
10nF 1.4kHz 1.37kHz
1 nF 10.1kHz 13.7kHz

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.