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Project Notes

#145 Variable Duty-Cycle 555 Timer

Test a 555 oscillator circuit that allows easy manual duty cycle adjustment while minimising the change to frequency.

Here’s a quick video of the circuit in action, in which the basic frequency has been slowed to around 2 Hz so that the duty cycle variation is clearly visible:



For many applications we want variable duty cycle while maintaining a fixed frequency.

The basic 555 timer astable configuration allows for duty cycles from ~50% to near 100%, and wide frequency range from <1Hz to hundreds of kHz. However, it is difficult to adjust duty cycle without affecting frequency and vice versa. This circuit is one approach for achieving continuously variable duty cycle while maintaining a relatively fixed frequency.

The formulae for calculating timings are given in the 555 datasheet:

t(high) = 0.693 x (R1 + R2) x C1
t(low) = 0.693 x R2 x C1
frequency = 1.44 / ((R1 + 2 x R2) x C1)
duty cycle high % = t(high) : t(low) = (R1 + R2)/(R1 + 2 * R2)

In order to adjust duty cycle without adjusting frequency, it follows that:

  • if we increase R1 by ∆Ω, we must reduce R2 by ∆/2 Ω
  • if we decrease R1 by ∆Ω, we must increase R2 by ∆/2 Ω

My objective here is “easy manual duty cycle adjustment” so I just want one control. This sounds like a job for a dual pole potentionmeter. NB: an alternative approach with diode-controlled discharge circuit is covered on the learnabout-electronics site.

If I had a dual-pole pot with one core of say 50kΩ and the other of 25kΩ, that would be perfect. Unfortunately I don’t. But I do have a 50kΩ dual pole. So I just need to configure it so one pole has its resistance halved.

Dual-pole Pot

One approach is to put one core in parallel with a fixed 50kΩ ressitor. This results in a slightly non-linear relationship between the limits, so it’s only an aproximation:

R2 variation

Here are the predicted minimum, maximum and mid-point duty cycles and corresponding frequency:

Set R1 R2 C1 Duty Cycle High % Frequency
min 1kΩ 35kΩ 10nF 51% 2028Hz
mid 26kΩ 27kΩ 10nF 67% 1800Hz
max 51kΩ 10kΩ 10nF 86% 2028Hz

The maximum frequency drift occurs near the midpoint, when R2 equals 26.67kΩ. It’s only 11% down, which is about as accurate as I could hope for, in return for a duty cycle being continuously variable from 51% to 86%.

Actual Results

It turns out my 50kΩ dual-pole pot only goes to a max of about 44kΩ. So I’m using a parallel R2 resistor of 47kΩ and my R1/R2/C1 values are slightly off theoretical. According to my frequency counter, the circuit is running slightly faster than predicted by theory. However the stability of the frequency, and the variation as the resistance is changed are as expected.

Set R1 R2 C1 Duty Cycle (calc) Frequency (calc) Frequency (actual)
min 970Ω 32.3kΩ 10nF 51% 2.2 kHz 2.75 kHz
mid 23kΩ 24.9kΩ 10nF 65.8% 2.0 kHz 2.55 kHz
max 45.1kΩ 9.79kΩ 10nF 85% 2.2 kHz 2.75 kHz

Visual Demo

To be able to see the duty cycle variation, I slowed the circuit down by replacing the 10nF C1 with a 10µF electrolytic capacitor. This would predict a basic frequency of around 2.2Hz. In practice, I’m seeing around 2Hz, as demonstrated in the video.



The Schematic

The Build

Credits and References

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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.