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Project Notes

#517 555 Timer Fixed-duty VFO

Using an unconventional configuration of a variable frequency 555 timer oscillator with fixed duty cycle (and precise duty cycle fine adjustment).


Here’s a quick demo:



The conventional configuration of the 555 timer - such as in LEADP#016 Astable Oscillator - suffers from a correlation of frequency and duty cycle when variable timing elements are used.

Is there an easy way to produce a variable frequency 555 oscillator with a fixed duty cycle? There are quite a few “close but no cigar” candidates, but my interest was piqued when I saw Julian Ilett’s “555 Timer LED Flasher (with weird connections)”:


I played this out on a breadboard for myself, and found that in the basic configuration, duty cycle remains solidy fixed at 60% over the full frequency adjustment range.

I’ve added a control voltage adjustment to the version of the circuit described below, which allows duty cycle to be precisely adjusted between about 10% and 90%.

I call this duty cycle fine adjustment, because adjusting the duty does affect the frequency. but once a duty cycle has been selected, the freuqncy can be adjusted without changing the duty cycle.


Duty cycle adjustment as provided by VR2: a simple voltage divider with C2 for stability, setting the upper threshold limit of the 555 via the control voltage input.



Breadboard Test

I used 1µF C1 to shift up the oscillator for an easier time-base to plot on a scope:


The default duty cycle (without any control voltage input) is 60%, VCC is 5V. The scope shows:

  • CH1 (Yellow) - discharge pin - oscillates between 0V and 3.84V
  • CH2 (Blue) - C1 cathode i.e. threshold/trigger pin - 1.68V to 3.36V
  • CH3 (Red) - control pin - 3.36V
  • CH4 (Green) - output pin - oscillates between 0V and 4.64V


With control voltage input to adjust duty cycle to 50%, VCC is 5V. The scope shows:

  • CH1 (Yellow) - discharge pin - oscillates between 0V and 3.84V
  • CH2 (Blue) - C1 cathode i.e. threshold/trigger pin - 1.28V to 2.88V
  • CH3 (Red) - control pin - 2.88V
  • CH4 (Green) - output pin - oscillates between 0V and 4.72V


Protoboard Build

I have some new 8mm strawhat LEDs, so I thought I’d give them a go with this circuit on a bit of protoboard:


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.