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#206 CD4047 Astable Oscillator

Test the astable operating mode of the CD4047

The Build


The CD4047 is capable of running in astable or monostable configurations, with operating frequency configured by an external RC network. So in one sense, sounds like the 555 timer!

Unlike the 555, the CD4047 provides a fixed 50% duty cycle with good frequency stability (+/- 2% @ 100KHz).

Astable Oscillation Frequency

This is a test of the astable operating mode. The frequency of oscillation is (roughly)

f = 1/(4.4RC)

Here are some measurements from the breadboarded circuit:

C R Measured f Theoretical f
1nF ceramic 220kΩ 936.3Hz 1033Hz
1nF mylar 220kΩ 996.2Hz 1033Hz
10nF ceramic 220kΩ 147.9Hz 103.3Hz
10nF mylar 220kΩ 102.1Hz 103.3Hz
68nF mylar 220kΩ 15.11Hz 15.19kHz
100nF ceramic 220kΩ 15.38Hz 10.33Hz

Astable Gating

The astable oscillation can be gated with pins 4 and 5:

pin 4 -ASTABLE pin 5 ASTABLE Oscillator
HIGH LOW LOW & stopped
LOW HIGH Running
LOW LOW Running

So it is possible to gate the oscillator using two complementary schemes:

  • Hold pin 4 HIGH, and pin 5 is the gate function (HIGH=ON, LOW=OFF)
  • Hold pin 5 LOW, and pin 4 is the gate function (HIGH=OFF, LOW=ON)


Here’s how the oscillator appears on the scope with C=1nF mylar and R=220kΩ

  • CH1 - Q output
  • CH2 - OSC OUT oscillator output (2*f, but duty cycle not guaranteed)


Close-up of the rising edge:




The Schematic

The Build

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.