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Project Notes

#206 CD4047/AstableOscillator

Test the astable operating mode of the CD4047


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

About LEAP#206 CMOS/TTLOscillators
Project Source on GitHub Project Gallery Return to the LEAP Catalog

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