#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:

Notes

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.

Circuit

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

• LM555 Datasheet
• 555 Timer LED Flasher (with weird connections) - Julian Ilett