Sometimes you find things you have not even been looking for…
A chaotic oscillator is an electronic circuit that can exhibit “chaotic“, nonperiodic behavior. A commonly cited example is Chua’s circuit, but there are many others.
I always regarded these as carefully designed, rather academic, examples. So I was a bit surprised to observe apparently chaotic behavior in a completely unrelated experiment.
A while ago a took an interest in recreating an ancient logic style based on discrete transistors, resistor-transistor-logic (RTL), with todays components. I discussed some of my findings earlier and continued to work on transistor selection and circuit optimization afterwards in collaboration with Yann from Hackaday TTLers. The tool of choice to evaluate the switching speed of different logic gate designs is to build ring-oscillators from chains of inverters.
One random variation I tried, was to use LEDs in series with the base of the transistor. In combination with PMBT3904 transistors, this surprisingly yielded an oscillator that switched between frequencies that were several orders of magnitude apart when varying the supply voltage. A closer look revealed a region with unpredictable behavior close to the switching voltage. You can find the chronological investigation, including my (limited) understanding of the principles, in this log entry.
One question of interest was how easy it is to recreate these results. Was it just a random fluke? It turns out, it is extremely simple.