Post by keurslagerkurt on Mar 24, 2021 8:27:34 GMT
Was in search for a small & simple LFO to make on the breadboard/Braedboard for some extra modulation. Found this super-simple oscillator circuit based on the CD4069 hex inverter chip. The circuit uses 3 inverters and the CD4069 has 6 of them, so you can make two LFO's with one IC, some resistors and two caps. (source: scott.joviansynth.com/fatman/4069_lfo.html )
With the exact values mentioned on the circuit, the LFO stopped oscillating when turning the pot to higher values (making the LFO slower). When it went low it stopped oscillating until I plugged the 1K resistor in and out..
In the end I managed to make it work conistently with a 50k pot, a 10uF cap (instead of 1.33) and a 200k resistor instead of the top 110k one. I'm not sure why my values have to be so different to make it work (I mean, the bigger cap makes sense, but I dont get why the circuit wasn't stable with the 110k resistor). However, when it works, it is quite a fun circuit for being so simple! LFO is not really triangle shaped but more S-curved, which has some cool musical uses too. However with this values it can't go in the really low cycles/frequencies
In order to get a more of a grip on the circuit, I tried to simulated it in Falstad circuitjs, but it doesn't seem to work there, or at least, it oscillates very quickly.. Don't know why the simulation can't handle the circuit? (simulation txt file is added if you want to try it out yourself)
If anyone could share some experience or extra understanding, I'd be very interested!
The resistor change to 200k is fine! Think it as a voltage divider together with the 100k, feeding the input 1. Looking at both states at pin 4, you see that you get a certain voltage change at pin1. With both resistors having the same value, there would be effectively no change -> no oscillation... Maybe 110k was the minimum working value for SG's design, but this depends on chip tolerances. Maybe 150k is also safe? The 1k is fine as limit of the upper frequency; otherwise it would oscillate with some very high frequency and almost impossible to control with the upper 3% of the pot range.
It's basically the circuit of my old single LFO module; with 10uF and 1M pot the frequency was quite low (around 0.1 Hz); if you want it even slower, just increase the cap ;-)
"And then, when AE Modular takes over the Eurorack world in a few years ..." - ahem, what crazy fantasies are this...?? ;-)