Tonight I begin oscillator testing. A simple square wave astable multivibrator has worked for my oscillator needs in the past (although those were LFO’s triggering an LED, I wonder if these oscillators will sound good at audio rate. Perhaps sounding ‘good’ doesn’t really matter. If they generate sound then I’m happy.). I’ve just spent the day recording a band with a friend, I am quite exhausted but I recognise the urgency of this exhibition so I’m pushing myself to fit in time for experimentation whenever I can. I have a week and a half to create SOMETHING, and I may have a lead on a model railway I can borrow….
2 hours later:
Learnt a lot today, but I still haven’t landed on an exact design that is usable using components I have lying around. I can’t seem to get my pots to work as I think they should be working, but that may be the tiredness – I suspect that components can sense tiredness and have some method of altering their functionality when in the presence of too much of the stuff. I learnt a rough formula of ratios of feedback resistor+drain capacitor to create different frequencies of oscillation, I learnt that there seems to be a lower bound value for my feedback resister (10 ohms is below that threshold, 500 is above – I have no finer resolution resistors and have no desire to work this exact figure out at this time), I learnt that despite the schematic being labelled with polarised capacitors I can use non-polarised mylar-poly capacitors with much the same effect (I have an abundance of low nano-farad non-polarised capacitors which will come in handy as I do not have many high value resistors – high value being relative to the tuning of this particular oscillator topology).
Despite all of this great knowledge, I still haven’t figured out a combination of potentiometer and capacitor that will achieve the desired frequency sweep range. The only working combo I have found uses a 500kOhm logarithmic pot which is too fine a control to use reliably as the upper bounds of Ohmage (new word?) occupy such a small sweep of the wiper and such a large frequency range. I’m talking ~1mm rotation controlling around a quarter of the audible frequency range.
Experiments will continue tomorrow evening (hopefully I will find time).