So, as you all kinda know I've been enjoying the idea of a 555-based oscillator a bit too much. So today, I built the perfect multi-waveform 555 oscillator. As if it wasn't enough fun...
So, we'll just start with the heavy stuff, here's the basic circuit:
So, about the saw wave: due to the capacitor's effect, an exponential rise and fall is created, with a peak at both ends. This is somewhat similar to how a saw wave behaves, except that saws are straight rises with linear falls. Unless... we look back into synthesizer history...
Take a look at any old Oberheim saw wave. Actually, I have one here:
Now, we still have three issues with this oscillator: how do you make it voltage controlled, how do you change the pulse width, and how do you make all outputs the same volume? I've figured this out, too.
First, voltage control. Nowadays, we have these fancy things called vactrols. This makes voltage control cheap and easy: the voltage is applied to an LED, whose amount of light is determined by the voltage. This shines onto a photoresistor, which changes resistance according to the amount of light hitting it. More voltage=more light=more resistance. Doepfer actually uses vactrols in some of their modules. Replace the resistors/potentiometers with the resistor end of a vactrol, and you've got voltage control.
Finally, amplitude. All that filtering causes amplitude problems, especially for the triangle and sine waves. The fix? Use an amplifier! You can amplify the signals with either a buffer amplifier using an opamp, or use an actual audio amplifier, like a 386. I'd use the opamp, though, because even though it needs a unique power source (which is easy to build), it won't clip as easily as a 386 will.
So basically, this is the simplest analogue multi-wave oscillator you can build, as far as I know. Sure beats something like the Aries 317 or (heaven forbid you build this) the MiniMoog oscillator. Except, well, those don't have the same problems this oscillator does, but whatever.
Hey, next maybe I'll show you how I built my multi-type analogue filter! Actually... *starts writing next post*