This is a simple opamp-based circuit which generates square and triangle waves, and can run on 9V. I built this to use as a basic audio probe (not to be confused with an actual precision test signal generator), but it could be used anywhere you need a basic oscillator. For example, you could stack several for use in a drone synth.
Schematic
What’s basically going on
Essentially, this is the example “Voltage Controlled Oscillator” circuit from the LMx58 datasheet, with the voltage control parts stripped out, and values tweaked. As shown, it runs from sub-audio range (under 10Hz) to around 8kHz.
Previously to that VCO example circuit, the datasheet also shows a “Squarewave Oscillator” circuit, which shows the basic idea this circuit is based on as well. In the datasheet circuit, the opamp is configured as a comparator, and works like an opamp-based version of a Schmitt trigger oscillator (also used in some of the noisebox circuits on this site). R1 and C control the frequency of the oscillator via the charge/discharge rate of the capacitor. In this tone generator circuit, U1a is still used in basically the same way, but the capacitor part works a little differently.
The second part of this circuit is the integrator created with the second opamp (U2a). Without getting too technical (see this link if you want to go deeper), in this circuit it’s essentially performing the same function as the timing capacitor in the squarewave circuit. Here, the capacitor charge/discharge cycle being part of the opamp feedback loop means we also get a nice triangle wave at the output of this opamp.
Each output is then AC coupled with a capacitor, and that’s about it. As we can see from the following oscilloscope screen grab, the shapes aren’t perfect, but certainly usable:
Note: as I was fiddling with this, I came to realize that it is pretty much the same as a “Simple Dual LFO” circuit I found in the Experimentalists Anonymous archives many years ago, though that one is tuned for use as an LFO (as the name implies). There is no name on the EA schematic, but I felt it was only right to mention it.
