Afroman’s digital capacitor writeup is mostly theory, but it has some good applications and looks like it could be pretty useful for future projects

“By manipulating the input/output states on a microcontroller, you can effectively switch capacitors in and out of circuit, as long as the circuit requires one end of the capacitor to be connected to ground when it is in use. When you set a PIC’s tristate buffer bit to be 1, it makes the pin an input and makes it high impedance as if it is an open circuit. When you set a PIC’s tristate buffer bit to be 0 and set the data bit to 0, it makes the pin an effective ground. With this system you can adjust capacitance to whatever you want, with range being dictated by your choice of capacitors and the resolution being dictated by the number of microcontroller pins you have available. This system is suitable for a large number of low power circuits. Here are some crappy pictures explaining it:”