Step 20: Power sources
I used a dual power supply for the TL084 that can be converted to a single power supply for the LM324.
On the image we can see that I used a couple of voltage regulators a 7812, for +12 V, and a 7912, for -12 V. The capacitors are, as usual, used to stabilize the levels and their values are the ones suggested in the datasheets.
Obviously to have a ±12 V we have to have at least about 30 V on the input because the voltage regulators require a higher input to provide a stabilized output. Since I did not have such power supply I used the trick of using two 15 V power supplies in series. One of the two is connected to the Arduino power connector (so it feeds both the Arduino and my circuit) and the other directly to the circuit.
It is not an error to connect the +15 V of the second power supply to the GND of the first! This is how we get a -15 V with isolated power supplies.
If I do not want to carry around an Arduino and two power supplies I can still use the +5 V provided by the Arduino changing those jumpers (and using the LM324).