Well, actually I need both 12 V and 5 V... like an ATX power supply, but with a lot less power output. Maybe I could just drop down the voltage from 12 to 5 with a resistor because the currents are so small? I don't have much experience with low current electronics... don't worry, I know how to handle mains power.
Not really, you can feed the 12V into both the OP amp, and arduino, and then get a 5V regulator, to get the other 5 volts.
Maybe I could just drop down the voltage from 12 to 5 with a resistor because the currents are so small?
DONT DO THAT, it will be a waste of energy, go with the regulator.