As an exercise in getting more familiar with op-amps, I have built this blinking LED circuit:
With the values above the circuit works fine on the breadboard, and in the Falstad simulator:
If I change R3 to 5 MΩ however (trying to increase the time constant), the simulator still works however the breadboard version doesn't. I assume this is due to the current being too low through the capacitor, and parasitic effects or leakages taking over (e.g. if I put a single multimeter lead on one of the capacitor's pins the circuit starts working...).
My question is: Is there anything I can do to improve that circuit to get slower blinking rates? I am doing this as a learning experiment, so I want to keep the LM358 / single power supply core setup.
I have large electrolytic capacitors that I could use, but I don't know how to avoid the reverse voltages.
Other suggestions welcome - thanks!