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As an exercise in getting more familiar with op-amps, I have built this blinking LED circuit:

schematic

With the values above the circuit works fine on the breadboard, and in the Falstad simulator:

Simulation

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!

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    \$\begingroup\$ Try taking the lower pin on the 100 nF capacitor directly to ground/0 volts/battery negative. \$\endgroup\$
    – Andy aka
    Apr 10 '20 at 11:46
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Increasing the capacitance is the obvious way to do what you want.

I think what @Andyaka is trying to say is that if you connect the negative terminal of an electrolytic capacitor to ground you won't have the problem of applying reverse voltages to it. However, electrolytic capacitors tend to be much more leaky than ceramic or other solid dielectric capacitors, so you may also need to reduce the value of R3.

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  • \$\begingroup\$ Works like a charm, thanks to both of you. I started from circuits with 2 power sources so going down to the virtual -2.5V rail felt like cheating, but I already had to do it for the load to get enough voltage across the LED anyway... \$\endgroup\$
    – Franck
    Apr 10 '20 at 13:45

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