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I have tried many times testing some Op Amp LED flashing circuits on a breadboard from the internet. So far, none of them works. The photo above is one of my attempt using a LM358. I have double checked my wiring and swap the 100k with 10k and 56k resistors. what is the problem here? why it's so hard to get a led flasher working with an Op Amp? Also I notice some of the circuit diagrams in the internet don't have the left 100k resistor between v+ and non-inverting input. why is that?

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  • \$\begingroup\$ You could start by drawing the schematic in a more conventional way instead of everything chaotically all over the place. And also explain why you think that circuit would do what you want it to do, so the (possible) error in that reasoning can be found. My guess would be 4V is not enough. \$\endgroup\$
    – PlasmaHH
    Commented Sep 30, 2016 at 9:15

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The LM358 might swing its output to within 2V of the supply rail on a good day. That means on a 4V supply it might be able to muster 2V on the output. It barely reaches half Vcc in other words. Try lowering the 100 k resistor from the non-inverting input to ground to maybe 33k and see what happens. In effect you are lowering the target voltage which the capacitor has to charge up to to make it switch polarity on the output.

Wording above slightly changed due to an error pointed out by Michael Karcher.

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  • \$\begingroup\$ works like a charm. I don't need the left 100k if I have higher voltage? and for the right 100k resistor between output and inverting input, what is its purpose? I have replaced it with a 10k and it still works. \$\endgroup\$
    – user83582
    Commented Sep 30, 2016 at 10:15
  • \$\begingroup\$ The 100k to the inverting input controls the rate at which the cap charges and discharges hence a lower value means the led flashes more quickly. \$\endgroup\$
    – Andy aka
    Commented Sep 30, 2016 at 10:36
  • \$\begingroup\$ @Andyaka You mean "non-inverting input to ground", I suspect. \$\endgroup\$ Commented Sep 30, 2016 at 20:30
  • \$\begingroup\$ @michael which resistor directly controls capacitor charge rate and to which pin does it connect to LOL.. \$\endgroup\$
    – Andy aka
    Commented Sep 30, 2016 at 21:25
  • \$\begingroup\$ Oh, in the question.... Yes thank you for pointing that out dude. I shall fix it. \$\endgroup\$
    – Andy aka
    Commented Sep 30, 2016 at 21:27

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