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I have the following circuit with a photodiode and an LM358P where I intend to blink an LED at around 20kHz. I want to work on it. I have made a calculation through 1/(2piRC), and got something around 33khz. Is this correct? Will this circuit work with a 20kHz frequency ?

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  • \$\begingroup\$ Just try it. Then try it with 47 or 68 pF at C1 and see which works better. \$\endgroup\$ – The Photon Feb 13 at 16:26
  • \$\begingroup\$ What is the photodiode capacitance at the expected bias? \$\endgroup\$ – Spehro Pefhany Feb 13 at 19:58
  • \$\begingroup\$ @SpehroPefhany sorry the late answer, if i check correctly it is 18pF, so i had to sum with the 100pF when doing the calculations correctly ? \$\endgroup\$ – FourZeroFive Feb 15 at 22:41
  • \$\begingroup\$ @ThePhoton that wouldn't reduce the frequency ? \$\endgroup\$ – FourZeroFive Feb 15 at 22:43
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got something around 33khz. Is this correct? Will this circuit work with a 20kHz frequency ?

It's nearer 34 kHz but yes this will do the job adequately with some rounding of the output signal.

However, the output signal will clip at around 3 volts typically with an LM358 and, that op-amp will introduce some extra low pass filtering due to its small bandwidth and slew rate limitations on the output: -

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It can work if your LED generates enough right wavelength light to the diode. I suspect that other lights disturb the function substantially. To avoid it you should use a circuit which doesn't amplify DC and the flicker of mains AC operated lights. For good sensitivity you should have a filter which removes unwanted signals. It should be placed in front of the actual amp if your LED is weak compared to ambient light. 47kOhm resistor is quite small, but that can well work in strong light or with sensitive diode.

BTW your capacitor causes lowpass filtering which doesn't help at all against low frequency nor continuous disturbing lights.

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