I made an optocoupler by shrink tubing a generic photo resistor and a cold white led.

Then used it in the following setup to be able to control audio level to achieve tremolo effect:


simulate this circuit – Schematic created using CircuitLab

However with each LED blink a brief noisy crunch is heard. I have no idea, but it almost seems that photoresistor generates a small spike of current. Can this be true? Why does that happen and how can I prevent it?

Unfortunately I don't own an oscilloscope, so I can't confirm my theory

  • \$\begingroup\$ You should first verify that no DC exists on the Analog_in / Analog_out line. It should be zero volts when measured with a DC voltmeter. There are other causes of a transient as well as this one. \$\endgroup\$ – glen_geek Nov 1 '18 at 22:21
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    \$\begingroup\$ Also - you may not own an oscilloscope, but you're dealing with audio frequencies, so a soundcard is all the oscilloscope you need :) \$\endgroup\$ – anrieff Nov 1 '18 at 22:22
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    \$\begingroup\$ cold white LED ..... that is an UV LED with a white phosphor strip ..... it is difficult to say how a photo resistor responds to that ..... try using an LED that does not use phosphor to generate light ...... also, you are missing a resistor that is required to produce a voltage divider \$\endgroup\$ – jsotola Nov 1 '18 at 22:37

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