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Issues with opto-coupler inside AC Triac dimming circuit - Electronoobs Q&A

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My questions are based on this project

I have built your AC triac dimming circuit with a potentiometer, and 110VAC 60Hz mains voltage.  The first time I ran the program, the lamp dimmed properly.  However, once I turned off power and turned in on again, the lamp did not turn on.  I changed out the optocoupler it fixed the problem, but it happened again.  I placed my oscilloscope across D8 (connected to the EL817 optocoupler) and GND and measured a sine wave.  I understand this should be a square wave, which is what I measured when the lamp actually dimmed.

Can you please help me understand why I am getting a sine wave across D8 and ground?  Why does the circuit work once and then stops working when I go to use it again?  How can I prevent these issues from happening in the future?  

Thank you in advance
asked in Arduino by (120 points)

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