Please see images for the setup that I use. The mega is powered from a 7812 and is generating a pwm wave at pin 7 (10us on time and 100us offtime) using delayMicroseconds and toggling the pin. I took the measurements across the 7805 output with the arduino disconnected from 7812. It has a 15mv p-p …

arduino making 7805 output noisy

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