BondBreaker

I'm building a switching power supply and i've been studying commercial ones. I've noticed that underneath the photo coupler, there is usually a hole in the PCB. What is the purpose of this? Thanks

Thank you

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I have a rectangular object removed from an old modem, it has 4 pins and the markings are: UT6-1B 40.320MHZ USI 9318 I want to use this crystal as the clock of a microcontroller. I haven't used 4-pin crystals before and any help with finding the datasheet and/or usage would be appreciated. THx!