Here's my tip for the 555 timer: Learn what's inside it! As you can see on the "Inside the 555" page, there are fewer than 10 functional components inside and three of them are resistors.
For some reason I always struggled to remember the different operating mode configurations, what they are called, and how to set them up. But one day I was trying to build a specific thing and decided to sit down and actually understand the 555. To my surprise, it's really simple in operation and requires relatively little electronics theory to understand and derive the different configurations yourself. Once I did that, I haven't forgotten it and I can come up with more creative uses for the 555.
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