Just another reminder to myself about how to convert 5V, like provided by Arduino, USB, and any number of wall warts, to 3.3V as needed by the ESP-01 board, using a resistive divider.
It's a little hard to see what's going on. Here is a schematic:
This only shows 2 resistors. In the photo, I'm using 2 1kΩ resistors in series to make 2kΩ resistance. The top yellow wire is Vin, R1 is the left most resistor, which is 1kΩ, R2 is the combination of the right 2 resistors, also both 1kΩ. The formula for the math is