Every one of these started because I wanted to try something out. Most of them did not work the first time, so I wrote down what went wrong too, because that is where I actually learn.
I put three LEDs on a breadboard, red, yellow and green, and an Arduino lights them up in order.
The first time I tried it all three came on together, because I had put the waiting line in the wrong place. I found it by reading my code line by line. Then I changed the timings so the yellow one stays on for less time, which is how the real ones work.
I wrote a game in Python where the computer picks a number and you try to guess it. It gives you a hint after every guess.
I made it in Scratch first, then I tried writing the same thing in Python. If you typed a letter instead of a number the program crashed, and fixing that took me the longest. Now it counts how many guesses you needed too.
My circuit was sitting on a breadboard in the middle of a messy desk, so I drew a box for it in Tinkercad and printed it.
The first box came out two millimeters too narrow and the board would not go in. I measured again with a caliper and printed a new one, and the second one fit exactly. There is a hole in the lid for the cables and a small window on top so I can see the LED.