In this project, I'll make a small circuit capable of controlling a servo, it is a servo tester made out of a 555 timer IC.

Over on YouTube our hacker [VIP Love Secretary] shows us how to make a simple MOSFET tester. This is a really neat, useful, elegant, and simple hack, but the video is kind of terrible. We found that ...

The improved one-BJT current-limiting supply is not much different than the primitive circuit, though its one additional resistor does change the circuit behavior. The improved circuit is shown below.