A display is a nice thing to have in many projects. A very cheap solution for only 2$ to 4$ is a 4 digit display from Qifei that can be found e.g. at www.Aliexpress.com. Of course, for this price you do not get a user manual! But what beats me is that I could not find a usable description on the internet. Therefore I had to rely on trial and error. Here is what I learned.
The device has 2 cascaded 74HC595 shift registers, one for the digit select, the other for segements select. By this the deviceu can only display one digit at a time. Therefore, you must time multiplex the for digits fast en