Inspired by some things I had recently learned about LEDs and shift registers, I decided to make a compact, self-contained Persistence-of-Vision board.
This device flashes its seven LEDs on and off rapidly so that if moved quickly it appears to write text in the air.
I designed, fabricated, and coded it completely from scratch with no special tools.
The etching process to make the PCB was very interesing. After designing it, I printed the traces on glossy paper with a laser printer, then used an iron to transfer the toner to a copper-clad board. Next I placed the board in a solution of hydrochloric acid and hydrogen peroxide, which etched the uncovered copper off the board, leaving only the traces. Finally, I drilled the holes and soldered in the components. The process produces much coarser boards than the ones that can be ordered commercially, but it is practically free and has no lead time. For simple single- or even double-sided boards used for prototypes or single-application projects, the process is effective.
The circuit for this project is also useful and interesting because it allows for control of the seven LEDs on the board simultaneously using a shift register, which can in turn be controlled by only three Arduino pins. This is detailed slightly more in my code. Since the interface to the Arduino is also broken out into Tx and Rx pins on my board, for one application I was able to add a bluetooth module so I could control the text with a simple app I wrote for my phone using MIT App Inventor.
Also check out my relay board made with the same technique.