When we work on electronic circuits, there are some tools that get frequent use. Things like multi-meters, oscilloscopes, and waveform generators. Working with digital circuit has some use for those, but brings with it different requirements that require different tools. We'll have a look at some of these. These are fairly simple and can be made quite easily.

Update:

When I designed, prototyped, and even when I laid out several revisions of the PCBs, my thought was always "these will be useful when people are playing around with the TTL chips I'll talk about in this guide series."

Then it hit me.

These are incredibly useful whenever you're hacking on any 5v microcontroller board. That includes

• ATmega328 boards such as Arduino UNO, Nano, Pro Mini
• ATmega32U4 boards, such as the Arduino Leonardo, Micro, 
• ATmega2560 boards such as the Arduino Mega2560

If you want to use the I/O boards with a 3.3v microcontroller board, you generally can't do it directly. Instead, you have to use a level shifter. The majority of Adafruit's breakout boards have level shifters builtin, but these I/O boards don't. You are in luck, though: Adafruit sells a levelshifter breakout that can be used between your 3.3v board and the I/O boards.