Designing a PCB to Teach Digital Logic Gates
A fellow student at my University asked me the other day if I could design a circuit board for a workshop teaching how to solder. I immediately accepted because I've been wanting more experience with designing boards for the CNC machine at our school. Deciding what I wanted to make was hard, because it had to meet the obvious criteria of being cheap to produce and having a low part count, but it also had to be fun. I thought about sound circuits since that was how I first learned electronics and later went on to design Analog Synthesizer boards.
In the end however, I decided I wanted to be more educational about things. So I choose to combine the basic comparative logic circuits onto one PCB so they could see how they work. The final design allows the user to use a 8 Pin DIP Switch Package as inputs to the circuits and see the result on an LED.
Note that in the process of making the board I pulled up a pad with a solder sucker, rendering the AND gate broken, but trust me it's there and works ha.
Circuit Explanation
The basic logic gates are comprised of two transistors chained together. The base of the transistor can be considered the ‘input’, so each of the transistors receives a full 9V from the battery when switched on. The gates implemented on this board are the AND, OR, NAND, and NOR gates. To learn more about these standard gate configurations, I recommend checking out this write up on Hyper Physics
You can almost tell how it's layed out if you looked at the Hyper Physics page. Each row, of resistors and transistors is a gate. The 8 resistors are the inputs into the base of transistors. In the AND and OR configurations, the drop down resistor is placed at the bottom, and in the NOT configurations they are at the top. Finally, each output signal is passed through a current limiting resistor before heading into the LEDs.
The user can then flip the switches to turn on or off a gate.
Resources
If you want to build this too, I have the EAGLE schematic and board files on my Github Page. The board was designed specifically for CNC milling, but I may go ahead and make a version thats more compact and suitable for fabrication in the future.