Who needs a 1920×1080 OLED display when you can have an 8×8 matrix of LED goodness? That’s the question [Kathy] asked when she built this LED matrix light pen project. It looks simple enough – a 64-LED matrix illuminates as the pen draws shapes. But how does the circuit know which LED is under the pen? Good old fashioned matrix scanning is the answer. Only one LED is lit up at any time.
[Kathy] used a pair of 74LS138 3-to-8 line decoders to scan the matrix. The active low outputs on the ‘138 would be perfect for a common cathode matrix. Of course [Kathy] only had a common anode matrix, so 8 PNP transistors were pressed into service as inverters.
The pen itself is a phototransistor. [Kathy] originally tried a CdS photoresistor, but found it was a bit too slow for matrix scanning. An LM358 op-amp is used to get the signal up to a reasonable level for an Arduino Uno to detect.
The result is impressive for such a simple design. We’d love to see someone use this platform as the start of an epic snake game.
Alright, 3D Printers exist. They’re machines you can simply buy for a few hundred dollars, set them on your desk, and have them start churning out plastic parts. A little pedestrian, isn’t it? How about something you can take into the field for a client, and print out some new parts right there? How about sending a printer to the latest humanitarian crisis? After all, all those humanitarian uses for 3D printers we’ve been hearing about won’t do any good without a 3D printer.
The project featured in this post is an entry in 
We’re pretty sure the Hackaday demographic is a a person who sees a giant tower crane lifting beams and girders above a skyline and says, “that would be fun, at least until I have to go to the bathroom.” Realizing the people who own these cranes probably won’t let any regular joe off the street into the cabin, [Thomas] and [screen Name] (see, this is why we have brackets, kids) 
