Italian maker “MadSimon” sent us info and a link to this video for his 3D freehand drawing rig. He explains:
It is an experimental DIY peripheral that behaves like a “3D pen.” Just hold the tip of the robo-arm, and act like you’re drawing, pressing keyboard buttons to lay down points and lines. The advantage here is that you can draw in three dimensions, thus creating solid figures. The trimmers placed at the four joints act as cheap, simple angular sensors. A Microchip PIC16F818 converts the signal from the trimmers into digital data, and sends it to the PC app through a parallel port. The software then makes sure the virtual arm follows every movement.
As for the mechanical part, the last two joints are made by the trimmers alone. The trimmer you can see being held by a fork is reinforced with a bearing keeping its rod in place. The base sensor isn’t visible, it’s hidden under the big car bearing at the bottom which holds up the whole thing. The drawer is usually powered by a 9v battery, but in the video, I’m using a converted PC power supply.