I have a schematic of a raspberry pi that is completely flat, no solid faces, just connected vertices.
Is it possible to render these lines somehow, so that it looks like a 2D drawing?
Maybe with freestyle?
The simplest way to render an object that has no faces is to change the material type in the internal renderer to a 'Wire' material:
The mesh on the left, when rendered, looks like the image on the right:
I also changed it to be 'Shadeless' in the shading panel of the material so that it wouldn't be affected by shadows and would be lit equally.
It seems as if freestyle doesn't care for wireframe meshes. Even faces aren't recognized if they have 0 surface area (edge extruded and escaped).
If you want to achieve that effect with freestyle then assign a material to your mesh (with faces) that has Z Transparency enabled. Alpha and Specularity should be set to 0.0.
To make the flat edgelines visible for freestyle, just extrude them all away from the camera. Then select the edges that you want to keep seeing with a Border Select and hit CtrlE - Mark Freestyle Edge. Set freestyle to render nothing but Edge Mark. The transparent material will hide the surplus geometry.
Alternatively you can set the material to Cast Only in the Shadow panel. It works if your scene doesn't have shadows.
Or you can disable Solid in the Layer panel of the Render Layers. Then nothing gets rendered except the freestyle pass. I get the best results if I disable Chaining and set the Caps to Round in the Freestyle Line Style panel.
The advantages: You don't have to display every edge. You can control line thickness and line style very creatively. Dashed lines are possible, to name just one option.
For the sake of completeness, there is also the possibility to use the skin modifier. It removes faces (if present) from your mesh and builds tubes intersecting at all vertices.
Below a screenshow showing original mesh (manually deleted faces), with skin modifier and last with skin modifier and subdivision modifier.
You can scale the tubing thickness at each vertices (use ctrl+A in edit mode).