One way you could do this is by using [freestyle][1]:

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1. Create a transparent material and assign it to your objects:

 ![enter image description here][2]

2. Enable freestyle in *Render settings*:

 ![enter image description here][3]

3. In *Render layers*, set the line visibility to QI range and set the end to 1:

 
 ![enter image description here][4]

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Or you could use wireframe materials:

![enter image description here][5]

See [this question][6] for details.


  [1]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Freestyle
  [2]: https://i.sstatic.net/mNjzV.png
  [3]: https://i.sstatic.net/s0rPP.png
  [4]: https://i.sstatic.net/buFCZ.png
  [5]: https://i.sstatic.net/mo2ZF.png
  [6]: http://blender.stackexchange.com/a/2785/599