see Thermo Plug
Errata
As mentioned on this forum post, the transistor pinout on the board is for a 2N2222/2N4401 type NPN transistor. The BC549 transistor shipped with a few units has the opposite pinout, and needs to be mounted 180° rotated w.r.t. what the silkscreen label says. See this weblog post for details.
All kits shipped as of 2011 include an NPN transistor which matches the board pinout.
This board has been replaced by the TP2.