An IR-sensitive phototransistor is used to sense soldering-iron temperature. The phototransistor must see t
he tip through an opaque tube to avoid stray light, and the phototransistor should preferably be fitted with an IR filter. An old photo negative, dark red plastic, or red or black glass can be used. The iron sits on a holder. When the iron is removed from the holder, the iron is not being viewed by the detector. The heat will increase, but the circuit has a lag time; if the iron is returned to its holder after each use, overheating should not be a problem.