My cheap glue gun has the bare minimum of parts, and did not include any means to indicate its power status. Several times I had assumed that my glue gun was ready to use, and strained at the trigger, only to realise that it was cold.
The reverse would happen as well: I would have finished with it, and assumed that it was switched off, while in reality it was hot and just waiting to burn me when I handled it carelessly, without realising that it was hot and dripping with molten glue.
The solution to this was obvious: Fit it with a light that would be shining whenever it was plugged in to the mains. This would be great for discovering those pesky loose mains sockets, which would accept the plug and hold it in but (treacherously) not supply power to it.
The glue stick is transparent - the ones I use are, anyway - so the light is arranged to shine into the glue stick , making the part sticking off the rear of the gun glow brightly.
Warning: The entire circuit is connected directly to the mains, and so is a shock hazard. All circuit components are at high potential with respect to ground. If using it in a device with exposed metal parts extra insulation with a minimum creepage distance of 6 mm is required between parts of this circuit and the metal parts that can be touched.
Step 1: Dismantle the gun
On the underside of the glue chamber is a thermistor which heats up to a constant temperature. It has a positive coefficient of temperature, and self regulates its temperature to the melting point of the glue. If it is colder, it draws more current and heats up. When it gets hotter, its resistance increases and so it draws less current and thus cools down. Thus the glue is effectively held at a constant hot temperature.
Since it gets hot solder would melt off, so the wires are crimped on to two metal plates contacting the heating element.
The modification involves soldering on two more wires to the mains cord so that the added lamp can run off the voltage coming into the glue gun.