I use hot melt glue a lot, but the warmup time is a problem. I turn the gun on, go away to do something else while it warms up, and may or may not remember the pending glue job. I’ve found the gun on 2 days later. Not good.
I solved the main safety problem a couple of years ago with a mechanical timer. Now the switch on the gun is always on, but it turns off after 10 minutes or so whether I remember to come back and use it or not. Of course when I run across the incomplete glue job an hour later, I have to start all over. An “it’s hot” notification would be very helpful.
While there isn’t easy feedback from the gun when it’s hot (yeah, a current sensor would work), open loop timing is good enough for the task. Sitting around and observing when the gun was up to temp gave the critical info: a little under 4.5 minutes.
Fifteen lines of Arduino code (plus a couple of #defines) for one of my general purpose ATtiny85s and a little piezo buzzer gave me a timer that beeped once when the gun was hot, twice one minute later, 3 times another minute later, etc. The dollar 5V wall wart from Goodwill that powers it is plugged into the same outlet as the gun, so the mechanical timer turns them both on at the same time. OK, wrapping it all in blue painter’s tape is pretty crude packaging, but it works.
When I upgraded recently from my drooling old Dremel 1200 glue gun to a Tec 805-12, warmup time dropped to just about 2 minutes. The old Arduino code was still safely in my sketchbook, so changing one #define and adding a couple of comments, and it was good to go again. I discovered with pleasure that power to the Tiny was plugged into the unmodified ICSP header, so reprogramming it was trivial. It works great again, and the reduced time ’til that first beep is icing on the cake.