Hot glue gun controller

As much as you wish to build all your foam models with epoxy and other light weight glues, sometimes you just really want to use hot glue for a quick bond. Unfortunately, if you're using a cheap glue gun, as I do, it's more than likely that it doesn't have a thermostat to control the temperature. And, when working with a material such as depron, this usually means that at some point, the glue is going to melt the foam.

After several hours of cursing and (ignoring) waiting for the glue to cool off to a usable state, I finally decided to do something about the issue. Now, the cheap and probably smart way to do this is to use a dimmer switch, and lower the current by increasing the resistance of the circuit through the gun.

However, that somehow did not seem like the most interesting way to solve this problem to me. So, ignoring the obvious solution, I decided to finally learn some electronics and make an Arduino-based feedback controller.

You can get these really cheap on Ebay, and they seem to be working just fine

My original idea is quite simple, connect a temperature sensor to the glue gun, and use a relay to control the power to the gun. As the plan grew (and I continued ordering semi-random components from Ebay), I also added a volume knob to control the target temperature, and an LCD screen to show the current status. For this project, I'm using a really cheap Chinese Arduino Nano.

One of the early breadboard prototypes

So, ordering all the components I needed from Ebay, I started designing the circuit. It's fairly simple, but being the electronics novice that I am it took me a few tries to solder everything together in a proper fashion (I also had some soldering iron issues along the way, so it needed to be replaced).

One of my early attempts at designing the circuit. It's still missing a few parts at this stage

Basically, I have a thermistor rated for up to 290°C attached to the hot end, and using a voltage divider I'm reading the temperature at the tip of the nozzle. I then compare this to the temperature set through a rotational potentiometer (a volume knob). When the current temperature reach the set limit, the controller will go into a cooling mode. This kills the power usign the realy, and waits for the temperature to drop 3°C before turning the power back off. These two values are displayed on the 20x4 LCD screen.

After attaching the arduino nano to the breadboard

I designed a box to fit everything in and cut it out from 4mm plywood. I made the casing wide enough to be able to rest the gun either on the sides, or along the back edge of the top plane, but I haven't decided exactly where to put the holder for it yet.

Project box top plane with fittings for the volume knob and a 20x4 LCD screen

The last issue that still has to be solved is how to properly attach the thermistor to the nozzle of the gun. Right now, I'm using a heat resistant silicone, but it has almost no thermal conductivity, causing the thermistor to heat and cool off slowly, leaving an undesired delay. I have ordered some kapton tape, and hopefully that will solve the issue.

Almost finished, just need to fix the thermistor attachment

I'll come back with some pictures of the finished assembly, especially the thermistor attachment, as I'm excited to see how the kapton works out when I finally get it. For now, I'm quite happy having a pretty cool way to control the temperature of my hot glue gun.