The Project

Most events ban fire and heat based weapons.  However, in MRCA (Midwest Robot Combat Association) heat is allowed so long as it is kept under 280ºC.  Many robots (including my own) use parts printed from TPU or UHMW as armor.

These are great materials – and I've been thinking about a way to counter to TPU.

One idea is to create a hot poker that will melt through it on contact and destroy soft internals (batteries, wires, ESCs, etc).  Many people have joked "just put a soldering on at the end of a stick", well that is basicaly what I intend to do.

For a basic proof of concept I need to mount the temp sensor to the element and that goes into the Arduino. Then the Arduino will send a PWM signal to the MOSFET, providing current to the tip.

A PID (or other) control loop will use the PWM signal to modulate the temp of the tip –  trying to keep it within a certain temp range.

Once I can prove that this can effecitvely keep the hot tip under 280ºC and that 280ºC is actually a usefully hot temperature, then I'll worry about packaging it as a weapon on a bot.

Design Goals

  1. Fast heat up time - by applying a lot of power at the start
  2. Prevent burning out the tip - limit power based on temp
  3. Prevent the tip from getting too hot - 280ºC is the lower limit of what would be useful IMO, but near the upper limit of what MRCA EOs feel safe with)
  4. Keep the tip hot during contact and provide rapid-reheats after contact (and the temp drops)

Parts List

Part Image
K Type Temp sensor Temp Sensor
Seeeduino (Tiny Arduino) Arduino
MOSFET Driver Mosfet Driver
Soldering Iron Tips Soldering Iron Tip

Build Log


The code for this seems pretty simple. I was thinking at first it would want a PID control loop, but I think a basic on/off would probably do it - at least to start.

The more I'm thinking about it, this could be a good module for The Saint rather than as a weapon on it's own bot.  Because of it's limited use against bots with metal armor, it would be liability. However, on The Saint, it could be really effective against other wedges and plastic bots.  

I prototyped the code for this. It is really simple and it isn't  complete (or tested), but a quick code sketch of how it might work.  Mentally replace the LED with the MOSFET controlling the heating element.


Today I worked  on prototyping  the temp control system.

This will use an arduino, a k-temp sensor, and a mosfet. I hooked the k-temp sensor hooked up to to the arduino and I validated the temps it was reading in some warm water with my actual k-temp sensor and they were spot on!

Aruino K-Type sensor reading the same as the stand-alone meter.

Next I hooked up some leds to represent when the mosfet should be on or off. Then I moved the sensor between two cups of water, one hot and one cold and I saw the leds toggle form “heating mode” to “ready mode”.

However, I ran into a problem.

I was in a good place: I had the temp sensor reading temps and I had the mosfet turning the soldering iron tip on and off.

However when I touched the temp sensor to the soldering iron tip, the system would brown out.

I discovered there is a shared ground in the system. I had assumed the mosfet would be fully isolated like a relay is.

I was wrong.

The soldering iron tip screws into a lamp base (like on a flashlight). The outside soldering iron is metal and continuous with the lamp base. I had the base connected to the positive out from the MOSTFET. As a result, when the  temp sensor, which was negative touched the soldering iron tip, it created a short.

It took me a while to realize this and I chased around several complex solutions.  

Then it dawned on me: Swap the polarity of the supply current to the soldering iron tip, its just a resistive load and doesn’t care which direction the current is flowing 🤦‍♂️