Joshua Paul Lyons

The Problem

Thermocell makes these great little devices that seem to work well to lower the amount of mosquitoes in a 15 foot area.

They have two consumables, the pads that have the repellent, and fuel to heat the pads. They work by putting in a pad and igniting the fuel. Then you walk away for 15 minutes.

Walking away is important because the devices make mosquitoes not want to be in the area, but if a warm bodied CO2 exhaling creature is in the area, mosquitoes will be attracted, diminishing their effectiveness.

Its easier to stop mosquito bites if you start with low to no mosquitoes in the area first. And if you want to cover a large area, you will need to set up 4 or 5 of these.

Next up, I hate these little butane canisters. I am always running out. I forget to turn them off and turning them on is a fiddly annoyance.

Lets find a way to start this remotely, and lets find a way to monitor lifespans of the pads.

Investigation

I am partial to doing something solid-state for this purpose, but how much heat do I need?

That sounds like something I should be able to calculate.. right? We know how long they last, we can figure out how much fuel is in there, so we should be able to figure out how much heat it puts out.

The canister is 1.64 oz and lasts 12 hours. The web says thats about 1640 BTU, over 12 hours that would be 136.67 BTU/hr.

Roughly, 1 BTU /hr is 0.293 watts. So about 40 watts per hour of heat generation.

This is where it hit me, shoot, I now need to figure out how much heat does this plate absorb, figure out the exact material it is made with, what the emissivity of that metal is..

There is a faster way, lets grab a contactless thermometer and just measure it when it is turned on for ~10 minutes.

And it measures to be... 250F, 121C, 394K. Simple! Lets figure out what can get to 250F and lets go.

Protoype One : Part One, Part Selection

A few more requirements: I want to be able to run everything with a single cable and use the power solution for other devices too. Let's consider 12V solutions with low-voltage direct burial cable.

I also want to avoid making a fire hazard, so I want a heater element that is self regulating, with auto shut off.

I've narrowed down my options for providing the heat to three choices:
A nichrome wire, PTC heater, or a silicone heating pad.

After doing a bunch of product searching, I decided to go with a PTC heater. It is self regulating, and can be powered by 12v. These are typically used in water heaters and should be able to survive some exposure to the elements.

The only downside is the PTC heaters I found that are cost effective, all max out at 200C, way too hot for my needs.

Ok, so temperature controlled on/off switch. A temperature-controlled on/off switch would be perfect. There are many purpose-built circuits available for this, and buying one on Amazon only costs a few dollars. Plus, they operate on 12V! Let's grab one.

An amazon order later, and I have a PTC heater and the temperature switch in hand. Lets put this together!

Putting it all together

We have heat, we have sensor, we have switch, how do we put it all together? Well, lets take one apart and see what we are working with.

A few screws later and we find that there is a simple metal plate I can attach the PTC heater to, and since there is no more butain canister, I can put the temperature switch in the body of it.

Now this runs off of NTSC thermister, I'll have to put this near the heater. On the right you can see the final version of this, with the thermister in the far right corner and ptc heater sandwitched between the orignal metal plates.

Put in a smart outlet switch and a 12v powersupply, and this works! I can turn it on and off from my phone, Good first version.

Prototype Two : Software time!

I'm a software developer remember, this hardware solution is cool, but lets go further.
Goals:

• Connect via Wifi
• Remotely Control the device
• Grab data from the device

Hello little ESP32 C6-WROOM board you can get for less than 8 bucks. Wifi 6 build in, GPIO. Lets go!

New Parts List

• ESP32 C6-WROOM
• DC to DC 5v Bulk Converter
• 1 Channel DC 3V/3.3V Power Relay Module

The ESP32 can run off of 3.3v, but it has more protection if it is ran off of 5v, so lets do that. I'll tap into the 12v line that is going to the relay, point that to the bulk converter, send that to the esp32.

Now the ESP32 can control the relay, and the relay is what turns the PTC heater on and off.

All I have to do now is write the software to control the relay, and the software to read the NTSC thermister, then set up some sort of server to control the devices.

Lets slap this together with a little breadboard I have left over from some things I made for my Flipper Zero. One side the esp32 dev board, and the other side the parts to interface with it.

I also put in a few plugs into the board for the NTSC thermister easily. All I need to do is plug in 12v, it steps down to 5v for the esp32 and 3v from the esp32 for the NTSC Thermister.

Oh boy, my plan was to do this using Platform.IO/Arduino and visual studio code... but I saw WIFI 6 and got overly excited and ordered the dev boards with out checking to see if Platfrom.IO supports it.

It does not. They are just too new to be on there yet. So I had to use the Expressif framework, in C. Never done that before =) Alot of nicities and ease of use are gone, but there is still alot of great things in it! I just have to look things up at a different location.

For example, the ESP32 has a built in webserver, so I can just use that to control the relay. I can also use that to read the NTSC thermister.

You can see an early sample of the webserver here.

Now we just have to write the code! Dont worry, I am aware of how long this has become, I will put up a GitHub repo for those who want to see some bad C code.

Here are some of the key points on getting the esp32 functioning using the Expressif framework:

• esp_wifi_start() : This lets you get the esp32 onto your wifi network after you have configured it.
• adc_oneshot_read() : lets you read the raw analog value from the ESP32, Converting to a tempiture can be done following the Steinart-hart equation.
• httpd_start(): lets you set up the webserver to respond with handlers you have already set up.
• gpio_set_level(pin,uint32) : Control the relay with this, 0 is off.

After I have all of the devices flashed, you then have to figure out how you are going to lay out the devices. In my case I ran power around the outside of my yard with 12v low voltage lighting wire. The picture to the left gives you a good idea of the current coverage in the yard/patio.

Ready for a gotchya? Do not forget about Ohm's law and get a higher guage wire than you expect. I found the last 2 devices in the long chain I set up were not getting enough power to get over 110'F, so I split the power circut into two and everything works fine now.

Whats Next

The current version of things work great, each of the devices go on the wifi network on their own, and since they are all wifi 6, I have not noticed an impact on the wifi performance. There is an opportunity to have them mesh together and use one device to connect to the network.

I am not in love with the ball of wires that the prototype is in so I have designed+ordered a PCB to replace it. I am still iterating through those designs but I will post those once I am happy where I end up at.

I have also been asked "Why not rust?". And yeah, I am going to give it a go and rewrite my little C app in rust.

I am also working on a esp32 powered rottery device to allow turing on and off the devices with out having to use a smart phone or app, just press the button.

An added bonus of having 6 devices outback that can all capture tempiture data is that I can then pipe that over to promethesus and display that on my dashboards.