Linkit ONE Wifi Meat Thermometer

Well there is a big bird in the fridge and my remote reading thermometer broke. I guess its time for my specialty, something overkill. How about a wifi thermometer that I can monitor on my iphone.

This will get plenty of use next year when the snow is gone and I start up the smoker up again.

When I first unpacked the Linkit One it was a bit hairy with the strip board antennas connected. I attached it to a robot proto board to keep the antennas at bay.

I soon realized I had some X-bee modules that used the same IPX U.FL connector. I also had a bunch of wifi antennas from scrapped access points and such. Luckily the pigtails and the antenna's had matching RP-SMA connectors. Since I have a sturdy all metal enclosure for the finished product the antenna had to be external.

These should do just fine......

I used commercial Meat thermometer replacement probes. They are pretty easy to find.Just search for AD-DTTC temperature probes.

These seem to be pretty popular in a few Arduino projects.

The schematic is as simple as can be. You just need three items. A Linkit One board, A Display, and temp probes. Since the Analog inputs are such a low impedance I had to put the sensors on the high side. I used small panel mount 3/32 headphone jacks from Radio Shack. I wired them right to Dupont connectors so they could plug directly into the headers on the Linkit One.

The headphone connectors have to be insulated from the chassis. The braided sensor leads will also be at +5V so be careful there too. You can insulate the antenna lead too. If your worried about shorts the 330 ohm resistor in the positive lead from the Linkit one does offer short circuit protection. if you can find a PTC that would be a better choice...

I really like VFD displays. They are a bit power hungry but so easy to see from a distance. I have a lot of Futaba 2X20 and 1X20 display lying around the workbench. I have one of each mounted to a plexi board for easy prototyping....

It should be simple enough to use any serial backpack out there. This is what I have....

Now that the display is working it is time to calibrate the sensors.

There is a line in the program you can un-comment to make it display the raw readings from the sensors. This make it easy to change the map command to get them reading the temperature. I just used a cup of ICE Water and another cup of boiled water....

The sensors may not be linear but for the range I need this is good enough for me. If you are a perfectionist look at the Steinhart-Hart equation.

OK, so now the proof of concept is done and it ts time to make it real.

I have a bunch of old modem cases. they are nicely made and easily gutted. It doesn't get more real than this. Drop one of these on your foot and you're looking at a serious bruise...

A quick shot of paint to get rid of the lettering and a little work with the nibbler to widen the display slot to fit the VFD.

I needed some spacers. I use these beads from the dollar store 400 for $1....

The back had a few too many holes so I used a self stick floor tile to cover them up. A little trimming on the belt sander and its done. I like these tiles since they compress a little and seem to keep everything tight.

I kept the little robot proto board since it really provides a nice and easy way to mount the controller. I just removed the factory antennas and put them away for future use....

For power I used an old linksys wall wort rated at 5V 2A. Overkill but its a reliable one and I don't want this thing to crash....

I used a micro USB connector attached to a coaxial power jack to hook it all together....

3 headphone jacks for the sensors. you need to be careful here the sensor cables are attached to 5V and if the hit the antenna connector the power supply will shut down. Probably should have added a fuse too.

The nice part of the vinyl tile is I can use it to isolate the probe jacks and antenna connector from the chassis.

I would have put the sensor on the ground side like I do with an Arduino but these Linkit's have a low impedance analog input.

I did jumper a few pins on the serial display to make it 9600,8,N,1. They come from the factory at 19200,7,E,1.

Once it's assembled its hard to make changes. Next project I may isolate the power feed from the usb port and just extend the USB to the rear panel for easy updates.

When it powers up it shows it IP address and then just loops updating the temps. If any probe is unplugged it will say "NA". After I plugged in the probes they were all with 1 degree of each other and within 2 degrees of my room thermometer. That's good enough for me.

The Web page is pretty sparse but it gets the job done......

All bolted together and one more test with boiling water. The window was open and it was in the 40's out there so it was dropping quickly.....

I left it running overnight to be sure it wouldn't crash on me.

I started playing with alarm settings so I extended the audio jack tpthe rear panel too. I cant decide what type of menu I'll make to set the target temperature. Setting the actual number of just a single button for meat type. I may also have different target for each channel. Maybe like that kiddie toy it will make animal sounds for each channel. Right now there is a single target temp variable in the program. When it is exceeded on any probe the board will play a klaxon sound. I just used an iphone speaker but I will mount one permanently inside the case. To turn off the alarm I turn off the speaker.

I'm leaning towards a simple one button menu that steps through "OFF - Beef/Rare - Beef/Medium - Poultry - Fish" Just now sure what Fish sound would work.....

There also a dive alarm at startup. You can put what ever sounds you'd like on the internal flash drive.