Long-Range Thermocouple Sensor Sips Battery Power

Sometimes you need to know the temperature of something from a ways away. That might be a smoker, a barbecue, or even a rabbit hutch. This project from [Discreet Mayor] might just be what you’re looking for.

[Discreet Mayor] remotely keeps an eye on the meat, but doesn’t blab about it.
It consists of a MAX31855 thermocouple amplifier, designed for working with commonly-available K-type thermocouples. This is hooked up to a Texas Instruments CC1312 microcontroller, which sends the thermal measurements out over the 802.15.4 protocol, the same which underlies technologies like Zigbee and Thread. It’s able to send radio messages over long distances without using a lot of power, allowing the project to run off a CR2023 coin cell battery. Combined with firmware that sleeps the system when it’s not taking measurements, [Discreet Mayor] expects the project to run up to several years on a single battery.

The messages are picked up and logged in a Grafana setup, where they can readily be graphed. For extra utility, any temperatures outside a preset range will trigger a smartphone alert via IFTTT.

Keeping a close eye on temperatures is a key to making good food with a smoker, so this project should serve [Discreet Mayor] well. For anyone else looking to monitor temperatures remotely with a minimum of fuss, it should also do well!

Power For Nothing And Your Kicks For Free

We all know that you can convert heat into electricity. Usually, you do that with some form of steam, but there are other methods, too, including thermocouples. If you’ve ever seen something producing waste heat, you’ll appreciate Penn State’s work to harvest power from hot pipes. The idea is simple in theory: create a flexible thermoelectric generator that can wrap around hot pipes or other surfaces to gather otherwise lost heat. The full (paywalled) paper is also available.

The devices can produce up to 150% more power per unit area compared to other thermoelectric generators. A three-square-inch test device produced over 50 watts. Scale that up to an industrial pipe hundreds of feet long, and you could create some serious power. To accomplish this, the scientists used strips of six thermocouples and connected them for a total of 72 thermocouples. Liquid metal between layers improved the device’s performance.

This isn’t a totally new idea. Russia was famous for making radios in the 1950s that operated using a generator that went around the flue of a kerosene lamp. Since the Russians were pulling this off in the 1950s, converting heat into electricity is obviously nothing new. Of course, your body creates heat, too, so why not use that?

Build Your Own High-Temp Oven Thermometer

Looking to keep an eye on the temperature inside his wood-fired pizza oven, [Giovanni Bernardo] decided to skip the commercial offerings and build his own high-temperature thermometer using a type-K thermocouple. The end result is a no-nonsense handheld unit with a surprisingly low part count that, at least in theory, can read temperatures as high as 1023.75°C. Though we hope he’ll be pulling the pizza out long before that.

Inside the 3D printed case we find just a handful of components. The 0.91″ OLED display mounted in the front panel is wired to a Digispark ATtiny85 development board, which in turn is connected to a MAX6675 breakout board. This takes the input from the thermocouple probe and converts it into a digital signal that can be read over SPI with an Arduino library from Adafruit. Rather than going through the added complication of adding a rechargeable pack, [Giovanni] is running this thermometer from a standard 9 V battery thanks to the 5 V regulator built into the Digispark.

We especially appreciate the attention to detail [Giovanni] put into his case design. Each component is nestled into a perfectly formed pocket in the bottom of the box, and he’s even gone through the trouble of using heat-set inserts for the front panel screw holes. It would have been quicker and easier to just model up a basic box and hot glue his components in place, but he took the long way around and we respect that.

This project is another example of an interesting principle we’ve observed over the years. Put simply, if somebody is going through this much trouble to check an object’s temperature, there’s a higher than average chance they intend on eating it at some point.

Turning Heat Into Electricity

You don’t really create energy, you convert it from one form to another. For example, many ways that we generate electricity use heat from burning or nuclear decay to generate steam which turns a generator. Thermocouples generate electricity directly from heat, but generally not very much. Still, some nuclear batteries directly convert heat to electricity, they just aren’t very efficient. Now researchers have developed a way of preparing a material that is better at doing the conversion: tin selenide.

Tin selenide is known to have good performance converting heat into electricity when in its crystal form. However, practical applications are more likely to use polycrystalline forms, which are known to have reduced conversion performance.

The material works well because it is not very thermally conductive and it has a favorable band structure that allows multiple bands to participate in charge transport. However, in polycrystal configurations, the results are not as good due to higher thermal conductivity. Yet crystalline tin selenide is difficult to manufacture and not very robust in real-world use.

The team worked out that the polycrystal material’s thermal properties were due to tin oxide films on the surface. Using a particular method of construction, you can remove the tin oxide and improve performance even better than the crystal version of tin selenide.

Creating this material might be beyond your garage lab, though. You need a fused silica oven that can reach a pretty tight vacuum. Although you might be able to swing it. Otherwise, you might stick with more conventional methods.

Practical Sensors: The Many Ways We Measure Heat Electronically

Measuring temperature turns out to be a fundamental function for a huge number of devices. You furnace’s programmable thermostat and digital clocks are obvious examples. If you just needed to know if a certain temperature is exceeded, you could use a bimetalic coil and a microswitch (or a mercury switch as was the method with old thermostats). But these days we want precision over a range of readings, so there are thermocouples that generate a small voltage, RTDs that change resistance with temperature, thermistors that also change resistance with temperature, infrared sensors, and vibrating wire sensors. The bandgap voltage of a semiconductor junction varies with temperature and that’s predictable and measurable, too. There are probably other methods too, some of which are probably pretty creative.

Bimetalic coil by [Hustvede], CC-BY-SA 3.0.
You can often think of creative ways to do any measurement. There’s an old joke about the smart-alec student in physics class. The question was how do you find the height of a building using a barometer. One answer was to drop the barometer from the top of the building and time how long it takes to hit the ground. Another answer — doubtlessly an engineering student — wanted to find the building engineer and offer to give them the barometer in exchange for the height of the building. By the same token, you could find the temperature by monitoring a standard thermometer with a camera or even a level sensor which is a topic for another post.

The point is, there are plenty of ways to measure anything, but in every case, you are converting what you want to know (temperature) into something you know how to measure like voltage, current, or physical position. Let’s take a look at how some of the most interesting temperature sensors accomplish this.

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Turning A Waffle Iron Into A Reflow Station

There are a ton of ways to go about building your own reflow oven. Most of these builds start with, well, an oven — usually a toaster oven — with a small but significant minority choosing to modify a hotplate. But this might be the first time we’ve seen a waffle iron turned into a reflow oven.

Of course, what [Vincent Deconinck] came up with is not an oven per se. But his “RefloWaffle” certainly gets the job done. It started with an old waffle maker and a few experiments to see just how much modification it would take to create the various thermal reflow profiles. As it turned out, the original cooking surfaces had too much thermal inertia, so [Vincent] replaced them with plain copper sheets. That made for quicker temperature transitions, plus created some space between the upper and lower heating elements for the SMD board.

As for control, [Vincent] originally used an Arduino with a relay and a thermocouple, but he eventually built a version 2.0 that used a hacked Sonoff as both controller and switch. Adding the thermocouple driver board inside the Sonoff case took a little finagling, but he managed to get everything safely tucked inside. A web interface runs on the Sonoff and controls the reflow process.

We think this is a great build, one that will no doubt see us trolling the thrift stores for cheap waffle irons to convert. We’ve seen some amazing toaster oven reflows, of course, but something about the simplicity and portability of RefloWaffle just works for us.

Peltier Device Experiments

Once an exotic component, solid state heat pumps or Peltier devices are now pretty mainstream. The idea is simple: put electricity through a Peltier device and one side gets hot while the other side gets cold. [DroneBot] recently posted a video showing how these cool — really cool — devices work. You can see the video, below.

Many things in physics are reversible, and the Peltier is no exception. The device is actually a form of thermocouple, and in a thermocouple a temperature difference causes a voltage difference. This is known as the Seebeck effect as opposed to the Peltier effect in which current flowing between voltage differences causes a temperature difference. It was known for many years, but wasn’t very practical until modern semiconductor materials arrived.

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