An Epic Tale Of Thermistors: Tricks For Much Better Temperature Sensing

July 21, 2019 by Kerry Scharfglass 25 Comments

For years [Edward] has been building professional grade underwater sensing nodes at prices approachable for an interested individual without a government grant. An important component of these is temperature, and he has been on a quest to get the highest accuracy temperature readings from whatever parts hit that sweet optimum between cost and complexity. First there were traditional temperature sensor ICs, but after deploying numerous nodes [Edward] was running into the limit of their accuracy. Could he use clever code and circuitry to get better results? The short answer is yes, but the long answer is a many part series of posts starting in 2016 detailing [Edward]’s exploration to get there.

The first step is a thermistor, a conceptually simple device: resistance varies with temperature (seriously, how much more simple can a sensor get?). You can measure them by tapping the center of a voltage divider the same way you’d measure any other resistance, but [Edward] had discarded this idea because the naive approach combined with his Arduino’s 10 bit ADC yielded resolution too poor to be worthwhile for his needs. But by using the right analog reference voltage and adjusting the voltage divider he could get a 20x improvement in resolution, down to 0.05°C in the relevant temperature range. This and more is the subject of the first post.

What comes next? Oversampling. Apparently fueled by a project featured on Hackaday back in 2015 [Edward] embarked on a journey to applying it to his thermistor problem. To quote [Edward] directly, to get “n extra bits of resolution, you need to read the ADC four to the power of n times”. Three bits gives about an order of magnitude better resolution. This effectively lets you resolve signals smaller than a single sample but only if there is some jitter in the signal you’re measuring. Reading the same analog line with no perturbation gives no benefit. The rest of the post deals with the process of artificially perturbing the signal, which turns out to be significantly complex, but the result is roughly 16 bit accuracy from a 10 bit ADC!

What’s the upside? High quality sensor readings from a few passives and a cheap Arduino. If that’s your jam check out this excellent series when designing your next sensing project!

DIY Industrial Oven Brings The Heat

July 21, 2019 by Kristina Panos 4 Comments

When [Turbo Conquering Mega Eagle] tried lost wax casting, he ended up with a fireball and a galvanizing sense of disappointment. There wasn’t enough heat to get all the wax out, and the paraffin ignited. Though a bit burned by the experience, it didn’t extinguish his desire to do lost wax casting. In a textbook case of project-spawns-project, this eagle decided to wing it and made his own high-temperature oven.

This is true, seat-of-your-pants DIY. For this project, [TCME] treated himself to a virgin sheet of mild steel, a metallic delicacy for a guy who seems used to using whatever is available. The oven consists of a welded-together box inside a larger box, with insulation between the two. The door is a shallower box filled with insulation, with hinges on the right and a sturdy-looking gravity catch on the left. [TCME] welded together a nice little box for the 12-volt, 1000 °C temp controller module, and tacked some tabs to the outside to help wrangle the wires. Lower your visor and click past the break to watch this hot box come together.

We hope [TCME] answers the burning questions of how well the thing loses wax, and how fast it bakes a pizza. Meantime, here’s a clay oven that’s built to pizza.

Continue reading “DIY Industrial Oven Brings The Heat” →

The Heat Is On With This ESP8266 Controlled Sauna

March 8, 2019 by Tom Nardi 14 Comments

We’ll be perfectly honest: sitting inside a heated box sounds just a bit too much like torture for our tastes. But if we did somehow find ourselves in possession of a fancy new sauna, we’d more than likely follow in the footsteps of [Al Betschart] and make the thing controllable with the ESP8266. After all, if you’re going to be cooked alive, you might as well do it on your own terms.

The sauna itself was purchased as a kit, and included an electric heater controlled by a thermostat. As explained in his detailed documentation, [Al] integrated a Sonoff TH16 into the original heater circuit so he could control power to the coils remotely. The TH16 also includes support for a thermal sensor, which allowed him to get a reading on the sauna’s internal temperature. The new electronics were mounted in a weather-proof box on the back of the sauna, complete with an external WiFi antenna to help get a good signal back to the house.

At this point the project could technically be done if all you wanted was remote control, but [Al] wanted to create a replacement firmware for the Sonoff that was specifically geared towards the sauna. So he came up with some code that uses MQTT to connect the heater to his home automation system, and allows configuring things like the maximum temperature and how long the sauna will run before turning itself off.

Interestingly, the company who makes these saunas thought the work [Al] did to integrate their product into his home automation system was so impressive they actually interviewed him about it and put it up on their site for others who might be inspired by his work. We’ve covered a lot of hacks to consumer devices here at Hackaday, and it’s exceedingly rare for a company to be so supportive of customers fiddling around with their products (especially in a case like this where there’s a real chance of burning your house down), so credit where credit is due.

The last time we brought you a sauna hack it was quite literally in a van down by the river, so the addition of an ESP8266 certainly brings this more into our comfort zone. Figuratively, if not literally.

[Thanks to Jon for the tip.]

The No-Parts Temperature Sensor In Your Arduino

February 26, 2019 by Brian Benchoff 25 Comments

[Edward], creator of the Cave Pearl project, an underwater data logger, needed a way to measure temperature with a microcontroller. Normally, this problem is most easily solved by throwing a temperature sensor on the I2C bus — these sensors are cheap and readily available. This isn’t about connecting a temperature sensor in your Arduino. This build is about using the temperature sensor in your clock.

The ATMega328p, the chip at the heart of all your Arduino Uno clones, has within it a watchdog timer that clicks over at a rate of 110 kHz. This watchdog timer is somewhat sensitive to temperature, and by measuring this temperature sensor you can get some idea of the temperature of the epoxy blob that is a modern microcontroller. The trick is calibrating the watchdog timer, which was done with a homemade ‘calibration box’ in a freezer consisting of two very heavy ceramic pots with a bag of rice between them to add thermal mass (you can’t do this with water because you’re putting it in a freezer and antique crocks are somewhat valuable).

By repeatedly taking the microcontroller through a couple of freeze-thaw cycles, [Edward] was able to calibrate this watchdog timer to a resolution of about 0.0025°C, which is more than enough for just about any sensor application. Discussions of accuracy and precision notwithstanding, it’s pretty good.

This technique measures the temperature of the microcontroller with an accuracy of 0.005°C or better, and it’s using it with just the interrupt timer. That’s not to say this is the only way to measure the temperature of an ATMega; some of these chips have temperature sensors built right into them, and we’ve seen projects that use this before. However, this documented feature that’s clearly in the datasheet seems not to be used by many people.

Thanks [jan] for sending this in.

Gesture Sensing With A Temperature Sensor

January 4, 2019 by Inderpreet Singh 8 Comments

Good science fiction has sound scientific fact behind it and when Tony Stark first made his debut on the big screen with design tools that worked at the wave of a hand, makers and hackers were not far behind with DIY solutions. Over the years the ideas have become much more polished, as we can see with this Gesture Recognition with PIR sensors project.

The project uses the TPA81 8-pixel thermopile array which detects the change in heat levels from 8 adjacent points. An Arduino reads these temperature points over I2C and then a simple thresholding function is used to detect the movement of the fingers. These movements are then used to do a number of things including turn the volume up or down as shown in the image alongside.

The brilliant part is that the TPA81 8-Pixel sensor has been around for a number of years. It is a bit expensive though it has the ability to detect small thermal variations such as candle flames at up to 2 Meters. More recent parts such as the Panasonic AMG8834 that contain a grid of 8×8 such sensors are much more capable for your hacking/making pleasure, but come with an increased price tag.

This technique is not just limited to gestures, and can be used in Heat-Seeking Robots that can very well be trained to follow the cat around just to annoy it.

Two-Cent Temperature Sensors

April 16, 2018 by Elliot Williams 85 Comments

When they need to add temperature control to a project, many hackers reach for a K-type thermocouple for their high-temperature needs, or an integrated temperature-sensing IC when it doesn’t get that hot. The thermocouple relies on very small currents and extremely high gain, and you pretty much need a dedicated IC to read it, which can be expensive. The ICs aren’t as expensive, but they’re basically limited to boiling water. What do you do if you want to control a reflow oven?

There’s a cheaper way that spans a range between Antarctic winter and molten solder, and you’ve probably already got the parts on your shelf. Even if you don’t, it’s only going to run you an extra two cents, assuming that you’ve already got a microcontroller with an ADC in your project. The BOM: a plain-vanilla diode and a resistor.

I’ve been using diodes as temperature sensors in three projects over the last year: one is a coffee roaster that brings the beans up to 220 °C in hot air, another is a reflow hotplate that tops out around 210 °C, and the third is a toner-transfer iron that holds a very stable 130 °C. In all of these cases, I don’t really care about the actual numerical value of the temperature — all that matters is reproducibility — so I never bothered to calibrate anything. I thought I’d do it right for Hackaday, and try to push the humble diode to its limits for science.

What resulted was a PCB fire, test circuits desoldering themselves above 190 °C, temperature probes coming loose, and finally a broken ramekin and 200 °C peanut oil all over my desk. Fun times! On the other hand, I managed to get out enough data to calibrate some diodes, and the results are fantastic. The circuits under test included both best practices and the easiest thing that could possibly work, and the results are pretty close. This is definitely a technique that you want to have under your belt for most temperature ranges. The devil is in the details, of course, so read on!

Continue reading “Two-Cent Temperature Sensors” →

Pocket-Sized Multiduino Does It All

March 13, 2018 by Kristina Panos 16 Comments

How many times have you wished for a pocket-sized multimeter? How about a mini microcontroller-based testing rig? Have you ever dared to dream of a device that does both?

Multiduino turns an Arduino Nano into a Swiss Army knife of portable hacking. It can function as an analog multimeter to measure resistance, voltage drop, and continuity. It can also produce PWM signals, read from sensors, do basic calculator functions, and display the health of its rechargeable battery pack.

Stick a 10kΩ pot in the left-side header and you can play a space shooter game, or make line drawings by twisting the knob like an Etch-A-Sketch. Be sure to check out the detailed walk-through after the break, and a bonus video that shows off Multiduino’s newest functions including temperature sensing, a monophonic music player for sweet chiptunes, and a virtual keyboard for scrolling text on the OLED screen. [Danko] has a few of these for sale in his eBay store. They come assembled, and he ships worldwide. The code for every existing function is available on his site.

More of a maximalist? Then check out this Micro-ATX Arduino.

Continue reading “Pocket-Sized Multiduino Does It All” →