Automatic Resistance: Resistors Controlled by the Environment

Resistors are one of the fundamental components used in electronic circuits. They do one thing: resist the flow of electrical current. There is more than one way to skin a cat, and there is more than one way for a resistor to work. In previous articles I talked about fixed value resistors as well as variable resistors.

There is one other major group of variable resistors which I didn’t get into: resistors which change value without human intervention. These change by environmental means: temperature, voltage, light, magnetic fields and physical strain. They’re commonly used for automation and without them our lives would be very different.

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Taming A Variac With A Thermistor

The folks at TOG, Dublin Hackerspace, have a large variac. A variac is a useful device for testing some fault conditions with AC mains powered equipment, it allows an operator to dial in any AC output voltage between zero, and in the case of TOG’s variac, 250V.

Their problem was with such a magnificent device capable of handling nearly 3KW, it presented an inductive load with a huge inrush current at power-on that would always take out the circuit breakers. Breakers come with different surge current handling capabilities, evidently their building is fitted with the domestic rather than the industrial variants.

Their solution was a simple one, they fitted an NTC surge limiter in series with the variac input. This is a thermistor whose resistance falls with temperature. Thus on start-up it presented an extra 12 ohm load which was enough to keep the breaker happy, but soon dropped to a resistance which left the variac with enough juice.

This is a simple fix to a problem that has faced more than one hackerspace whose imperfect lodgings are wired to domestic-grade spec. In a way it ties in neatly with our recent feature on mains safety; making the transformer no longer a pain to use means that it is more likely to be used when it is needed.

Via: TOG, Dublin Hackerspace.

Nixie Thermometer Destined for Custom PC Case

There’s no denying the retro appeal of the warm glow of a set of Nixies, and when a friend was looking for a unique touch for the case of his new liquid-cooled PC, [Luca] pitched in with this sweet Nixie thermometer.

From the look of [Luca]’s detailed blog entries, he’s been at this build since the New Year. He starts with a list of requirements, including the oddly specific need for a round PC board. For the thermometer, three Nixies are enlisted for the display, two for the temperature and one for the units. Everything was prototyped on perf board before committing to a PCB design, but even with careful planning, the Nixie sockets on the final PCB came out a tiny bit too close together. Luckily the tubes still fit, even if they are snuggled together some. And yes, the tube bases all include the hated RGB LEDs – hey, it’s what the customer wanted. The specs are for the colors to change at the touch of a button; we’d like to see a color gradient linked to the temperature – blue for “nice and cool”, red for “leave the room.” You can see the finished thermometer in action below the break.

The recent run of Nixie projects continues unabated, and this one has a nice look that’s sure to complement the finished case. We’ve asked [Luca] to keep us up-to-date on the project, so hopefully we’ll get a look at why a round PCB is needed. While we wait for that, check out an earlier Nixie thermometer build with a bar graph twist.

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A Pair of Toaster Reflow Oven Builds


For some reason or another, the Hackaday tip line sometimes sees a short burst of submissions for the same project. The latest one of these was for toaster oven reflow stations. They’re both great builds and different approaches to making a useful tool out of home appliances.

First up is [Richard]’s build. he ended up with a fairly high-end build using a Rocket Scream Reflow Oven Controller Arduino shield. This shield accepts a normal K-type thermocouple and controls an external solid state relay with the Arduino’s PID library. [Richard]’s build has a few neat additions – a properly dremeled enclosure, computer fan, and a welding blanket for insulation. Now that we think about it, it’s odd we’ve rarely seen any sort of insulation in these reflow oven builds.

Next up is [Ray]’s version of a Black & Decker reflow oven. While not as fancy as [Richard]’s build, this one does have a few features that make it very interesting. Instead of messing around with thermocouples, [Ray] simply took a digital kitchen thermometer – a neat tool that already a thermistor in a compact metal probe – and read the analog value with an Arduino. To control the power, [Ray] is using a cheap 433 MHz radio transmitter to control a few remotely operated power sockets. It’s a very clever and inexpensive replacement for a SSR, especially since [Ray] had these power sockets just lying around.

So there you go. The same tool, built two different ways. A great demonstration of how you can not only build anything, but you can build anything any way you want.

Tracking cicadas with Radiolab and an Arduino


Once every 17 years, a population of cicadas ranging from Connecticut to the Appalachian highlands of North Carolina emerges to annoy everyone within earshot. The last time east coasters saw this brood was in 1996, making 2013 yet another year of annoying insect pests. The only question is, when will we start to see this year’s cicada brood?

Radiolab, the awesome podcast and public radio show, has put together an awesome project that asks listeners to track when the cicadas in their area will emerge. Cicadas generally enter their loud and obnoxious adult stage when the ground temperature 8 inches below the surface reaches 64º F. Armed with an Arduino, thermistor, and a few wires and resistors, any Radiolab listener can upload soil temperature data to Radiolab servers where all the data will be correlated with documented cicada sightings.

After following the page’s instructions for wiring up a bunch of LEDs and a thermistor to an Arduino, just upload the most well-commented code we’ve ever seen and go outside to take soil temperature measurements. The temperature is displayed in a pseudo-binary format on nine LEDs. To decode the temperature without counting by powers of two, Radiolab has an online decoder that also allows you to upload your data and location.

That’s not beer! A biofuel fermentation controller project


Any home brewer will recognize the setup pictured above as a temperature controlled fermentation chamber. They wouldn’t be wrong either. But you’re not going to drink what results. This project is aimed at providing a temperature controlled environment for fermenting biofuel.

[Benjamin Havey] and [Michael Abed] built the controller as their final project in his microprocessor class. The idea is to monitor and control the mini-refrigerator so that the strain of Saccharomyces Cerevisiae yeast produce as much ethanol as possible. An MSP430 microcontroller was used. It monitors a thermister with its analog to digital converter and drives a solid state relay to switch mains power to the fridge. At 41 degrees Fahrenheit this is down below what most lager yeasts want (which is usually in the low fifties). But the nice thing about using a microcontroller is you can set a schedule with different stages if you find a program that gives the yeast the best environment but requires more than one temperature level.

Who knew all that beer making was getting you ready to produce alternative fuels?

Toaster oven reflow control without modifying the oven

[Eberhard] wanted his own reflow oven but didn’t really want to mess around with the internals that control the heating element. He put his microcontroller programming experience to work and came up with an add-on module that controls the oven by switching the mains power.

The image above shows a board in the midst of the reflow process. If you’re not familiar, solder paste usually comes with a recommended heat curve for properly melting the slurry. [Eberhard] managed to fit three of these temperature profiles into his firmware.

The ATtiny45 which makes up the controller samples oven temperature via the thermistor seen next to the board. A PID algorithm is used to calculate when to switch mains power on and off via a relay. One button and one LED make up the controller’s user interface for scrolling through the three preprogrammed temperature profiles.

It looks like it works great, see for yourself in the clip after the break.

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