A Ridiculous Way to Light an LED: Candle-power

If you have ever entertained yourself by reading comprehensive electronic-theory textbooks you’ll have seen references to technologies that sound really interesting but which you will rarely hold in your hand. They may be dead-ends that have been superseded by more recent innovations, or they may be technologies that have found uses but in other fields from those in which they originally showed promise. What if you could take these crazy parts and actually build something?

[Fedetft] has an interesting project that combines two of those intriguing textbook references, he’s created a thermopile that lights an LED through an inverter whose oscillator is a tunnel diode. Dig out the textbook.

If you’ve used a thermocouple thermometer or a semiconductor thermoelectric generator then you’ll have encountered the thermoelectric effect. Perhaps you’ve even operated a Peltier cooling element in this mode. When a circuit is made with two junctions between different types of conductor with a temperature difference between the two junctions, a current will flow in the circuit which is dependent on both the scale of the temperature difference and the properties of the conductors.

A thermopile is a collection of these thermoelectric junction circuits between metal conductors, arranged in series to increase the voltage. [Fedetft]’s thermopile uses chromel and alumel wires taken from a K-type thermocouple. He’s made six sets of junctions, and supported them with small pieces of mica sheet. Using the heat from a candle he found he could generate about 200mV with it, at about 3.7mW.

The RCA tunnel diode inverter circuit
The RCA tunnel diode inverter circuit

Such a tiny source of electricity would be of little use to light an LED directly, so he needed to build an inverter. And that’s where the tunnel diode comes in. Tunnel diodes have a negative-resistance region that can be used to amplify and oscillate at extremely high frequencies in extremely simple circuits, yet they’re not exactly a device you’d encounter very often in 2016. [Fedetft] has a Russian tunnel diode, and he’s used it with a toroidal transformer in an inverter circuit he found in an RCA tunnel diode manual from 1963. It’s a two-component Joule Thief. The RCA manual is a good read in itself for those curious about tunnel diodes.

The resulting circuit produces a 15kHz oscillation with 4.5v peaks, and has just enough power to light an LED.

While it might seem pointless to barely light an LED from a brightly lit candle, the important part of [Fedetft]’s project is to gain some understanding of two of those technological backwaters from the textbooks. And we applaud that.

It’s the mark of a truly esoteric technology that it features rarely on Hackaday, and neither of these two disappoint. We’ve only mentioned tunnel diode in passing when looking at diodes in general, and we’ve tended to use “thermopile” in another sense to refer to thermal imaging cameras.

A Light Painting Infrared Ray Gun


[Noe] over at Adafruit has a really great build that combines the Internet’s love of blinkey LEDs and rayguns with the awesome technology behind extraordinarily expensive thermal imaging cameras. It’s a light painting infrared heat gun, used for taking long exposure photographs and ‘painting’ a scene red or blue, depending on the temperature of an object.

While this isn’t a proper FLIR camera, with a DSLR and a wide open shutter, it is possible to take pseudo-thermal images by simply ‘painting’ a scene with the light gun. This is an absurdly clever technique we’ve seen before and has the potential to be a useful tool if you’re looking for leaks around your windows, or just want to have a useful cosplay prop.

The circuit inside this raygun is based on a contactless infrared sensor connected to an Adafruit Gemma, with the LEDs provided by a NeoPixel ring. There are two 3D printable cases – your traditional raygun/blaster, and a more pragmatic wand enclosure. With either enclosure, it’s possible to take some pretty heat map pictures, as seen in the video below.

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DIY Thermal Imaging Camera

Thermal imaging cameras – those really useful devices that give you Predator vision – are incredible tools. If you’re looking for heat escaping your house through a window, or just trying to figure out where your electronics project will explode next, they’re invaluable, if expensive, tools. [Kaptein QK] figured out an easy and cheap way to make your own thermal imaging camera using nothing just a few dollars worth of parts.

[Kaptein] based his camera off of a non-contact IR temperature gun. This device is useful for spot checking temperatures, but can’t produce an IR image like it’s $1000 cousins. By taking the thermopile out of this temperature gun, adding an op-amp, an A/D converter, and connecting it to an Arduino Nano with pan and tilt servos, [Kaptein] was able to slowly scan the thermopile over a scene and generate an image.

In the video below, you can see [Kaptein]’s scanning camera in action reading the ambient temperature and creating an imaging program for his PC. It works very well, and there a lot more [Kaptein] can improve on this system; getting rid of the servos and moving to mirrors would hopefully speed everything up, and replacing the 8-bit grayscale display with colors would give a vastly improved dynamic range.

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A real thermal imaging camera for $300

If you want to check your house for hot air leaks, take pictures of the heat coming off a rack of equipment, or just chase the most dangerous animal, [Arnie], through the jungles of central america, a thermal imaging camera is your friend. These devices normally cost a few thousand dollars, but the team behind the Mu Thermal Camera managed to get the price down to about $300.

The basic idea behind the Mu Thermal Camera is overlaying the output of an infrared thermopile – basically, an infrared camera – on top of the video feed of a smart phone’s camera. This is an approach we’ve seen before and something that has even been turned into a successful Kickstarter. These previous incarnations suffered from terrible resolution, though; just 16×4 pixels for the infrared camera. The Mu thermal camera, on the other hand, has 160×120 pixels of resolution. That’s the same resolution as this $2500 Fluke IR camera. After the indiegogo campaign is over, the Mu camera will eventually sell for $325.

We have no idea how the folks behind the Mu camera were able to create a thermal imaging with such exceptional resolution at this price point. The good news is the team will be open sourcing the Mu camera after their indiegogo run is over. W’e’d love to see those docs now, if only to figure out how a thousand dollars of infrared sensor is crammed into a $300 device.

Building a thermal imaging sensor from scratch

[Rob] lives in a 100-year-old house, and with these antique lath and plaster walls and old window frames comes a terrible amount of drafts. The usual way to combat this energy inefficiency is with a thermal imaging camera, a device that overlays the temperature of an object with a video image. These cameras are hideously expensive so [Rob] did what any of us would do and built his own.

The build centers around a Melexis MLX90620 far infrared thermopile that can be had for about $80. Basically, this sensor is a very, very low resolution camera (16×4 pixels) that senses heat instead of light. By sticking this sensor on a breadboard with an Arduino Mini and WiFly network adapter, [Rob] is able to pull the data down from the IR sensor to his iPhone and overlay it on the feed from the camera.

The result, as seen in the video above, is a low-resolution but still very useful thermal imaging camera, perfect for looking for cold drafts in an old house or tracking down [Arnie] just like a Predator.

Tip ‘o the hat to [Ronald] for sending this one in.

Heat-seeking firebot drowns out the flames

This robot can find and extinguish fires automatically. It is the culmination of an Embedded Design class project from last school year. [Dan] and his classmates developed a turret that holds both a spray nozzle and heat sensor which would be a fantastic building block for a real-life tower defense game.

The jewel of the sensor array is a TPA81 thermopile array. Note the use of the term ‘array’ in the name. This is more like eight temperature sensors aligned with each other. By monitoring them all, the direction from which the most heat is coming can be determined. Once it’s zeroed in on the fire getting water to the right place can be a difficult task. That’s where the other sensors come into play. An accelerometer allows the bot to determine the angle of the spray nozzle (a weed sprayer was used in this case). An ultrasonic range finder and few algorithms let the Arduino which drives it all make sure that the arc of the water lands on the hot spot. This is all shown quite clearly in the clip below the jump.

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