POV Display with an Element of Danger

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Persistence of vision displays are always cool, although we must admit this one looks like it could very well explode at high speeds…

Safety concerns aside, this desk fan based display provides a great starting point for learning about making POV displays. It makes use of an old cellphone battery, an ATmega8, some LEDs, Veroboard, assorted wires and solder and of course, a high-speed desk fan.

[shparvez001] also provides the full code on his blog for the project, making it very easy to replicate. Though we might also suggest you keep it small enough that the original fan cage still fits on top.

From an aesthetic point of view, the display looks fine in the dark — but when the lights are on you might get some odd looks. We can see this project being greatly improved by mounting the LEDs through one of the fan blades, and the control electronics on the back side of the other blades. Maybe throw in some wireless charging for the battery while the fan is off too?

Anyway, stick around after the break to see the display in action. If you want a more permanent fan POV try adding display hardware to a ceiling unit.

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Hacking Digital Scales for the Disabled

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[Jan] works with both physically and mentally disabled individuals, some of whom cannot read, making many of their tasks more difficult. Although [Jan] is not in a position to teach reading or writing skills, he was able to build an add-on device for the scales used in repackaging sweets to provide simple feedback that the user can interpret.

The device has three LEDs—red, green, and yellow—to indicate the package does not weigh enough (red), weighs too much (yellow), or lies within an acceptable range (green). The industrial scales at [Jan's] workplace each have a serial output to connect to a printer, which he used to send data to the device. An ATMega8 controls the lights and an attached LCD, with the usual trimpot to change the display’s contrast and a rotary encoder to adjust the device’s settings. Everything fits snugly into a custom-made frosted acrylic enclosure, laser-cut at a local hackerspace.

[Jan] provides a rigorous guide to approaching each step on his Instructables page, along with source code and several pictures. See a video overview below, then enjoy another scale hack: building one from scratch.

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AVR Atmega based PID Magnetic Levitator

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[Davide] saw our recent post on magnetic levitation and quickly sent in his own project, which has a great explanation of how it works — he’s also included the code to try yourself!

His setup uses an Atmega8 micro-controller which controls a small 12V 50N coil using pulse-width-modulation (PWM). A hall effect sensor (Allegro A1302) mounted inside the coil detects the distance to the magnet and that data is used by a PID controller to automatically adjust the PWM of the coil to keep the magnet in place. The Atmega8 runs at 8Mhz and the hall effect sensor is polled every 1ms to provide an updated value for the PWM. He’s also thrown in an RGB LED that lights up when an object is being levitated!

So why is there a kid with a floating balloon? [Davide] actually built the setup for his friend [Paolo] to display at an art fair called InverART 2013!

After the break check out the circuit diagram and a short demonstration video of the device in action!

Oh yeah, those of you not impressed by magnetic levitation will probably appreciate acoustic levitation.

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Build a Sensorless Brushless DC Motor Controller

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[Davide Gironi] shows us how to implement a sensorless brushless DC motor controller (sensorless BLDC) using an ATmega8 microcontroller. In order to control a BLDC motor you need to know its rotational sequence position and speed so you can calculate and apply the correct current phase sequence to the motor windings at just the right time.

Simply said, sensorless BLDC means you’re not using a purpose built sensor to determine the motor’s position and speed, however, you are sensing the motor’s sequence position using the back EMF signal coming from one of motor’s coils that is not currently receiving power. When this back EMF signal crosses zero voltage a microcontroller can calculate the rotational speed and when to switch to the next power sequence. This technique is not good for position control motors but is great for continuous motors like computer fans and drives were the slightly reduced wiring costs make this type of BLDC control favored.

If you want to build a BLDC controller we recommend starting with [Davide’s] last project on sensor controlled BLDC motors. You can also checkout these interactive demonstrations for more understanding on the different BLDC configurations.

Follow along after the break to watch the video demonstration of [Davide’s] sensorless BLDC controller controlling a motor from CD-ROM drive.

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Building a brushless motor controller around an ATmega chip

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You know when you see something like this it’s just going to be awesome, and we weren’t disappointed by our first impression. [Davide Gironi] built a brushless motor controller from the ground up using an ATmega8 as the brain. If you want to understand every aspect of a subject this is how to do it. Lucky for us he explains what each portion of the prototype does.

Brushless motors have no brushes in them (duh). But what does that really mean? In order to spin the motor a very carefully crafted signal is sent through the motor coils in the stationary portion (called the stator), producing a magnetic field that pushes against permanent magnets in the rotor. A big part of crafting that signal is knowing the position of the rotor. This is often accomplished with Hall Effect sensors, but can also be performed without them by measuring the back EMF in the coils not currently being driven. The AVR-GCC compatible library which [Davide] put together can be tweaked to work with either setup.

Get a good look at the system in action after the break.

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Drinking games and digital logic

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For those of you who might have forgotten, let’s go over the rules of Centurion. The object of the game is for every minute, for 100 minutes, drink a shot of beer. It doesn’t sound like a lot, but after completing the challenge you’ll have had 3 liters of beer (or about eight and a half 12 oz cans) in just under two hours. When [Peter] played Centurion, he found the biggest problem was – understandably – keeping track of the time and who drank what. For an upcoming weekend of drinking, [Peter] decided to solve this problem once and for all with shift registers and seven-segment displays.

[Peter]‘s Centurion score box comes in two parts. The first and largest part of the build is the main board housing an ATMega8 microcontroller and a huge two digit seven-segment display to keep track of the countdown until the next shot. Two other boards house eight additional two digit seven-segment displays for each player, incremented every time a player presses a giant arcade button.

The entire build is designed around a small travel case that also holds a large battery for cordless drinking parties. Let’s just hope the project is reasonably water-resistant; we can see a lot of spills happening in the future. Check out the video demo below.

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Building a replacement for a broken dehumidifier controller

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We’ve thought of doing a project like this ourselves as the dehumidifier we ordered online runs the fan 24/7 no matter what the humidity conditions. But it wasn’t that [Davide Gironi] was unhappy with the features on his unit. It’s that the dehumidifier controller stopped working so he replaced it with one of his own design. The original humidity sensor was mechanical and simply broke. He used an AVR along with a humidity and frost sensor to get the appliance up and running again.

A DHT22 humidity sensor is polled by the ATmega8 chip and compared to the user-adjustable trimpot value. If it is above that threshold the unit is switched on using one of the relays seen in the image above. The one problem you have to watch out for when using compressor cooled appliances is ice accumulation on the radiator. [Davide] uses a thermistor for temperature feedback, switching the compressor off when it gets below 7C and turning it back on again when it is above 12C.

The replacement still uses the reservoir sensor and indicator LEDs. We, however, would recommend using the watchdog timer on the chip to ensure that it is reset if something goes wrong in the code.