From The Readers: Low Battery Cutoff Solutions

December 1, 2011 by Mike Szczys 25 Comments

We got a lot of really great feedback about low battery cutoff options in the comments section of Monday’s replacement battery post. To refresh your memory, some power tool batteries were replaced by Lithium Polymer units which can be damaged if drained too low before recharging. We had thought that many Lithium cells had cutoff circuitry these days. The consensus is that these batteries didn’t because they’re for RC applications where weight is an issue. But we did get a ton of people sending in commercially available drop-in solutions, mostly from RC hobby outlets, so search around for those if you’re interested.

[Christopher] sent us a link to the cutoff circuit he built for his bike light. You can see the schematic for it above (direct link). He sourced an ATtiny45 to drive a MOSFET which disconnects the battery when it gets too low. This would be easy to adapt to other uses, but note that there’s a voltage regulator involved as well as a few other passives… not a difficult solution but also not all that simple.

This same concept can be adapted. A few commentors mentioned using a transistor (or MOSFET) with the base driven by a voltage divider including a zener diode. This way the voltage rating of the diode would effectively shut off the gate when that threshold was reached.

We also enjoyed reading about [Bill’s] human-controlled cutoff circuit. It also uses a zener diode, but this time in series with a resistor and and LED patched into the trigger of the tool. The LED will shine brightly when the battery is in good shape. It will dim near the end, and fail to light when the critical limit has been reached. Just make sure you’re paying attention and you’re in good shape.

Beer Dispenser Talks To Customers, Announces Office Parties Via Twitter

November 25, 2011 by Mike Nathan 15 Comments

arnie-intelligent-beer-dispenser

Just about the only thing better than beer is free beer.

Staff at the Arnold Worldwide ad agency are free to imbibe in the office’s lounge area, but a few employees thought that it would be pretty awesome to have their beer stash offered up by a vending machine. Using a grant that the company sets aside for “creative projects”, they built [Arnie], the interactive beer dispensing machine.

The machine was stocked with company-branded brews, and each employee carries an RFID key fob pre-loaded with beer credits. When the urge hits, staff members swipe their fob in front of the machine and select their preferred drink from the large, front-mounted touch screen. [Arnie] speaks with his customers and also uses Twitter to announce parties in the making, when a handful of bottles have been vended over a short period of time.

The project was a great use of money if you ask us, and we think that every office should have one of these babies in-house.

Continue reading to see a short video of how [Arnie] came to be.

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Making A Privacy Monitor From An Old LCD

November 25, 2011 by Mike Nathan 94 Comments

privacy-screen

[dimovi] had a spare LCD monitor sitting around and thought it would be great to convert it into a “privacy” monitor.

The process is simple enough for anyone comfortable with disassembling electronics. He took apart the monitor’s plastic frame, cutting out the polarized film with a utility knife. Once the film was removed, he spent some time removing the film adhesive from the glass panel using a combination of Oops cleaner and paint thinner.

He reassembled the monitor, which now shines a bright white regardless of what is actually being displayed on the screen. He removed the lenses from a pair of theater 3D glasses, replacing the plastic with the film he removed from the monitor.

Now, [dimovi] is the only one who can see what’s he is doing on his computer, which is just the way he likes it.

While there’s not a lot of magic going on behind the process, we think it’s a neat way to reuse an old monitor.

Experimenting With An Air Muscle And Sensor Feedback

November 21, 2011 by Mike Szczys 29 Comments

Check out this setup that [Ruenahcmohr] is using in his air muscle experiments. The orange mesh contains an air bladder that is connected to a hose on the right side. The bladder can be filled, or emptied with two solenoid valves not seen here (but you can get a good look in the video after the break). The muscle attached to chain on the other end and is kept under constant tension by a spring. The chain bends 180 degrees around a gear which is connected to a potentiometer. This gives feedback to the ATmega32 which controlling the whole thing. This way, the slider seen above can be used to control the apparatus.

We don’t know if [Ruenahcmohr] has a use in mind for this setup, but it certainly looks promising! We’ve seen these air muscles used for haptic feedback before, but right now we’re drawing a blank when it comes to ideas. What would you use it for?

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Getting Useful Data From A Dirt Cheap RFID Reader

November 19, 2011 by Mike Nathan 21 Comments

cheap-rfid-reader

[Aaron] was looking for a cheap RFID reader that had some easy to follow documentation and a standardized interface. Most everything he saw was pretty expensive, so he decided to buy a cheap $10 reader from eBay to see how easy it would be to work with.

The reader came with very little documentation, but [Aaron] did know that the device identifies itself as a USB keyboard, outputting scan tag data into a text editor. That functionality wasn’t incredibly useful, so he took it apart to see if he could interface with it in some other manner. Exposing the PCB revealed an unknown IC for which he could find no documentation, but the board did include some breakout pins, so [Aaron] started by probing those for data.

He tried reading the data in both a terminal program and with a logic analyzer, but nothing seemed to make a whole lot of sense. He turned the sampling rate of the sniffer down, and things started looking a little better. After comparing the data from the sniffer with known tag codes, he noticed that each digit had an offset of 39 applied, so he whipped up a bit of code to correct the numbers.

[Aaron] did a good amount of legwork to get usable data from the reader, but at a cost of $10 it can’t be beat. We certainly know what we’re going to be hunting for on eBay this afternoon…

Lighting LEDs With Raindrops

November 18, 2011 by Mike Nathan 40 Comments

[Steven] had one of those musical gift cards laying around, and thought he might as well reuse the piezo speaker inside it. Without a particular project in mind, he soldered an LED to the piezo and tapped on it, which caused the LED to illuminate as expected. He started to wonder what quantity of force would be required to light the LED, and if it could be done by a raindrop.

He first tested his theory in the shower, and as you can see in the video below it actually worked, though the light was dim and sporadic as you might imagine. He eventually discovered that for optimal lighting, the piezo worked best when struck by single droplets falling with pauses in between, from a minimum height of 4 feet. To achieve a water flow within those specifications, he built a rain funnel so that he can control the droplet frequency and intensity.

It seems to work pretty well from what we can see. Off the top of our heads we can’t seem to come up with any practical applications of the water powered LED, but it is an interesting set of experiments nonetheless.

Have an idea to use this setup that we totally missed? Let us know in the comments!

[Thanks, Rob]

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A Head-mounted Tesla Coil – What Could Possibly Go Wrong?

November 18, 2011 by Mike Nathan 26 Comments

We can hear the commercial now…

“Reeeeeeal men of geniuuuuuuus! Here’s to you Mr. no-fear, singing Tesla coil on your hat wearing guy…”

Call him a genius or call him crazy – all we know is that [Tyler’s] Tesla coil hat is awesome! Even though it’s the middle of November, we couldn’t pass up this Halloween costume.

[Tyler’s] creation essentially boiled down to taking a standard dual resonant solid state Tesla coil and shrinking it down to a reasonable size for mounting on his skull. The mini Tesla features a pretty compact boost core which worked reasonably well, at least for a little while. He says that the boost never truly worked properly and needs a redesign, which is something he’ll tackle when he gets some free time.

Boost issues or not, we think that the video below speaks for itself. The hat is certainly an attention getter, and we think it’s great – even if wearing a Tesla coil on your head is not necessarily the safest thing to do.

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