Multivibrator In Theory And Practice

December 1, 2011 by Mike Szczys 60 Comments

We must admit that we’ve been guilty of using a microcontroller to make two LEDs blink alternately in the past. It’s not the worst transgression, but it stems from our discomfort with analog circuits. Luckily, [Ray] published an illustrated guide on building multivibrator circuits. This is a simple method of assembling a two-output oscillator. All it takes is a pair of NPN transistors, which are then switched by on and off based on a resistor-capacitor (RC) timer.

[Ray] does a good job of walking us through how the circuit works at each stage of one complete cycle. You’ll need to read carefully, but the supplementary schematics he uses to water down snap shots of the various electrical states really helped us understand.

Of course, blinking LEDs isn’t the sole purpose of a multivibrator. It is a method of producing a clean square wave which can be used as a clock signal for TTL logic chips. Oh, who are we kidding, see the blinky goodness for yourself in the video after the break.

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Make Your Own Spray Paint Cans

December 1, 2011 by Mike Szczys 48 Comments

[Mikeasaurus] found a way to build his own refillable spraypaint canister. The donor vessel used here is a plastic soda bottle. It’s a great choice since it is engineered to house a pressurized liquid and you can find them for free by intercepting a satisfied soda consumer before they reach the recycling bin.

He repurposed the spray nozzle from a commercial spray paint can. By first releasing all of the pressure from the empty paint he could then use a hack saw to remove the top disk. He used Sugru to attach it to the bottle cap which has a hole drilled in the center to accept the feed straw. We wonder if there wouldn’t be a better way to attach this from the inside of the cap for better resistance to bottle pressure?

The final piece of hardware is a Shrader valve from a bicycle inner tube. This lets you pump up the pressure in the bottle. You’ll need to dilute the paint you use to make it sprayer-friendly. [Mikeasaurus] diluted his six to one which might have been a bit too much judging from the drips seen in the video after the break.

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Making Sweet Floppy Drive Music With A Calculator

December 1, 2011 by Mike Nathan 14 Comments

floppy-music-ti83-style

[Chris] says that he’s been pretty busy lately, leaving little opportunity for hacking. However he did manage to find a little time to put together a small project that has occupied his to-do list for a while – a floppy drive music controller.

We have seen hacks that use microcontrollers to actuate floppy drive motors before, but we can’t remember anything that used a calculator to do the job instead. While a microcontroller gives you plenty of I/O pins to play with, [Chris’] Ti-83+ only has two.

Even with the calculator’s I/O limitations, he didn’t find the task too overly difficult as he merely needed to hold a pair of the drive’s pins low, while pulsing two others. He modified a media player written for Ti calculators to output the necessary control signals, then he cranked out some tunes.

As you can see in the video below, his simple setup works quite well – not bad for just a few hours’ work.

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From The Readers: Low Battery Cutoff Solutions

December 1, 2011 by Mike Szczys 26 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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