Fail Of The Week : Measuring DC Current Has To Be Easy, Right?

[DainBramage] needed a DC ammeter to check how long his amateur radio station would be able to stay powered on battery backup power. The one’s he already had on hand were a Clamp Meter, which could only measure AC, and another one that measured just a few milliamps. Since he didn’t have one which could measure up to 25A, he decided to build his own DIY DC Ammeter with parts scavenged from his parts bin. Measuring DC current is not too difficult. Pass the current to be measured through a precision resistor, and measure the voltage drop across it using a sensitive voltmeter.

I = V/R

So far, so good. If it’s late at night and you’ve had a lot of coffee, busy building your DC ammeter, things could head south soon. [DainBramage]’s first step was to build a suitable Shunt. He had a lot of old, 1Ω, 10W resistors lying around. He made a series-parallel combination using nine of them to create a hefty 1Ω, 90W shunt (well, 0.999999999 Ohms if you want to be picky). This gave him a nice 1 Volt per Amp ratio, making it easy to do his measurements.

Next step was to hook up the shunt to a suitable voltmeter. Luckily, he had a Micronta voltmeter lying around, ripped out from a Radio Shack product. Since he didn’t have the voltmeter data, he hooked up a 10k resistor across the meter inputs, and slowly increased the voltage applied to the meter. At 260mV, the needle touched full-scale and the voltage across the inputs of the voltmeter was 33mV. [DainBramage] then describes the math he used to calculate the resistors he would need to have a 10A and a 25A measurement range. He misses his chance to catch the fail. His project log then describes some of the boring details of putting all this together inside a case and wrapping it all up.

A while later, his updates crop up. First thing he probably realized was that he needed more accurate readings, so he added connectors to allow attaching a more accurate voltmeter instead of the analog Micronta. At this point, he still didn’t catch the fail although it’s staring him straight in the face.

His head scratching moment comes when he tries to connect his home made ammeter in series with the 12V DC power supply to his amateur radio station. Every time he tries to transmit (which is when the Radio is drawing some current), the Radio shuts off.  If you still haven’t spotted the fail, try figuring out how much voltage gets dropped across the 1Ω shunt resistor when the current is 1A and when it is 5A or more.

Fail of the Week: Hair Dryer as Light Switch

Home electrical, it’s really not that hard. But when you’re dealing with the puzzles left for your by someone else things can get really weird. [Daniel’s] sister and her husband ran into this recently. The video demonstration of their fail includes a lot of premature laughter, but it’s worth hanging in there… you’ll quickly see why she can’t contain her amusement.

The project at hand is a replacing a bathroom fan with a simple light fixture. Once the swap was made the light switch works just as anticipated. But a second switch which used to control a different light now behaves strangely. It doesn’t activate the original light, but instead switches the new fixture. Even stranger is that the original switch apparently now acts as a bizarre dimmer when the second switch is on. That’s odd, but the coup d’état of the fail is when they plug in a hair dryer and switching it on illuminates the light but doesn’t activate the hair dryer.

As with all Fail of the Week segments, the goal here is not to criticize but to commiserate. What do you think is causing this? We can’t wait to see what you come up with. Posting simple diagrams is encouraged (you can use HTML img tags in the comments). Ladies and gentlemen, start your conjecture.

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Fail of the Week: WinCE is a Noun and a Verb

A few years ago, [localroger] found some incredible hardware on sale: a very tiny laptop with a seven-inch screen, full keyboard, trackpad, Ethernet, WiFi, USB (with support for a lot of HID devices), and a battery that would last hours. They were on sale for $30 USD, and [localroger] bought four of them. A great deal, you say? These machines ran Windows CE. No, owning a WinCE device is not the Fail of the Week.

Figuring he should do something with these machines, [roger] thought, ‘a clock will do’, and began to figure out how to program or write an app for these things. These tiny netbooks did come with a programming language, JavaScript, in the form of the built-in IE6 web browser. This was actually a really, really good solution – WinCE apps formatted for portrait displays just didn’t work with the ‘widescreen’ laptop, and a hand-coded HTML table is probably the best solution anyone could have hoped for.

These machines – [roger] used three of them over the years as alarm clocks – did their job well, even if NTP had been left out of the OS image. The real fail here comes from buying a $30 WinCE netbook, and using it for something as mission critical as an alarm clock. The displays burned in, the batteries began puffing up, one unit somehow wouldn’t allow IE to run (probably a bad Flash chip), and the trackpad in another one sent the cursor on a random walk. You get what you pay for.

These WinCE netbooks have finally been put out to pasture, hopefully the same one laser printers go to. It’s all for the best, though; [roger] made a much better alarm clock with Nixies.

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every now and again. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

Fail of the Week: Cat6 != Coax

With a new Kenwood 5.1 receiver acquired from questionable sources, [PodeCoet] had no way to buy the necessary coax. He did have leftover Cat6 though. He knew that digital requires shielded cable, but figured hacking a solution was worth a try.

HAD - Coaxfail4To give hacking credit where credit is due, [PodeCoet] spent over a decade enjoying home theatre courtesy of a car amp rigged to his bench supply. Not all that ghetto of a choice for an EE student, it at least worked. To hook up its replacement he pondered if Cat6 would suffice, “Something-something twisted pair, single-sideband standing wave black magic.” Clearly hovering at that most dangerous level of knowledge where one knows just enough to get further into trouble, he selected the “twistiest” (orange) pair of wires in the cables. Reasonable logic, one must select the strongest of available shoelaces for towing a car.

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Fail of the Week: Battery Packin’

[NeXT] got himself an IBM ThinkPad TransNote and yeah, we’re pretty jealous. For the uninitiated, the TransNote was IBM’s foray into intelligent note transcription from roughly fifteen years ago. The ThinkPad doesn’t even have to be on to capture your notes because the proprietary pen has 2MB of flash memory. It won an award and everything. Not the pen, the TransNote.

Unfortunately, the battery life is poor in [NeXT]’s machine. The TransNote was (perhaps) ahead of its time. Since it didn’t last on the market very long, there isn’t a Chinese market for replacement batteries. [NeXT] decided to rebuild the replacement battery pack himself after sending it off with no luck.

The TransNote’s battery pack uses some weird, flat Samsung 103450 cells that are both expensive and rare. [NeXT] eventually found some camera batteries that have a single cell and a charge controller. He had to rearrange the wiring because the tabs were on the same side, but ultimately, they did work. He got the cells together in the right configuration, took steps to prevent shorts, and added the TransNote’s charge controller back into the circuit.

Nothing blew up, and the ThinkPad went through POST just fine. He plugged it in to charge and waited a total of 90 minutes. The charging rate was pretty lousy, though. At 94% charge, the estimated life showed 28 minutes, which is worse than before. What are your thoughts on the outcome and if it were you, what would be the next move?

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every Wednesday. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

Fail of the Week: Robotic 1950 Mercury Boogies, Won’t Come Back From Dead Man’s Curve

[Dave] wanted to make an Arduino robot out of a remote-control 1950 Mercury. He removed the RC portion from the car and kept the drive and steering motors. The idea was to use three ultrasonic rangefinders in the grille real estate and move the car forward based on the longest distance detected.

He initially used a Seeed motor controller and some Grove cables soldered to his sensors to power the steering. It went forward, but only forward, and [Dave] decided the motor controller and the car’s steering motor weren’t playing well together.

[Dave] had the idea to use relays instead to both power the motor and determine polarity. Now, the Merc was turning and avoid obstacles about half the time, but it was also getting dinged up from hitting walls. He figured out that his sensor arrangement was making the car turn immediately and decided to give the program information from the wheels with a reed switch and a rare earth magnet. The only problem is that the caliber of magnet required to trip the reed switch is too heavy and strong. [Dave] and has concluded that he simply can’t exercise the kind of control over the car that he needs. and will build his own robot chassis.

Update: Check out a video of [Dave]’s car after the break.

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every Wednesday. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

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Fail of the Week: This Inanimate Titanium Rod

You saw [Chris] cast aluminium on the cheap using Kinetic Sand a few weeks ago, didn’t you? He recently got his meaty hands on some titanium through the magic of modern transactional methods and was bowled over by its strength, hardness, and poor heat transfer.

He thought he would cast it into a nice, strong bottle opener. As you can probably guess, that didn’t go so well. First off, it wasn’t easy to saw through the thin rod. Once he did get it split in twain, it was surprisingly cool to the touch except at the tip. This is nasty foreshadowing, no?

[Chris] takes a moment to help us absorb the gravity of what he’s about to do, which of course is to send several hundred amps through that poor rod using a DC arc welder. Special precautions are necessary due to the reaction between oxygen and heated titanium. His trusty graphite crucible is grounded to the bottom of a big aluminium tub, and a cozy blanket of argon from a TIG welder will shield the titanium from burnination.

Well . . . the titanium didn’t melt. Furthermore, the crucible is toast. On the up side, vise-enabled cross-sectional examination of the crucible proved that there was still gold in them there walls.

Do you have any (constructive, on-topic) suggestions for [Chris]? Let him know below.

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