Triple Sensor Mailbox Alert Really Delivers

Messing with the U.S. Mail is not something we generally recommend. But if you build your own mailbox like [Bob] did, you stand a much better chance of doing what you want without throwing up any flags.

Speaking of throwing up flags, one of the coolest parts of this project is the toy mailbox inside the house that monitors the activity of the real box. When there is mail waiting, the flag on the toy mailbox goes up. Once [Bob] retrieves the mail, the flag goes back down automatically. A magnet in the real box’s flag prevents false alarms on the toy box provided the Flag Raised On Outgoing protocol is followed. Best of all, he built in some distress handling: If the mailbox door is left hanging open or the battery is low, the toy mailbox waves its flag up and down.

So, where do the three sensors come in? A magnetic reed switch on the wall of the real mailbox pairs with a magnet in the flag. To determine whether the door is open, [Bob] initially used another magnetic reed switch on the underside of the box. This didn’t work well in wet weather, so he switched to a mechanical tilt sensor. An IR LED on the ceiling and a phototransistor on the floor of the box work together to detect the presence of mail.

[Bob]‘s homebrew mailbox has a false back that hides a PIC 16F1825. When the door opens, the PIC wakes up, turns on a MOSFET, and checks the battery level. It waits two minutes for the mailman to do his job and then reads the flag state. After comparing the IR LED and phototransistor’s states, it sends a message to the toy mailbox indicating the presence or absence of mail.

The toy mailbox holds a modified receiver board and a servo to control its flag. [Bob] has made the code and schematics available on his site. Walk-through video is after the jump.

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SDR: Satellite Death Receiver

Halloween may be over, but [happysat] has found a way to listen to the dead. Satellites, that is, specifically those in the 136-138 MHz and 150-400 MHz ranges. He’s using an RTL-SDR dongle and a QFH antenna to detect the death throes of decommissioned navigation and space research satellites.

[happysat] was listening to NOAA/Meteor on the 137MHz band when he made this discovery. When a satellite is near end of life, the last bit of fuel is used to push it into graveyard orbit. This doesn’t always work, however, and when the light is just right, a chemical reaction makes the long-dead batteries conduct and these satellites in purgatory transmit once more.

They’re not sending out anything proprietary useful, just unmodulated carrier that sometimes interferes with currently operational satellites on the 136-138 MHz band. [happysat] captured some audio from two of the oldest satellites that are still broadcasting, and links to a TLE set of dead satellites he created. Check out his frequency database for SDR# as well. Don’t have a weather satellite-capable antenna? Build one!

[via /r/RTLSDR]

Retrotechtacular: The Jet Story

A plane from Britain is met in the US by armed security. The cargo? An experimental engine created by Air Commodore [Frank Whittle], RAF engineer air officer. This engine will be further developed by General Electric under contract to the US government. This is not a Hollywood thriller; it is the story of the jet engine.

The idea of jet power started to get off the ground at the turn of the century. Cornell scholar [Sanford Moss]‘ gas turbine thesis led him to work for GE and ultimately for the Army. Soon, aircraft were capable of dropping 2,000 lb. bombs from 15,000 feet to cries of ‘you sank my battleship!’, thus passing [Billy Mitchell]‘s famous test.

The World War II-era US Air Force was extremely interested in turbo engines. Beginning in 1941, about 1,000 men were working on a project that only 1/10 were wise to. During this time, American contributions tweaked [Whittle]‘s design, improving among other things the impellers and rotor balancing. This was the dawn of radical change in air power.

Six months after the crate arrived and the contracts were signed, GE let ‘er rip in the secret testing chamber. Elsewhere at the Bell Aircraft Corporation, top men had been working concurrently on the Airacomet, which was the first American jet-powered plane ever to take to the skies.

In the name of national defense, GE gave their plans to other manufacturers like Allison to encourage widespread growth. Lockheed’s F-80 Shooting Star, the first operational jet fighter, flew in June 1944 under the power of an Allison J-33 with a remarkable 4,000 pounds of thrust.

GE started a school for future jet engineers and technicians with the primary lesson being the principles of propulsion. The jet engine developed rapidly from this point on.

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Delicious Dash Pi Driving Data

A few weeks ago, [sentdex] described how Python has changed his life. In particular, it has allowed him to mine Bitcoin automatically, teach other people programming, and realize a full in-car computer for less than $100 using a Raspberry Pi.

It’s based on a model B, which he’s enclosed in a beefy Pi camera case  that sits on the dash of his Honda S2000. The screen is a $17 internet special with composite in, which keeps the BOM way down. A 3A switch wired into the ignition ensures that power to the Pi is not rudely interrupted.

A script takes the Pi directly into desktop mode when [sentdex] starts the car. His main goals for the project were setting up a dash cam and communicating with the OBD computer. The Pi pulls various data points including the throttle position, and the user moves through the list with the arrow keys of one of those roll-up keyboards.

In the future, he’d like to upgrade it to live graph the throttle position and add a sensor to show the brake position. Be sure to check out the walk-through/demonstration video after the break.

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Push Button, Receive Candy (or Death)

Will you be handing out candy on Halloween? Maybe you have a party to attend or kids to take around the neighborhood and can’t be home to answer the bell. You don’t want to be The Dark House With No Candy, ’cause that’s a good way to get TP’d. We’re not exactly sure what [Ben]‘s catalyst was aside from trying to avoid tempting would-be thieves with an unattended bowl on the porch. Whatever the reason, we’re happy to present Candy or Death, his gamified candy (or death)-dispensing machine.

Okay, so it only dispenses candy for now. [Ben] hasn’t quite worked the kinks out of his death ray. He designed it to sit behind a porch-facing window so it can’t be messed with. All trick-or-treaters can do is push the button and take the candy. It’s built around a cereal dispenser that’s modified to be cranked by a piece of round rod driven with a NEMA-17 stepper motor and an Arduino Uno with a motor shield. The candy slides down a length of aluminium rain gutter into a plastic stacking bin, and the whole thing is built into a nice wood frame.

A few adjustments were necessary to keep it from jamming. The dispenser’s hopper uses rubber blades to govern the flow, and he ended up removing a few and trimming the others. [Ben] has an album up of all his build pics and put his code on the gits. Stick around to see videos of the machine from the front and rear.

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Retrotechtacular: Fire Control Computers in Navy Ships

Here is a two-part Navy training film from 1953 that describes the inner workings of mechanical fire control computers. It covers seven mechanisms: shafts, gears, cams, differentials, component solvers, integrators, and multipliers, and does so in the well-executed fashion typical of the era.

Fire control systems depend on many factors that occur simultaneously, not the least of which are own ship’s speed and course, distance to a target, bearing, the target’s speed and course if not stationary, initial shell velocity, and wind speed and direction.

The mechanisms are introduced with a rack and pinion demonstration in two dimensions. Principally speaking, a shaft carries a value based on revolutions. From this, a system can be geared at different ratios.

Cams take this idea further, transferring a regular motion such as rotation to an irregular motion. They do so using a working surface as input and a follower as output. We are shown how cams change rotary motion to linear motion. While the simplest example is limited to a single revolution, additional revolutions can be obtained by extending the working surface. This is usually done with a ball in a groove.

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Build a Better Something During the Deconstruction 2014

Couldn’t make it to Detroit for the Red Bull Creation Contest? Its founder, [Jason Naumoff], has a separate event called The Deconstruction and 2014 marks the second year of this lively, worldwide competition.

The Deconstruction is taking place November 14-16th and will be broadcast live online. That’s right, you can participate from anywhere in the world, and your team’s progress will be live streamed from The Omni Commons in Oakland, CA. Registration began October 16th and the friendly competition is absolutely free to enter.

Creativity is the name of this game. The idea is to build something awesome using what you already have access to, including teamwork skills. Ideally, what you build will be a deconstruction of something you’d like to see rethought, though there is no stated topic. There are no age limits, no boundaries, and very few rules. A number of great things came out of last year’s event, more of which you can check out at their site. What are you waiting for? Go show the world how you’d make it better.