Ask Hackaday: Whatever Happened to LED Light Sensors?

If you’re a long-time Hackaday reader like we are, you’ll certainly remember a rash of projects from around ten years ago that all (mis-)used an LED as a light sensor. The idea wasn’t new, but somehow it made the rounds and insinuated itself into our collective minds. Around the same time, a cryptographic cipher with an exceptionally small memory footprint was also showing up in hacker projects: TEA (Tiny Encryption Algorithm).

This old project by [Marcin Bojanczyk], [Chris Danis], and [Brian Rogan] combines both the LED-as-light-sensor meme and TEA to make a door-entry keyfob that works over visible light. And they do so using almost nothing — a few LEDs and just over 2Kb of code. It’s pretty sweet.

Which brings us to the question: where are they (LED-sensors and TEA) now?

ledtouch_photoLED-as-light-sensor was just cool. We certainly loved the idea back in 2006. But [Forrest Mims] had been using the phenomenon for decades back then. It certainly makes sense when you’re trying to squeeze as much as possible out of as little as possible, or when budget is a main concern and you just can’t afford an extra photodiode.

But our own experience with LEDs as light sensors is that the results are extremely variable across different LEDs. Code that works with water-clear red LEDs might not work with the ones that come in red-tinted plastic, for instance. Is that why they went extinct?
4746123271_7888160588Similarly, the TEA family of ciphers showed up in a bunch of projects around this time, from the badge for the HOPE conference in 2010 to a widely used RFM12B radio library. There are a couple of attacks on XXTEA, but they only affect reduced-round versions of the cipher, and rely on a tremendous amount of intercepted data — more than we’d see in a home-automation network over years.

Over the last five years or so, there’s been a lot more Internet of Things, which means using standard Internet-style encryption methods (AES and so on) that are widespread on non-memory-constrained computers. Is that what happened to XXTEA?

Anyway, we got tipped off to a project that combined a few of our favorite (old) ideas in one, so we thought that we’d share. Thanks [Blue Smoke] for the walk down memory lane. Any of you out there keeping the flame(s) alive? Have you used sensing LEDs or XXTEA? Are those projects still going, or do you have any future projects planned with these tricks still up your sleeve? Let us know in the comments below.

This Car Lets You Fistbump to Unlock

In the dark ages, you had to use a key to lock and unlock your car doors. Just about every car now has a remote control on the key that lets you unlock or lock with the push of a button. But many modern cars don’t even need that. They sense the key on your person and usually use a button to do the lock or unlock function. That button does nothing if the key isn’t nearby.

[Pierre Charlier] wanted that easy locking and unlocking, so he refitted his car with a Keyduino to allow entry with an NFC ring. What results is a very cool fistbump which convinces your car to unlock the door.

Keyduinio is [Pierre’s] NFC-enabled project, but you can also use a more conventional Arduino with an NFC and relay shield. The demo also works with a smartphone if you’re not one for wearing an NFC ring. Going this round, he even shows how to make it work with Bluetooth Low Energy (BLE).

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Hacked Apartment Intercom Barks at You or Buzzes You In

Forgot your apartment keys? If you’ve got a ritzy building with a doorman, no problem. If your digs are a little more modest, you might only have an intercom panel that calls up to your apartment so someone can buzz you in. But if nobody is home, you’re out of luck. That’s why [Paweł] spent an hour whipping up an intercom connected automation system pack full of goodies.

entryphoneThe design is pretty simple – an ATMega328P to snoop on the analog phone ringer in the apartment when the intercom call button is pushed, and a relay wired in parallel with the door switch to buzz him in. For added security, the microcontroller detects the pattern of button presses and prevents unwanted guests from accessing the lobby. Things got really fun when [Paweł] added a PCM audio module to play random audio clips through the intercom. As you can see in the video below, an incorrect code might result in a barking dog or a verbal put-down. But [Paweł] earns extra points for including the Super Mario Bros sound clip and for the mashup of the “Imperial March” with “The Girl from Ipanema”.

True, we’ve seen a slightly more polished but less [Mario] version of this project before, but the presentation of this particular hack has us grinning from ear to ear.

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Fingerprint Scanner Both Simplifies And Complicates Opening Garage Door

Fringer Print Scanner Garage Door Opener

Opening a garage door by hand is a lot of work and a hassle, hence the advent of the garage door opener. Nowadays, some people may even say just pushing the button of a remote control requires too much effort. [nodcah] is one of those people so he came up with a fingerprint scanner that controls a pre-installed garage door opener. All kidding aside, it is a cool project that lets you into your garaage, keeps unknown people out and doesn’t require you to remember to carry a key or remote.

In the center of this project is an ATmega328 that runs a custom Arduino code. This ATmega328 is responsible for controlling a 16 character, 2 line LCD screen as well as communicate with an off the shelf fingerprint scanner from Sparkfun. The fingerprint scanner has a built in CPU, can store up to 20 fingerprints and does all its own processing of fingerprint scans. It then communicates to the ATmega328 with simple commands over serial Tx and Rx lines.

The ATmega328, LCD and fingerprint scanner are all mounted outside the garage in a 3D printed enclosure. If the wires for the internal-garage open/close button were just run straight into this outdoor module, anyone could open it up, short the wires and get into the garage. To prevent this, if the ATmega328 gets the ‘OK’ from the fingerprint scanner, then it sends a signal to an ATtiny85 that is inside the garage. If the ATtiny85 receives the correct signal, it will then actuate the garage door opener by shorting the open/close button contacts. This prevents anyone from sneaking into the garage.

[nodcah] did a great service to the community by making all of the part list, schematics, instructions and Arduino code available so anyone can easily put this project together.

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Raspi Z-Wave Automation is Automated


Home automation keeps popping up here at Hackaday, so [Cristian Zatonyl] decided to share his Raspberry Pi-based system with us. This build takes a firm stance on the “automated” side of the automation vs. control debate we had last week: no user input necessary. Instead, [Cristian] relies on geofencing to detect whether he has driven outside the set radius and automatically turns off the lights and locks his door.

The build takes advantage of Z-Wave products, which are your typical wireless remote-control gadgets, but tacks on a third-party “RaZberry” board to a Raspi to give it control over off-the-shelf Z-wave devices. The final step is the integration of a custom iOS app that keeps tabs on the geofence boundaries and signals the Pi to control the lights and the front door lock.

[Cristian’s] tutorial covers the basics and admits that it’s a proof of concept without any security features. Judging by his other YouTube videos, however, we’re sure more developments are underway. Check out the video below for a demonstration of the system, then feel free to speculate on security concerns in the comments. Our article on Z-wave security from a few years ago might be a good starting point.

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School Bus Keyless Door Lock Conversion


When a school bus has finished its life ferrying children around, it often ends up as someone’s pet project. Most buses in the US, however, have annoying back doors that lock only from the inside, which isn’t very convenient if you’re loading/unloading gear. Drawing inspiration from another project that fit a simple deadbolt upgrade, [Leonard] took the build one step further and hacked together his own keyless entry deadbolt system.

He started by removing the white safety bar that normally covers the long red handle and attached a slide bolt to the door. The slide bolt serves only as an extension for the deadbolt, which would otherwise get whacked by the red handle. [Leonard] made a few modifications to the slide bolt so it can sit flush against the bus’s lock bar, then went to work attaching the keyless deadbolt. At 2.5″, the bus’s door is actually thicker than standard doors, not to mention this build needs the deadbolt to move along the door’s surface to push the slide bolt fitted to the door. [Leonard] decided to throw in a chunk of wood as a kind of “simulated door,” which mounts next to the slide bolt and houses the deadbolt’s guts.

Check out the video after the break to make sense of the door’s operation and swing by [Leonard’s] blog to see what else he’s done to the bus. If you’re in the mood for more transportation hacks, make sure you see the Raspberry Pi CarPC.

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Passive Bluetooth keyless entry system

Modern smart keys allow you to keep the key fob in your pocket or purse while you simply grab the handle and tug the door open. [Phil] decided he would rather ditch the fob altogether and instead implemented a passive Bluetooth keyless entry system with his Android phone. It’s probably unlikely for car manufacturers to embrace phone-based keys anytime soon, and [Phil] acknowledges that his prototype poses a landslide of challenges. What he’s built, however, looks rather enticing. If the car and phone are paired via Bluetooth, the doors unlock. Walk out of range and the car automatically locks when the connection drops.

His build uses an Arduino Mega with a BlueSMiRF Silver Bluetooth board that actively searches for his phone and initiates a connection if in range.  Doors are unlocked directly through a 2-channel relay module, and an LED indicator inside the vehicle tells the status of the system. A pulsing light indicates it’s searching for the phone, while a solid ring means that a connection is established.

We hope [Phil] will implement additional features so we can make our pockets a bit lighter. Watch a video demonstration of his prototype after the break, then check out the flood of car-related hacks we’ve featured around here recently: the OpenXC interface that adds a smart brake light, or the Motobrain, which gives you Bluetooth control over auxiliary electrical systems.

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