An interesting trick you can do with a a fast CPU and a GPIO pin mapped directly to memory is an FM transmitter. Just toggle a pin on and off fast enough, and you have a crude and kludgy transmitter. [Brandon] saw a few builds that turned a Raspberry Pi into an FM radio transmitter and realized a lot of toy remote control cars use a frequency in the same range a Pi can transmit at. It’s not much of a leap to realize the Pi can control these remote control cars using only a length of wire attached to a GPIO pin.
The original hack that turned a Pi GPIO pin into an FM transmitter mapped a GPIO pin to memory, cycled through that memory at about 100 MHz, and added a fractional divider to slightly adjust the frequency, turning it into an FM transmitter. Cheap RC cars usually listen for radio signals at 27 and 49 MHz. It doesn’t take much to realize commanding RC cars with a Pi is possible.
The only problem with this idea is that most RC cars use pulse modulation. For an RC transmitter to send the command for ‘forward’, a synchronization pulse is sent, then a series of pulses and pauses. The frequency doesn’t change at all, something the originally FM code doesn’t do. [Brandon] realized that if he just moved the frequency up to something the RC car wasn’t listening to, that would register as a zero.
All that was left was to figure out the command codes for his RC truck. For this, [Brandon] decided brute force would be the best option. Armed with a script and a webcam, he cycled through all possible combinations until the webcam detected a moving truck. Subtlety brilliant, if you ask us. Of course more complex commands required an oscilloscope, but now [Brandon] has a git full of all the code to control a cheap RC car with a Pi.
[Emilio] has a Raspberry Pi with a few sensors running totally headless. It’s a great way to gather data and post it on the Internet, but for the rare occasions when the Pi needs to be turned off for maintenance, [Emilio] needs to connect a monitor, a keyboard, and a mouse. Not a perfect solution when a simple pushbutton and indicator LED would suffice. There’s one problem with adding a simple button and LED combo: there’s only one GPIO pin available in the setup. That’s nothing a few resistors won’t fix.
After wiring up a very simple circuit on a piece of perfboard, [Emilio] met his design goal of being able to tell if the Pi was running and giving it a software reset button using only a single GPIO pin. The circuit requires only two resistors, and the software to make everything run – a simple Python script – toggles the pin between input and output, checking if the button is held down for five seconds. If it is, the Raspi powers off for [Emilio]’s routine maintenance.
For both the Raspberry Pi and BeagleBone Black, there’s a lot of GPIO access that happens the way normal Unix systems do – by moving files around. Yes, for most applications you really don’t need incredibly fast GPIO, but for the one time in a thousand you do, poking around /sysfs just won’t do.
[Chirag] was playing around with a BeagleBone and a quadrature encoder and found the usual methods of poking and prodding pins just wasn’t working. By connecting his scope to a pin that was toggled on and off with /sysfs he found – to his horror – the maximum speed of the BBB’s GPIO was around three and a half kilohertz. Something had to be done.
After finding an old Stack Overflow question, [Chirag] hit upon the solution of using /dev/mem to toggle his pins. A quick check with the scope revealed he was now toggling pins at 2.8 Megahertz, or just about a thousand times faster than before.
About a decade ago, [Mansour] learned of the Linksys WRT54G, a wireless router that’s been shoved into just about every project under the sun. After learning of this device’s power, he decided a firmware upgrade was in order. Unfortunately, he accidentally bricked this router and left it sitting on a shelf for a few years.
Idle devices are the devil’s playthings, and when [Mansour] discovered a Samsung hard drive with a an SDRAM that was compatible with the WRT54G, he decided he would have a go at repairing this ancient router. There was only one problem: the most popular utility for programming the router through the JTAG header required a PC parallel port.
No problem, then, as [Mansour] had a Raspberry Pi on hand. The parallel port utility bit-banged the new firmware over to the router, something the GPIO port on the Pi could do in spades. By adding Pi support to the debricking utility, [Mansour] had a functional WRT54G with just a little bit of patience and a few wires connecting the GPIO and JTAG header.
[Doug Jackson] makes word clocks, and he must be doing quite a bit of business. We say that because he put together a programming and test bed for the clock circuit boards.
This is a great example to follow if you’re doing any kind of volume assembly. The jig lets the populated PCB snap into place, making all the necessary electrical connections. This was made possible by a package of goods he picked up on eBay which included rubber spacers to separate the board from the acrylic mounting plate, pogo pins to make the electrical connections, and a spring-loaded board clamp seen to the left in this image.
The switch in the lower right connects power to the board and pulls a Raspberry Pi GPIO pin high. The Python script running on the RPi polls that pin, executing a bash script which programs the ATmega169 microcontroller using the GPIO version of AVRdude. We looked through his Python script and didn’t see code for testing the boards. But the image above shows a “Passed” message on the screen that isn’t in his script. We would wager he has another version that takes the hardware through a self test routine.
We first saw one of [Doug’s] word clocks back in 2009 and then again a few months later. The look of the clock is fantastic and it’s nice to see the project is still going strong.
[Kees] wanted a remote for an XBMC audio system. He had a classic T65 Dutch telephone in one of his project boxes and thought this phone with the addition of a Raspberry Pi he could have a functional media remote with classic lines and 70s styling.
Each of the digits on the phone were wired up to a small solderless breadboard. With a handful of resistors, [Kees] set up a simple pull up/pull down circuit feeding in to his Raspi’s GPIO input.
With a short Python script, [Kees] managed to map the buttons to XMBC’s play/pause, volume up/down, next, and previous commands. There were a few buttons left over, so those were mapped to online radio stations, playlists, and a strange setting known only as ‘moo’. We’re not sure what that button does, but you can see the other functions of this XMBC phone remote in action in the video below.
Continue reading “Turning a phone into a media center remote”
So you’ve got a project running on an x86 board and you’d like some GPIO pins. Whether you want to read a few buttons, light up a few LEDs, put an accelerometer in your computer or whatever, you’ve got a problem. Luckily there’s an easy way to get 24 GPIO pins on an x86 board using a PCI card for just a few bucks.
The key component of the build is a PCI TV Tuner card made by Hauppague under the WinTV brand. If you’ve got one of these cards with either a Brooktree bt848, bt849, bt878 or bt879 video capture chip, having 24 GPIO pins is just a spool of magnet wire, a soldering iron, and a steady hand away.
It’s a great build if you’d like some GPIO action without going through the usual parallel port mess, and especially useful since these WinTV capture cards can be had from the usual Internet suppliers for just a few bucks. You’ll need a driver, of course, but the relevant Linux kernel driver – bt8xxgpio – should be included any reasonably modern distro.
Special thanks to [Dex Hamilton] for notifying us of this build.