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Hardware hacking project idea + questions

geeksunny

Hey guys! I've been plotting out a Raspberry Pi project recently and wanted to describe it here along with a few questions before I move forward with it.

 

First, a little background on myself. I am a software developer by trade. Most of my experience lies in the web & with Python. I have a little experience with some basic electronics work. I know how to solder, completing a circuit, etc. I will be doing reading and trying to teach myself more along the way!

 

As for my project idea, I feel like this should be a fairly simple concept... Hopefully I can explain it properly. I have a Griffin Amplifi 2.1 iPod Speaker Box that I would like to turn into a network connected music player. The box...

http://ipods.techfresh.net/wp-content/uploads/2007/01/griffin-amplifi-ipod-speaker-dock.jpg

 

The (important) details about this box:

  • The front knob has a digital volume control by turning it to the left and right.
  • Pressing in on the knob toggles the power to the box.
  • Its powered by an external power brick. (The exact voltage/amperage, I do not know off the top of my head.)
  • There is a 3.5mm audio-input jack on the back-side.
  • When there is nothing plugged in to this jack, it falls back to the iPod dock on the top. (30-pin Apple connector.)

 

What I envision going on in this project:

  • The Raspberry Pi board will ideally be powered by box's power supply.
  • The audio jack on the Pi would be hooked in to the audio-input signal for the speaker box. This will be accomplished by either soldering directly to the box's circuit board, or a 3.5mm cable ran out the back and plugged in... soldered to the board would be preffered.
  • Wires connected from the Pi to the box's volume control, allowing my software to adjust the volume. I dont know exactly how this would be done. I assume it would connect to the Pi's GPIO pin header? (This will be a major point of my research during the project.)
  • Lastly, I'd like some wires from the Pi to be able to toggle the power on the box, replicating what would happen if you push in on the knob. (Again, I assume this is done through the Pi's pin header.)

 

That all covers the hardware side of this project. As for the software, a fairly basic Linux distribution will be used. (I haven't decided which one, yet.) A combination of a few software packages will be installed for handling the audio sources (AirPlay, local network MP3s, Pandora, etc.) And I will be writing my own client/server software for controlling the box, including control over the audio-source, the speaker's volume control, and power state.

 

Well, thats about it... Does this all sound reasonable / possible? Are there any tips that anyone out there could share? And any resources for helping me along the way? I would be very greatful!

 

Thanks for reading!

  • Looks like a fun project.  Yes, you use GPIOs to interface with the Griffin.  A good place to learn about RasPi GPIOs -- both hardware and software --  is the RasPi Hardware Wiki.

     

    General comments:

     

    Be very careful about incompatible voltages.  RasPi GPIOs are 3.3V max and going above that can easily destroy your RasPi.

     

    Check the Griffin's power supply carefully.  RasPi requires a well-regulated 5V +/- 5% supply, which is met by a compliant USB supply.  However, a lot of "wall warts" are unregulated.  If it says 6V at 400 mA, it may very well give you 9V at lower currents, which is plenty to destroy RasPi (as is 6V, for that matter).  A multi-meter will help you determine if it's OK, but some are so poorly regulated that there's a lot of AC ripple which you won't see without a 'sillyscope.

     

    Test RasPi GPIO behavior and software using LEDs with 1K current-limiting resistors (to start).  LEDs are a good, cheap way to see if you're doing the right thing.  Don't hook GPIOs up to something real until you're pretty sure it's working OK with LEDs.

     

    Your volume control is very likely a quadrature encoder.  If that's the case, it should be pretty easy to spoof with RasPi GPIOs driving small NPN transistors like 2N2222 with a 1K resistor to base.

     

    You'll need to see if the Griffin's on/off switch is turning on a large current through a decent size switch or just pushing a micro-switch that's an input to the Griffin's micro-controller.  OTOH, it may be that the Griffin doesn't use much power if it's not making any sound, so maybe you just want to leave it on all the time but drive the Griffin's LED from RasPi image

  • A few more thoughts (to add to John's points):

     

    • You'll need to take a look at where the switch interrupts the power.  If the switch is between the wall wart and all of the electronics, you'll be interrupting power to the Raspberry Pi also (i.e., turning it on will always be manual).
    • The PrFace might be useful in this case as it has a couple built-in relays.'
    • It's probably a good idea to test the power usage of the device before modifying it.  Manufacturers tend to use power supplies which provide "just enough" power.  Adding a Raspberry Pi to the load may be more than the PS can handle.
    • I'd also be wary of overheating.  Putting the Raspberry Pi in an enclosed box with other electronics may require additional venting.

     

    All in all though, it's an interesting project.

  • in reply to packetgeek

    I think you are missing an important point here - the audio quality from the jack socket on the Pi is grim - they claim the good audio is on the HDMI connector. There are several projects concerned with getting good audio from Pis and most seem to use some kind of external DAC.

     

    It would seem a shame to do a lot of work for it to sound awful.

     

    MK

  • John & Tim, thank you guys for your tips! I will keep these in mind when things start to kick off. image

     

    Michael, thanks for letting me know. I hadn't read this anywhere before. I did a little searching for a Pi/DAC solution, and this little tutorial showed up featuring an Adafruit MCP4725 12-Bit DAC. Would this be enough for some decent quality output? I see a lot of people talking up expensive USB-based DAC boxes, which seems like an odd choice since USB can be so CPU intensive.

     

    If USB would be the way to go, would a "USB Sound Card" be a viable alternative? I have an old Soundblaster X-Fi USB that I can rip apart if needed.