Beagleboard hardware interfacing

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Beagleboard hardware interfacing Richard Krehbiel 9/17/12 4:19 PM
I'm using a Beagleboard (original, C4) as a Dalek voice modulator.  In a Dalek.  Yes, really*.

The voice changer is working fine.  I'm weak at electronic circuits, but a pretty strong programmer, which is why the voice changer is software, not a 25Hz oscillator and ring modulator.  :-/  So when I look for instructions on hardware interfacing for a Beagleboard I go over my head pretty quickly.  I could use a bit of help.

Daleks "ears" blink when they speak, so I'm trying to add this feature.  I have a pair of "high intensity" white LEDs, which claim 40ma each, so I need to drive about 100ma at 3.3v.  I only need on/off, not PWM or analog.  Is there something cheap and simple I can do?  Thanks.

I used the custom installation builder at http://narcissus.angstrom-distribution.org/ to build Angstrom Linux, kernel 2.6.32.

*  Wanna see it?  The Dalek has a Facebook fan page:


Re: Beagleboard hardware interfacing Toybuilder 10/5/12 4:55 AM
Your white LED's will have a "typical" forward voltage.  Many white LED's I've seen have Vf of somewhere around 3.4 volts.    The datasheet specs for typical or test current is the amount of current that the manufacturer normally expects people to use when driving the LED.  So, in your case, it's 40 mA.

You'll want to switch that 40 mA on/off using a transistor of some kind.  If you just want something quick and dirty, grab a 2N3904 from Radio Shack or any local electronics store. 

I'll assume you are going to drive the LED's from the 5 volt power rail.  Subtract about .3 V for the voltage across the transistor (Vce sat) when it's turned on, and 3.4 volts for the LED, and you have about 1.1 volts left over.  That's the voltage drop you will place across your load resistor to get the desired 40 mA -- 1.1 volts / 40mA = 35 ohms.  Find a resistor close to that value (36 ohm), or find something a little larger for somewhat less current (49 ohms or 50 ohms should be easy to find).

Here's a schematic I just grabbed off the net -- I'd probably just use a 1 K resistor for R4, and set R3 to 36 ohms (per above calculation).  And the voltage source is the 5V rail instead of 3.3v.