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I'm thinking of improving my Google Glass by DIY building a micro usb to 3.5mm headphone jack with a battery connected to a neck strap out of parts from the internet.

I've already built the neck strap with gorilla clamps and a sewn in pouch at the back to house my 2600mAh USB pocket Power pack Charger External battery pack. It's nice and slim at 95mm x 25mm and round so sits on the back of the neck comfortably.

What I do want help with is the electronics. From what I have researched Google Glass uses a Maxim integrated MAX14532E chip to switch 3 functions, USB, TTY, Audio. In audio mode the pins represent the following.

  • Pin 1: +5V
  • Pin 2: Right audio out
  • Pin 3: Left audio out
  • Pin 4: Detect
  • Pin 5: GND

Also I've read that a 500kΩ resister will switch audio to stereo and 1mΩ will switch to mono.

Sooo my theory is that if I connect it all up like the following I should have a wearable addon battery pack that should double the battery life and a 3.5mm stereo jack for my beats earphones ?

  • Pin 1: + from battery pack
  • Pin 2: Right audio out
  • Pin 3: Left audio out
  • Pin 4: 470kΩ 1/4W 0.25W ±1% metal film resistor (Couldn't find 500kΩ)
  • Pin 5: GND (resistor and battery)

Total cost for this if it works is about £35, £70 with some good earphones ! Just I'm in the UK and don't want to brick my glass lol I can't get another pair.

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  • \$\begingroup\$ Where did you read all the info? On the part datasheet the control is done via two pins, and it seems to me that one of them is hooked to \$V_{BUS}\$ through a diode, but that's just the typical application circuit, who knows what's actually in the glasses. If you consider your source trustworthy, go for it. Just find the rigt R: there's the \$499k\Omega\$ in the E96 series (1%), or put two \$1M\Omega\$ in parallel. These reisistors usually need to be precise. \$\endgroup\$ – Vladimir Cravero Apr 27 '14 at 12:47
  • \$\begingroup\$ My information was mostly from here. stackoverflow.com/questions/19738163/… and a few other posts about people successfully making them and testing. I cannot find the other post now but another guy confirmed this info by looking at the source code in the kernel for google glass. \$\endgroup\$ – drgr33n Apr 27 '14 at 14:32
  • \$\begingroup\$ So you already have your answer... \$\endgroup\$ – Vladimir Cravero Apr 27 '14 at 14:35
  • \$\begingroup\$ Well wanted to check the info with some people that really knew there stuff and you have changed my mind about the resistors so was worth asking. :D Wasn't sure about charging the device while in audio mode too. \$\endgroup\$ – drgr33n Apr 27 '14 at 14:40
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    \$\begingroup\$ Read a few things that would suggest it would. Hopefully it will. I'll keep you posted. \$\endgroup\$ – drgr33n Apr 27 '14 at 15:38
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The power connections above certainly seem to match the USB pinouts which is the most important thing to get right for a start. Maybe once constructed you could test that the charger is working with a cheaper device with a micro USB port to make sure you've got the pin ordering right.

I haven't verified the other information you've researched on Google Glass in particular but even if the device is in USB data mode I think it's relatively unlikely that placing headphones across the D+ / D- lines to ground would cause any damage. If you wanted to be cautious you could place a 1K resistor between the headphone ground and USB ground on the first test to limit the maximum possible current down to the sort of range used during normal USB communication. I'd expect that should be barely audible just for an initial test.

Finally I'm not sure how criticial it is for that device but to ensure that it is in the right mode it probably is worth shelling out for something closer than a 470k resistor. Most hobby type electronic shops won't sell them but 499k is the closest E192 value. Here's one at Farnell / Element 14 in the UK. I noticed the delivery charge is quite high but they probably have some other stuff you need or you may be able to pick them up (not sure how that works in the UK).

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    \$\begingroup\$ Well it seems we agree on everything... But the \$499k\Omega\$ resistor should be also in the E96 series \$\endgroup\$ – Vladimir Cravero Apr 27 '14 at 12:50
  • \$\begingroup\$ Can I just ask if this would be sufficient ? newark.com/te-connectivity-neohm/yr1b499kcc/… ? \$\endgroup\$ – drgr33n Apr 27 '14 at 15:26
  • \$\begingroup\$ Or this one newark.com/te-connectivity/lr1f499k/resistor-metal-499k-0-6w-1/… much cheaper on ebay :D \$\endgroup\$ – drgr33n Apr 27 '14 at 15:32
  • \$\begingroup\$ Or just get a couple of 470k 10% resistors, and use a multimeter to find out which one is closest to 500k. Because +10% is 517k and +5% is 493.5K, so your likely to find 500k in a 10% tolerance 470k resistor. Not that the IC needs 100% accurate resistors @drgr33n \$\endgroup\$ – Passerby Apr 27 '14 at 15:34
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I can now confirm this does indeed work. I used a LR1F499K RESISTOR, METAL , 499K, 0.6W, 1%. Sounds great with my beats headphones and lasts longer than 4 songs ;). Going to chop up the headphones now and make a unit that fits the device. Cheers for the advice guys ! Much appreciated.

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