I have been spending some time recently working on Android Open Accessory using an Uno and the Sparkfun USB host shield. I'm including a range of accessories, Geiger counter, ultrasonic distance meter etc. in my soon to be released (well Dec 2011) book Arduino + Android Projects for the Evil genius
Although the feature that the accessory should provide half an amp at 5V to charge the phone is all very well for AC outlet powered accessories, it means battery powered accessories need a big battery.
Or does it?
I decided to do a bit of experimenting with a surplus USB extension lead that came with a USB modem dongle and just extends the USB lead a couple of inches.
Firstly simply cutting the red 5V USB power lead. This stopped the accessory being recognised and the charge indicator on my Nexus One did not indicate any charging (no surprise there).
If, however, I put a 1k resistor in line on the red 5V wire, everything works fine as an accessory, but the phone does not draw anything significant from the accessories battery, allowing it to be powered from a small 9V battery. At least it does if the phone is well charged.
WARNING: This result was derived experimentally. This may well void your phone warranty, if you connect it up like this. It didn't hurt my phone, but do it at your own risk. Also your wire may not use the same color coding as mine did, so red might be the wrong lead for you.
The following sequence of photos shows the construction of the lead.
Carefully cut away some of the outer plastic with a sharp knife on one side and tease out the shielded bundle of wires.
Although the feature that the accessory should provide half an amp at 5V to charge the phone is all very well for AC outlet powered accessories, it means battery powered accessories need a big battery.
Or does it?
I decided to do a bit of experimenting with a surplus USB extension lead that came with a USB modem dongle and just extends the USB lead a couple of inches.
Firstly simply cutting the red 5V USB power lead. This stopped the accessory being recognised and the charge indicator on my Nexus One did not indicate any charging (no surprise there).
If, however, I put a 1k resistor in line on the red 5V wire, everything works fine as an accessory, but the phone does not draw anything significant from the accessories battery, allowing it to be powered from a small 9V battery. At least it does if the phone is well charged.
WARNING: This result was derived experimentally. This may well void your phone warranty, if you connect it up like this. It didn't hurt my phone, but do it at your own risk. Also your wire may not use the same color coding as mine did, so red might be the wrong lead for you.
The following sequence of photos shows the construction of the lead.
Carefully cut away some of the outer plastic with a sharp knife on one side and tease out the shielded bundle of wires.
Scrape a gap between the shielding to get at the +5V wire.
Cut the red wire and strip and tin the ends.
Solder the 1k resistor into place. Then wrap it all up with insulating tape. Wrap the part behind the tape carefully, so that there is no chance of the bare leads from either side of the resistor making contact with the shielding wires or foil.
About the Author
These are my books. Click on the image below to find out more about them.
These are my books. Click on the image below to find out more about them.
