Observations about the universe, life, Lausanne and me

Saturday, November 22, 2008

iBrick Mk. II

A couple of weeks ago I quickly cobbled together the iBrick, and external battery for my iPod mini. It was ridiculously heavy, but powerful enough for many hours of musical goodness.

Meanwhile I have graduated to an iPhone, which has it's own power-issues: Under heavy GPS, WiFi or 3G data usage, it's battery won't last the day. This called for a more protable form of the iBrick, the iBrick Mk. II

The design of the iBrick Mk. II revolves around a little chip called 7805 SR-C, a variant of the well-known 7805 voltage regulator. It takes an input voltage of 6-35 Volts, and outputs 5 Volts. Only the 7805 SR-C does this with a ridiculous efficency of nearly 95%.

This is the circuit I dreamt up. The battery will consist of 8 AA rechargeable Ni-MH accumulators, wired in series. It delivers around 8 to 12 Volts, depending on it's charge, which the 7805 transforms down to the 5 Volts required by my power-hungry iPhone. Unfortunately, for some reason only known to apple engineers, you can't just have 5 Volts and ground on the USB connectors power pins. It also reads the data pins, and expects to see 1.8 Volts and 2.5 Volts on the D- and D+ pins respectively. So we have to add a voltage divider network, consisting of three 270 kOhm and one 180 kOhm resistors.

All the components lined up below the circuit drawing:

From left to right: Two 4x AA battery holders, a female USB-A connector, a contact for the batteries, a switch, the 7805 SR-C, and resistors. Below the little Cassis-box that I will use as an enclosure.

First thing to do was prepare the battery holders:

 The two holders were soldered together in series, and held together by some masking tape I had lying around. I had to cut up the battery connector for it to fit.

Next up: Soldering together the voltage divider network:

Think yourself lucky that you can't see the actual solder below. It's been a long time since I have soldered much, and the results weren't pretty. The five wires are: Voltage supply (pink with black stripes), +5 Volts (pink), ground (black), 2.5 Volts (green) and 1.8 Volts (blue).

Putting everything into the enclosure:

No, I did not leave the positive leads uncovered - no short circuits in something that will be connected to my iPhone, thank you very much! They are covered in transparent resin, since I neglected to filch some shrink-tubes from the lab. Later on I also covered them with tape, just to be sure.

Aaand here I proudly present: The iBrick Mk. II, charging my iPhone! Ok, time for some rough calculations. The iPhone's battery is 1150 mAh, at 5 Volts. If I use 3000 mAh Ni-MH accumulators in my iBrick, I'll have 3000 mAh, but at 9.6 Volts! Since I am converting at nearly 95% efficiency, that means I have an equivalent of more than 5000 mAh of juice avialable. Plus it's small enough to fit into my pocket! Well, if it is a large one. I'll let you know how it turns out in reality.

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