Saturday, September 12, 2015

Powering the laptop

I am hoping to have the laptop ready for the Adelaide Mini Maker Faire on the 1st of November.  This means I need to make some rapid progress.

As shown in the previous post, I have started to get the hang of the laser cutter, and am starting to work out the cutouts in the various panels, starting with the keyboard.

I have already ordered a nice battery management board that will charge the batteries, and also provide output power from the batteries.  To this I will add a cheap 12V adapter that I bought from Jaycar Electronics. So, I can make use of power once I can get it into the laptop.

Next is to work out how I am going to get power into it to charge the batteries.  I use 2009-2012 model macbook laptops at work and home already, so my preference is to make it compatible with Apple's magsafe connector.  To this end I have obtained a dead macbook and given it a magsafeectomy, and started trying to understand how the magsafe connector works. I have discovered a few things:

1. The magsafe DC-in board really is just a way to get to the 5 pins on the connector, there is essentially no smarts in the DC-in board.  This is good.

2. Magsafe power supplies don't turn on their full voltage until they see a ~37K resistance between the Vcc and GND pins.  After seeing that for 1 second, they switch on the full voltage.  What I don't yet know is if the resistor can stay there. From a power perspective, 37K Ohm at 20V will dissipate only about 0.5mW, so it could indeed remain there.  What I don't know is if the Apple power-supply expects it to disappear after a while. I will also have to work out if I have to isolate the battery manager etc until the full voltage is available. It would be nice if all I have to do is put the resister in place, and then just use it.

3. The LED on the magsafe connector is controlled using the 1-wire protocol on the centre pin. This allows setting the LED either orange, green, or both, which makes yellow.  It would be nice to implement this for the MEGA65P prototype, but it probably won't happen for the first revision. Someone has managed to control the LED via an Arduino, so there is no real technical barriers to doing this -- provided I can get an indication from the battery management board about the charge state of the battery, i.e., full or charging.

4. The connector on the end of the 5 pins is a bit too small to be convenient, so I will probably replace it with a 0.1" 5-pin header.

So all up, it looks like I should be able to use magsafe on the prototype.


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