consumer and otherwise electronics questions

jeremy at jeremy at
Thu Aug 23 00:36:19 EST 2001

On 23 Aug, Damien Elmes wrote:
> hey everyone!
> thanks to a very kind person on this list, i now have an old laptop to
> mount in my car for the mp3 player i'm building. you can see a bit
> about it here:
> i've a couple of questions i was hoping some of the people here might
> know, as electronics isn't my forte:

> * i need a regulated 5V power supply for the LCD screen. what is the
>   line level of a laptop serial port? the alternative is regulating
>   the DC output from the car, but that's beyond my abilities. i'd need
>   to find schematics on the net somewhere.

Some details on serial ports can be found here:

As you can see, that's not too helpful.  But serial ports were never
meant to provide power to the devices.  (Ok, USB is)

The technical reference manual will have more detail.

A regulator is extremely easy, and can be done with three components.
For better quality, Jaycar/Dick Smith has what you want.

> * i'm also on the look-out for a laptop battery for an IBM thinkpad.
>   is there a way to determine if the notebook is running off 'mains'?
>   this would allow my laptop to shut down properly when power is cut. 

Yes.  The laptop will generate APM events, which your kernel will trap
if you have APM compiled in.  It in turn calls apmd, which can execute
scripts that you tell it too.

There is also a suite of programs that can poll the thinkpad bios to
find this out directly.  Included in Debian.

I/O, I/O,
It's off to disk I go,
A bit or byte to read or write,
I/O, I/O, I/O...

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