On Tue, Nov 1, 2011 at 9:35 PM, Mouse <mouse at rodents-montreal.org> wrote:
The one clock I've dug into in enough detail to
say anything about this
is multiplexed by a factor of two, driven off mains power. ?Half the
segments are (potentially) driven during one half of mains power, the
other half of the segments during the other half of the power cycle.
This is a very common arrangement of one-chip inexpensive LED clocks
in the US. I did some clock hacking a while back because I wanted a
24-hr clock at the South Pole and those are hard to find from US
sources (not so in NZ or Oz, but those are 220V - the 50Hz vs 60Hz is
often a jumper option, but the voltage is usually a fixed-winding
transformer, not so easily hacked).
I know that clock because I used it to make a _big_
clock to hang on
the wall: I opened up a commercial clock, cut the ribbon cable between
the main PCB and the display, probed the display to deduce its pinout,
and wired up a bunch of LEDs in an electrically similar configuration
forming digits about ten inches high. ?The original clock board fits,
with plenty of space to spare, in a corner of the result.
My hack was to take a larger clock (3" digits) implemented as a bunch
of 805-sized SMT LEDs on a PCB and populate the missing LEDs (onto
bare pads) to allow the 10s-of-hours digit to render 2 as well as 1
(cheap American clocks can only render blank or 1). The clock chip
won't try to render a leading 0 when in 24-hr mode, so only six of the
seven segments were on copper on the board.
I got the 805 LEDs from a pile of dead Quantum IDE hard drives - the
size and color of the activity lights were a match for the bare dice
and flying wire LEDs installed on the clock at the factory.
I even managed to find the 12/24 mode pin, so I
didn't have to resort
to logic hackery to convert a 12-hour-plus-am/pm-bit display into a
proper 24-hour display.
Yep. I've hacked 3-4 types of cheap alarm clocks and the 12/24 mode
pin is usually right at one end. Some clock PCBs even have solder
pads to make an easy jumper. The only issue I've seen is the cheapest
clocks only have 2 segments populated in the 10s-hours place. If you
made your own digits, then you wouldn't run into that.
I'm proud of the result. :)
As you should be - reverse engineering, repurposing, extending
functionality... hacking all.