Demo of multiplexing

[nickgammon]

The video below shows how multiplexing works by slowing it right down.

The display shown is a "scrolling LED" display I bought at (I think) Jaycar a while ago.

I opened it up, and disconnected its internal processor and connected up a Uno (Ruggeduino actually).

Controlled by a potentiometer off camera, I slowly increase the multiplexing speed.

The display has 72 columns of LEDs and 7 rows.

Internally it has 10 x 74HC595 shift registers. Nine of them are used for the columns (9 x 8  = 72) and the last one selects which row is displayed.

As you can see initially, you see one row at a time, as each row is selected. However once running at full speed the persistence of vision effect makes it look like all LEDs are active at once.

http://vimeo.com/80312101

Some photos of the internals:

[Pedro147]

A nice bit of "hacking for educational purposes" there Nick. Thanks for sharing  XD  Would it be possible to see your code as I am interested in how you used the pot to adjust the multiplexing speed.

[nickgammon]

I'm working on describing the project in more detail. Basically the multiplexing was slowed down by a simple delay in the inner loop (between each row).

[Pedro147]

I look forward to seeing it when completed

[nickgammon]

Here is my write-up of the process involved in hacking into, and taking control of, the LED strip:

http://www.gammon.com.au/forum/?id=12298

Ultimately I needed to only cut three jumpers and solder wires on leading to the SPI pins on my Uno, plus Ground.

[Pedro147]

Nice well written write up Nick, thank you.

[nickgammon]

The page linked to above now has a schematic to make it clearer what is happening. Also this video demonstrates the sign in action scrolling words across the display, updating as it goes.

http://vimeo.com/80311021

The updating of the time is done in loop() and the display refreshing is done in a timer-driven interrupt service routine.