LED sequencer

Discussion in 'Electronic Design' started by Chris W, Oct 26, 2003.

1. Chris WGuest

I want to flash a group of LEDs in an interesting sequence. I was
wondering if there is some existing device that could do this, or if it
could be made with out too much difficulty. It's hard to describe the
sequence so I will use ascii art to illustrate. O = on, X = off.

OOOXXXXXXX
XOOOXXXXXX
XXOOOXXXXX
XXXOOOXXXX
XXXXOOOXXX
XXXXXOOOXX
XXXXXXOOOX
XXXXXXXOOO

Then repeat, or maybe reverse it and then repeat. This shows 10 LEDs,
but I may be using 15 or 20. I also have 2 different applications for
this. In one, each of the 15 to 20 positions will be a single high
intensity LED. The other requires more intensity, so each position will
be a group of 5 or 10 LEDs, so the circuit needs to be handle enough
current for than many LEDs.

--
Chris W

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

2. Robert MonsenGuest

You can use a shift register with a parallel out. They can be strung in
series for longer sequences.

The problem is starting it properly, and preloading it with data. If its a
parallel in parallel out shift register, you can just stuff the data in with
resistors to either ground or Vcc, and use the reset logic to clock it into
the register.

Look at the datasheets for various shift registers here
http://www.philipslogic.com/products/shiftregisters/ to pick one out that
works for you.

As to driving the LEDs, you can use an open collector buffer to drive
voltages greater than 5V, which you probably need for your 10 LEDs in
series. Again, look at the philips site
http://www.philipslogic.com/datasheets/ for information.

Regards,
Bob Monsen

3. CFoley1064Guest

I want to flash a group of LEDs in an interesting sequence. I was
The easiest, most straightforward way to do this is with a PIC and the ULN
2804, which has 8 open-collector outputs, each capable of handling over 250 mA,
and up to more than 1A of total current (depending on ambient temp). That will
give you 1 PIC plus 1 ULN for each 8 outputs, which is probably good enough for
a one-off display, considering that a 28-pin PIC can give you up to 24 I/O
pins. Especially if you only have 3 outputs on at a time per 2804, it should
be straightforward. If you're working exclusively with LED bundles, run a +5V
for the processor and a higher voltage for the LEDs, and just put all the LEDs
in series with appropriate current limiting resistors. Two PIC inputs for
count-up and count-down signal inputs, and bells&whistles to taste.

Have fun with your project. (By the way, questions like this are usually
posted on sci.electronics.basics. ;-)

Good luck.
Chris

4. John WoodgateGuest

You can do this in analogue using the LM3914. You need to add a
triangular wave voltage to the reference voltage so that 3 LEDs turn on
at once. Then if you put a much slower triangular or sawtooth voltage to
the 'input' pin, the patch of 3 LEDs will sweep up and down the row of
ten. For the triangular waves, you can generate square waves with a 556
(or two 555s) and integrate them using a 747 (or two 741s).

Devices chosen to exasperate Australians working in Belgium, of course.
But they are quite adequate for this application.

For higher current applications you can add bipolar high-current drivers
to the outputs of the LM3914.