Controlling hundreds of LEDs

[Danny T]

Hi All,

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

I'm not an electronics guy (I'm a software developer), so trying to
control x00's of LEDs from 8 pins looks a bit tricky...

There's no way to change them all at the same time, so I reckon I'll
need some sort of "addressing" system. Using some of the pins to select
which LED I'm controlling, and one for the data.

Trouble is, even using 7 bits as the "address", I've only got 128
channels, but I imagine I'd need a bit for "send" too, so I can set the
address and the data, then have it read in one go.

What are my options? And what components would I need to achieve
something like this (I imagine right down at the end, I'll need a number
of registers for the LEDs. If these come in chips with 8 output legs, I
guess I'll need 38 for 300 LEDs. The bit between my 8 data pins and the
computer isn't my area though, so I'm stuck!

Oh, and this is just a hobby thing, so the cheaper the better. I'm sure
I can already buy scrolling screens, but I don't have the money, and I
want the fun of building one!

 

[CFoley1064]

Subject: Controlling hundreds of LEDs

From: Danny T [email protected]
Date: 12/21/2004 11:41 AM Central Standard Time
Message-id: <[email protected]>

Hi All,

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

I'm not an electronics guy (I'm a software developer), so trying to
control x00's of LEDs from 8 pins looks a bit tricky...

There's no way to change them all at the same time, so I reckon I'll
need some sort of "addressing" system. Using some of the pins to select
which LED I'm controlling, and one for the data.

Trouble is, even using 7 bits as the "address", I've only got 128
channels, but I imagine I'd need a bit for "send" too, so I can set the
address and the data, then have it read in one go.

What are my options? And what components would I need to achieve
something like this (I imagine right down at the end, I'll need a number
of registers for the LEDs. If these come in chips with 8 output legs, I
guess I'll need 38 for 300 LEDs. The bit between my 8 data pins and the
computer isn't my area though, so I'm stuck!

Oh, and this is just a hobby thing, so the cheaper the better. I'm sure
I can already buy scrolling screens, but I don't have the money, and I
want the fun of building one!

Hi, Danny. Look up the datasheet for the 74HC595. You can control as many
LEDs as you want by shifting data out serially, limited only by how fast you
can shift. You'll only need three pins (use Data In, CLK and Latch) and have
the Data Out of the first chain to the Data In of the second, and so on. CLK
and Latch drive all the '595s. Just shift out all the data you want, clock
after every bit is asserted, then toggle the latch after you're done to set all
the data at all the outputs. Easy. One IC for every 8 LEDs (which can be
driven directly, source or sink, with only one series resistor per LED). 38
16-pin ICs for 300 LEDs.

Note that, if you're going over extended distances, you'll want to buffer the
signals to achieve sharp transistions. The '595 doesn't behave well with slow
logic edges. And watch for noise -- that can mess up your data.

You'll have no problem transferring from a printer port to a PIC.

http://www.fairchildsemi.com/pf/MM/MM74HC595.html

If system speed is a consideration, you might want to look into latching data
and multiplexing control of the LEDs. But for "real time" control, the '595 is
the easiest way from the hobbyist perspective.

By the way, you might want to look at Jan Axelson's "Printer Port Complete" for
an explanation of how to do this, as well as the website:

http://www.lvr.com/parport.htm

There's a lot of good stuff there.

Hobbyist questions of this type usually get a better reception at
sci.electronics.basics.

Good luck
Chris

 

[hderaser]

Danny:

My typical projects for micros have more outputs than pins that do not
need to respond all that fast, like milliseconds. I use 74HC595s. My
company buys them in 100s at $0.17 USD each.

The '595 has a serial input, a serial output, 8 parallel outputs and
some control pins. I believe each output could drive one typical LED,
especially if you don't have to be sure the output voltage stays in the
logic range.

You could daisy chain any number of them, serial out to serial in.
Many micros have pins that can operate general purpose or special
purpose like serial interface. The '595 works best with SPI output.
If your micro does not offer that, it is easy enough to emulate with
general purpose pins.

 

[Rolavine]

Subject: Controlling hundreds of LEDs

From: Danny T [email protected]
Date: 12/21/04 9:41 AM Pacific Standard Time

Hi All,

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

I'm not an electronics guy (I'm a software developer), so trying to
control x00's of LEDs from 8 pins looks a bit tricky...

There's no way to change them all at the same time, so I reckon I'll
need some sort of "addressing" system. Using some of the pins to select
which LED I'm controlling, and one for the data.

Trouble is, even using 7 bits as the "address", I've only got 128
channels, but I imagine I'd need a bit for "send" too, so I can set the
address and the data, then have it read in one go.

What are my options? And what components would I need to achieve
something like this (I imagine right down at the end, I'll need a number
of registers for the LEDs. If these come in chips with 8 output legs, I
guess I'll need 38 for 300 LEDs. The bit between my 8 data pins and the
computer isn't my area though, so I'm stuck!

Oh, and this is just a hobby thing, so the cheaper the better. I'm sure
I can already buy scrolling screens, but I don't have the money, and I
want the fun of building one!

You could use a bunch of series wired 8 bit shift registers, that allows one
data bit, one clock bit, and some control bits to run the whole show. Some of
these have a shift register and an output register, so you could change all the
LEDs at once. If you don't require maximum brightness, and you could find a
shift register in the 40xx or 45xx series logic parts you could get by without
needing a series resistor to limit current for each LED. The 74hc or 74hct
series parts have about 20 ma. of current output so you would need a resistor
for each led to limit current. An easy way to add the resistors would be to use
a resistor pack, the kind that have a common on pin one work well.

Another way if your up for it, is to use a Large IC that has a JTAG test port
and then send your data to the shift registers built into these chips for
testing. I did this a few year ago using an unprogrammed Xilinx CPLD as a shift
register. This is a bit tricky but is a good introduction to using these test
ports.

Rocky

 

[nospam]

Hi All,

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

Unless very high brightness is needed anyone designing for this requirement
would arrange the LEDs in a matrix, typically 8 rows and in your case about
40 columns.

The rows are driven from the top probably with P channel MOSFETS and the
columns driven from the bottom with a latched shift register. Others
suggested the 74HC595, there are better chips if you can obtain them such
as Allegro A6276ELW or ST STP16CL596 which are 16 bits and have high power
constant current LED drive outputs.

You need 6 or 7 control lines.

data, clock, and latch strobe for the column drive.
3 lines to select which row is driven
optionally another line to disable all columns or all rows which can be
used to control brightness and can ease timing requirements on switching
row drives.

You do have to multiplex the display in real time (typically 2ms per row)
which isn't hard for a PIC, software on a PC will likely be a bit jittery.

So for a 348 LED display you need 3 A6276ELW chips, 8 suitable P channel
MOSFETS, something like a 74HC4051 to drive them and a (big if you want
bright) 5v power supply.

It looks like you are in the UK, you can get the Allegro parts from
www.farnell.com about L3.50 each. They also do RDF15P05 MOSFETs which might
be suitable for driving the rows.

 

[Danny T]

Hi, Danny. Look up the datasheet for the 74HC595.

Someone in sci.electronics.basics suggested 74HCT259. Looking at them
both (I'm not that familiar with geek-speak in datasheets!), is the main
difference that one uses 3 bits for addressing, while the other
increments the bit I'm modifying using a single pin?

38 16-pin ICs for 300 LEDs.

But to control 38, I'll need another 5, right? So my parallel port
controls 5, which control the 38, attached to the LEDs?

I don't understand what you mean by "One IC for every 8 LEDs (which can
be driven directly, source or sink, with only one series resistor per LED)"

Directly, source or sink?

As for resistors, I'm currently running the LEDs directly from 5V on my
parallel port. Would the chip not output a similar voltage, and hook up
straight to the LED?

If system speed is a consideration, you might want to look into latching data
and multiplexing control of the LEDs. But for "real time" control, the '595 is
the easiest way from the hobbyist perspective.

While Googling earlier, I found lots of references to "demultiplexing",
but couldn't figure out what on earth it was. The context seems to
suggest it might help, but even google didn't link to dictionary.com for
it :-(

By the way, you might want to look at Jan Axelson's "Printer Port Complete" for
an explanation of how to do this, as well as the website:

http://www.lvr.com/parport.htm

There's a lot of good stuff there.

Hobbyist questions of this type usually get a better reception at
sci.electronics.basics.

Yeah, I posted there too, unsure exactly which was best. Had good
answers from both. Thanks

Danny

 

[Danny T]

So for a 348 LED display you need 3 A6276ELW chips, 8 suitable P channel
MOSFETS, something like a 74HC4051 to drive them and a (big if you want
bright) 5v power supply.

I've had a quick look at the data sheets, and 16 pin output would
certainly require less chips, but I don't understand what MOSFETs are,
or what(/how) to use the 74HC4051 for :-\

Daisy-chaining loads of 595's (or 74HCT259's as suggested in the .basic
group) looks easy - are there any advantages/disadvantages to the method
you suggest over this?

Thanks,

 

[Danny T]

Look up the datasheet for the 74HC595.

Another quick (possibly dumb) question...

If you go to www.rswww.com (the URL has a session ID, so prob won't
work!) and search for the 74HC595, and click through to the product
page, it lists several:

MC74HC595AN
SN74HC595N
74HC595D

but I don't understand the differences between them (there's a hefty
price difference!). What am I missing?

Thanks,

 

[CFoley1064]

Subject: Re: Controlling hundreds of LEDs

From: Danny T [email protected]
Date: 12/21/2004 3:22 PM Central Standard Time
Message-id: <[email protected]>

Yeah, I posted there too, unsure exactly which was best. Had good
answers from both. Thanks

Danny

Hi, Danny. I'll post answers on s.e.b. This is really too basic for this
forum.

Chris

 

[Stefan Heinzmann]

Danny T wrote:
[...]

If you go to www.rswww.com (the URL has a session ID, so prob won't
work!) and search for the 74HC595, and click through to the product
page, it lists several:

MC74HC595AN
SN74HC595N
74HC595D

but I don't understand the differences between them (there's a hefty
price difference!). What am I missing?

Different manufacturer.

The last few characters usually specify the package, so be careful to
check this. Each manufacturer has a different package code.


Regarding your original problem of controlling LEDs:

There's a whole lot of issues to consider:

You say that eventually a PIC will control the LEDs. If the software of
the PIC has some spare time and RAM, it is possible to use a multiplexed
driving scheme. This saves hardware, but uses the PIC to cycle through
the rows at regular intervals. This may be easy to do with a PIC but
hard with the parallel port of a PC. This is a good solution if hardware
cost is the paramount issue.

If the compatibility between parallel port and PIC is the overriding
issue, then you will probably prefer static drive. This needs no
software interaction as long as the LED display doesn't change. It is
somewhat more expensive on the hardware side, however.

I take it that if you say you've got 8 bits worth of data port you also
have got some control lines, like strobe and busy lines on the PC's
printer port. If the 8 data bits is ALL you've got, things are different.

Many PICs also have a built-in serial port (SPI compatible), which is
very handy when talking to shift register chips like the 74HC595. The PC
parallel port doesn't have this, so again this raises the question how
compatible you need to be.

In the case of static drive (as opposed to multiplexing) you also need
to know whether you want to be able to change a single LED or whether
you are prepared to output the complete status for all LEDs each time
anything changes.

If you accept to output the complete data for all LEDS each time
something (or everything) changes, you may consider shifting out the
data bytewise. You need 2 control signals in addition to your 8 data
signals. On the PC parallel port this could be STROBE and (for example)
INIT. With STROBE you'd clock out bytes, and with INIT, you'd latch the
bytes shifted out into the output register (of a 74HC595 or 74HC4094).
You would use 8 shift register chips in parallel, each being connected
to one of your data outputs. This gives you 64 outputs (for LEDs), and
you can expand this in increments of 64 by making the shift registers
longer, ad infinitum.

A more PIC-specific solution would be to string the shift registers up
in serial fashion rather than 8-bit wide fashion. This would save PIC
pins, but is only useful if the PIC has a built-in SPI port.

If you prefer changing individual LEDS without affecting any other, you
may be better off with the addressable latch method (using 74HC259
chips). You have already noticed that you need 7 bits for addressing 128
LEDs, plus one data line that controls whether the LED is set on or off.
You also need a strobe signal for timing. Again, using the 8-bit
parallel port for addressing, and one control signal each for strobe and
data, you could address 256 LEDs. This could be done with 32 chips of
type 74HC259 and 4 chips of type 74HC138, plus an inverter.

The hardware cost between those two methods is pretty similar, but they
are radically different regarding the way they are driven by the
software. The multiplexing method is different again and cheaper in
hardware. Make sure you understand the impact on software before you
make a decision.

 

[nospam]

I've had a quick look at the data sheets, and 16 pin output would
certainly require less chips, but I don't understand what MOSFETs are,
or what(/how) to use the 74HC4051 for :-\

Daisy-chaining loads of 595's (or 74HCT259's as suggested in the .basic
group) looks easy - are there any advantages/disadvantages to the method
you suggest over this?

Yes. Driving 384 LEDs requires 4 chips and 8 3 pin transistors (MOSFETS)
instead of 48 chips and 384 resistors. Depending on how you want to
physically arrange the LEDs it can reduce the required wiring by a factor
of 8 also.

Search the web for LED, matrix, and multiplex. You should something that
gives you the idea.

 

[Danny T]

Different manufacturer.

The last few characters usually specify the package, so be careful to
check this. Each manufacturer has a different package code.

There was a seperate column for package (I think I want DIP - it's going
into a breadboard for now!), so I'll just go with the cheap ones for now
(I'm sure I'll blow a few up ;o))

Regarding your original problem of controlling LEDs:

<snip>

Compatibility with the Parallel port isn't important, it's just my
current test bed. My PIC programmer should arrive this week, so as long
as it works, although tedio swapping the chip between programmer and
breadboard, that would do me fine. The chips I've got coming with it are

PIC16F627
and
PIC12F629

Multiplexing keeps cropping up. Is this sending data serially, and using
timing (or another bit) to denote the changes? If so, does this my chip
would send multiple signal across single pins, and another chip would
de-code these into more signals? Is there a limit to the number of
singals I can send in serial?

As for controlling invidiual LEDs, it'd be nice for testing, but I don't
think it's important - as long as it's not too slow to draw the whole
board, I can live with that (animations etc. would either involve most
frames being different, or some processing to decide which have changed,
which'll probably slow things down anyway).

The hardware costs aren't *too* important, it's just gotta be easy and
educational! I'll make a list of all the different methods being
suggested, and make an informed (!) decision

 

[Uwe Bonnes]

Hi All,

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

I'm not an electronics guy (I'm a software developer), so trying to
control x00's of LEDs from 8 pins looks a bit tricky...

Xilinx has a nice report for a project with many LED in one of the last
Xilinx journals.

Bye

 

[Stefan Heinzmann]

<snip>

Compatibility with the Parallel port isn't important, it's just my
current test bed. My PIC programmer should arrive this week, so as long
as it works, although tedio swapping the chip between programmer and
breadboard, that would do me fine. The chips I've got coming with it are

PIC16F627
and
PIC12F629

Multiplexing keeps cropping up. Is this sending data serially, and using
timing (or another bit) to denote the changes? If so, does this my chip
would send multiple signal across single pins, and another chip would
de-code these into more signals? Is there a limit to the number of
singals I can send in serial?

As for controlling invidiual LEDs, it'd be nice for testing, but I don't
think it's important - as long as it's not too slow to draw the whole
board, I can live with that (animations etc. would either involve most
frames being different, or some processing to decide which have changed,
which'll probably slow things down anyway).

The hardware costs aren't *too* important, it's just gotta be easy and
educational! I'll make a list of all the different methods being
suggested, and make an informed (!) decision

Ok, I think multiplexing really is the way to go for what you want to
achieve. It is also quite an interesting educational project.

The PIC16F627 has a USART port, but no SPI compatible port. In its
synchronous mode it can be used to drive shift registers, but I guess
you would rather like to use this port for a connection to a PC (using
RS-232) for controlling purposes. The PIC12F629 lacks even this, so you
would have to do everything by wiggling the port pins with software. The
PIC12F629 is also a bit low on SRAM, this will limit the number of LEDs
you can control (you need to store their current state somewhere).

In multiplexing, only one row of LEDs out of (say) 8 rows will be active
at any time and the PIC needs to step through the rows quickly enough
for the eye to perceive a steady state. Aim for cycling through all rows
within 12ms or faster.

As the LEDs are on at most 1/8th of the time, they need to be operated
with higher currents than normal, so that the perceived brightness
averages out to normal. Standard LEDs usually can take 60mA pulse
current easily, some are specified much higher. With an 8 row
multiplexing scheme your pulse currents needn't be too high, and you can
use standard LEDs and standard small signal transistors as column drivers.

For example, the column drivers could be standard supercheap NPN
transistors like the 2N3904, the row drivers would be higher power PNP
transistors, as they need to drive the current for an entire row of LEDs.

Let's say for example you want to drive 512 LEDs. That's a 64 x 8
matrix. You need 64 NPN transistors, 64 current limiting resistors, and
8 PNP transistors. When you operate at 60mA pulse current per LED, a row
will consume a maximum of 64 * 60mA = 3840mA. The PNP transistors will
thus need to be rated for 4A, keeping in mind that each will only be on
1/8th of the time. Something like a BD434 would be called for, but try
to get a transistor with better hfe if possible.

A fully illuminated matrix will draw 4A of current, so make sure that
your power supply is up for that.

In order to control these driver transistors, you can use ordinary 74HC
type logic chips. Use eight 74HC595 or 74HC4094 shift register chips for
controlling the column drivers and one 74HC138 for controlling the row
drivers. Use series resistors to limit the base current for the
transistors, but drive the base of the row transistors as hard as the
74HC138 allows.

The 74HC138 is controlled from the PIC, using port pins. More port pins
are used to shift the data into the column drivers. You can select
whether you want to string them together serially, using only one data
port pin on the PIC or whether you feed the data into each shift
register in parallel, using 8 port pins for the data lines. Clock and
control lines are common for all shift registers. The former is ideally
suited to using the USART port, the second uses more port pins on the
PIC, but is faster to drive with software.

 

[Bob Smith]

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

I'm not an electronics guy

Danny
Wiring all those parts will be the hard part. If
your electronics is up to it, you should at least
consider using the Maxim 7219. It is a 24 pin part
that directly controls 64 LEDs.

Would you mind posting whatever design you decide
to use?

The MAX 7219 data sheet is at:
http://pdfserv.maxim-ic.com/en/ds/MAX7219-MAX7221.pdf

Bob Smith

 

[Jeff]

Hi All,

I'm trying to control hundreds of LEDs from my parallel port (8 data
pins - will soon be replaced with the output of a programmed PIC, also 8
pins).

I'm not an electronics guy (I'm a software developer), so trying to
control x00's of LEDs from 8 pins looks a bit tricky...

There's no way to change them all at the same time, so I reckon I'll
need some sort of "addressing" system. Using some of the pins to select
which LED I'm controlling, and one for the data.

Trouble is, even using 7 bits as the "address", I've only got 128
channels, but I imagine I'd need a bit for "send" too, so I can set the
address and the data, then have it read in one go.

What are my options? And what components would I need to achieve
something like this (I imagine right down at the end, I'll need a number
of registers for the LEDs. If these come in chips with 8 output legs, I
guess I'll need 38 for 300 LEDs. The bit between my 8 data pins and the
computer isn't my area though, so I'm stuck!

Oh, and this is just a hobby thing, so the cheaper the better. I'm sure
I can already buy scrolling screens, but I don't have the money, and I
want the fun of building one!

Check out this web site:

http://home.wanadoo.nl/electro1/avr/dotmatrix.htm