Re: Controlling hundreds of Leds
From: Rodney Josey (rjos_at_dodo.com.au)
Date: 01/07/05
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Date: Fri, 07 Jan 2005 10:45:40 +1000
On Tue, 21 Dec 2004 17:41:51 +0000, Danny T <danny@nospam.oops> wrote:
>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,
Came across your initial post awhile ago - & have followed the threads
avidly. Most interesting!!! The majority of the replies have been
suggesting you use the 'serial-to-parallel' data transfer method and that
you also use either a PIC or an AVR micro - well I agree with the later (in
principle) BUT (there's always a but, isn't there ...). you need to be aware
that most of the PIC's and the AVR micro's have a "limited" amount of memory
available - especially when you most probably need to store the message or
graphic before you output it to the LED display - now in most of these
micro's your control program takes up a fair chunk of the memory thus
leaving a small (in some micro's - VERY samll) amount of free memory space
to store the other things like 'variables', 'tables', as well as your
'message/graphic' data.
I agree with the many suggestions regarding 'multiplexing' of the LED array
BUT driving the array by using the venerable 74LS595 (or 74HC595)
serial-to-parallel shift register method is OK if you have plenty of speed
and time to do the data setup & transfer in - with most of the micro's in
question you may find this method a bit tight on time &/or speed - AND - the
other thing to keep in mind is that your chip count CAN BE fairly large (38
or so 595's plus several control chips - already over 45 chips) which also
means a larger PCB for your system, more costs, etc.
After a few days twiddling with the concept I came up with the following -
you might want to consider another method which could save you in costs as
well as lower chip count and more FLEXIBILITY in what & how the LED MATRIX
can be utilised.
The proposed method uses around 18 or so IC's; 72 FETS for row & column
drivers; 1 x power FET - for PWM control of the LED supply; approx 90
resistors; a handful of small value capacitors; 384 LEDs - in a 8 x 48
matrix; plus either 1 or 2 PCBs.
As can be seen from the circuit diagram below - the 2 main IC's are the
62256 SRAM (32K x 8 bits static ram) & the microprocessor - an PICAXE 28X.
The LED MATRIX uses 6 x 74LS574s for column drive & 1 x 74LS574 for row
drive (these are 8 bit parallel tri-state registers).
The "flexibility" spoken about above is - each LED in the matrix can be
turned on or off individually - e.g. ALL on, 1 on, combination on, many on
(in any position), PLUS a 'moving' display - right / left / up / down, etc,
are all possible display options.
The row & column drive FETs are 2N7000 N-Channel fets - 60 VDC (max), 400mA
current (continuous) or 2A (pulsed), Rds(on) = 1.2 ohms, gate drive 2.4V, in
a TO92 package. With each LED drawing 20mA 'on' current - column FETs drive
8 x LEDs = 160mA (total - all on); row FETs drive 16 x LEDs (@ 3 x FETs per
row) = 320mA (total - all on). 2N7000 @ $0.35 in qty's 25+
1 x MTP3055V N-Channel Power MOSFET is used to PWM the LED positive supply
to the LED MATRIX - freq = 250 Hz, pulse width varies for brightness /
dimness. With ALL leds on the max current drawn from the LED supply is
approx 7.8A - the MTP3055V can handle 12A @ 60V DC.
The 62256 SRAM is used for storing the data to be displayed on the LED
MATRIX - 32K x 8 bits provides for up to 4600 different graphics displays -
the matrix is setup as 8 x characters @ 8 bits high x 6 bits wide (= 384
LEDs). A full (384 LEDs) graphic/character display takes up only 7 x data
bytes in the SRAM - 6 bytes for column drive; 1 byte for row drive.
The PICAXE 28X micro controls the LED display functions as well as the
updating of graphics data to the SRAM. PWM output 2 (pin 13) of the PICAXE
28X is used to drive the MTP3055V MOSFET.
The 74LS138 IC's provide gating & control signals to the rest of the circuit
as directed by the relevant PICAXE 28X Output portC pins.
I also suggest that the i2c pins (pins 14 & 15) of the PICAXE 28X be
utilised to provide communication with an external PC via an PICAXE 18X chip
(i2c pins 7 & 10) and a few extra IC's - all configured as a PC Parallel
port - to - i2c converter.
I hope this gives you some more useful insights into how 'multiplexing'
displays can be designed!!
The Circuit Diagram:
----------------------------------------------------------------------------------------
Ic9 LED MATRIX
.--. (8x48=384)
| | col col
Picaxe +--+ +-------------------+ 1 48
28X | | | | o -/----------------------------/- rw1
.-----. | '--' | o -/----------------------------/-
| | | .--.Ic10 62256 | Ic7 o -/----------------------------/-
| | | | | .------. | .--.o -/----------------------------/-
| +--+ | | |A0 D0| | | |o -/----------------------------/-
| | +--+ +-------+ +----+---| |o -/----------------------------/-
| | | | | | D7| | | |o -/----------------------------/-
| o-o| | | +----+ | | '--'o -/------------------------------ rw8
| | || '--' | | | |
| | || .--. | |A14 | | ######## ########
| | || | | | | | | .------. .------. .------.
'-----' |+--+ +--+ | Ic8 | | | Ic1 | | Ic2 | ------ | Ic6 |
Ic14 | | | '------' | '---+--' '---+--' + + + '---+--'
| | |Ic11 SRAM | | | | | | |
| '--' +---\--------+--------+-----+-+-+-----+
| Data Bus
| .--.Ic12
| | | ----------------------------------------------------------------
o---o | | LED Setup
| | | | Col
| '--' | +V 1
| .--.Ic13 | o-o-o-+|+-o-----o +
| | | | o = | |V| (Ic1-Ic2) # = |
o---o | | | === Fet1 +
| | | +--------+ ||-+
'--' | | | ||<- Fet X1
| | o-o-+|+-o-----o +--+-||-+
Ic1-7,10,11 = 74LS574 | | | |V| (Ic3-Ic4) o/p |
| | | === Fet2 Ic1 |
Ic8 = 62256 SRAM | +--------+ ===
| | | GND
Ic9 = 74LS244 | | o-o-+|+-o-----o Col
| | |V| (Ic5-Ic6) | x
Ic12-13 = 74LS138 | | === Fet3 |
| +---+--------+ / = o-+-----------|-o
Ic14 = Picaxe 28X | o/p | | Rw1X
| Ic7 +->|-+--+ |
Fets = 2N7000 | NOTE: Each o/p of Ic7 Led .-. |
(Total = 72) | drives 3 x Fets | | |
| for each ROW line Rx r | |
| '-' |
+---+
|
+
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
-----------------------------------------------------------------------------------------------------
Hope this helps...:)
Oz-Rod
- Next message: Joerg: "Re: Resonant Transformer"
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- Maybe in reply to: Rodney Josey: "Re: Controlling hundreds of Leds"
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