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Topic: Maxim7219 to contoll 256 LEDS (Read 370 times) previous topic - next topic

AllenPitts

Hello Arduino Forum,

Most of the info on using the Arduino for controlling hundreds of LEDs is
about either shift registers (74HC595) or multiplexers like Maxim7219.
The number of 74HC595s needed to do 250 LEDS causes a space issue.
So the 7219 was chosen. But the research on the 7219 turned up a lot of
data on using that chip to control seven segment LEDs or matrices.

A PCB has been developed to control 256 LEDS in a more linear display like
a movie marquee.

Have strung four of these together and have it working but only for
eight groups of LEDs at a time. Would like to turn on all 256 at once
but the Arduino will only provide only half an amp of current.

The spec on the LEDs is
Emitting color: Green
Diameter: 3mm
Lens color:Water Clear
Wavelength: 515 - 520 nm
Forward voltage(V): 3.2 - 3.4 V
Current(mA): 20

If the LEDS each use 20 mA (i think this is a maximum and they actually
use about half that but will use 20mA for design purpose) then
256 would require 5120 mA or a little over five amps.

A seven volt 10 amp power source has been acquired and should provide enough current
to drive the Arduino and the 256 LEDS but not sure how to connect
the power supply to the project. It is known that data on the Maxin7219
such as that found at
http://www.planetarduino.org/?p=2861
say if a separate power supply is used the grounds of power supply
and the Arduino ground should be connected.

The diagram

shows that on the PCBs there is a place in both V++ and Ground traces
between the input and capacitor C2 for power and ground input. But if the
power supply is connected at the PCB how
then should the Arduino be powered?

Should the V++ between the Arduino and the PCB be severed and with
the ground between the two systems connected as shown and a separate
connection be made between the power supply the barrel jack on the
Arduino?

A third system, a passive infrared detector, that turns the display
on also needs between 3 and 20 volts to send a signal to an input
pin on the Arduino.

Whats the best to provide power to the three systems?

Thanks.

Allen In Dallas

Wawa

If the LEDS each use 20 mA (i think this is a maximum and they actually
use about half that but will use 20mA for design purpose) then
256 would require 5120 mA or a little over five amps.
The MAX7219 drives an 8*8 matrix, so only eight of the 64 LEDs are on at a given time.
If... the MAX is set for 20mA (resistor), then one MAX only draws ~160mA.

A seven volt 10 amp power source has been acquired...
You should have ordered a 5volt supply. Now you have to drop it further to 5volt.

Whats the best to provide power to the three systems?
Any regulated 5volt supply with enough current capability,
connected to the 5volt pin of the Arduino, the MAX chips, and the PIR.
Share all grounds.
Leo..

Grumpy_Mike

Quote
A seven volt 10 amp power source has been acquired and should provide enough current
to drive the Arduino and the 256 LEDS but not sure how to connect
the power supply to the project.
The problem is that it will provide the current the voltage is too high to power the LEDs. The Max7219 requires 5V, so you have to regulate your 7V to that. Regulating an amp or so is but the 5A you need for your LEDs might be a bit more tricky. I would suggest either you get a 5V power supply or a couple of buck ( switching regulators ).

You can power your Arduino on the 5V pin and likewise for the sensor.

Then all 5V and ground lines can be shared. If you have a separate power supply then all grounds must be connected together only.


INTP

Cascading MAX7219's is what they were made to do.
 
But not what you want if you truly want all of the LEDs to be on at the same time.
Or, you just want them to APPEAR to be on at the same time. That's the trick and benefit of multiplexing.

AllenPitts

Hello Arduno forum,

Thanks to Leo, Grumpy_Mike and IEEE488 for their excellent replies.

Three ideas:

The Arduino Uno tech specs say

1.
Arduino Uno Rev 3 Tech Specs
Microcontroller ATmega328P
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limit) 6-20V

So I guess I could just connect the recently
acquired 10 amp, 5 volt power supply
to the barrel jack on the arduino.
Right?

2. But would not the voltage regulator
on the Arduino get fried by the 6 amps
being drawn by 256 LEDs?

3. Or should i connect the power supply
to the Arduino and the PCBs like

or for the double E types that prefer schamatics.


Either way I am going to stop typing and walk over and
connect it. Although I did fry and Arduino once the
great thing about low voltage you don't usually get
fireworks, it just doesn't work.

Allen




INTP

What part of "7-12V" tells you that 5V is in that range?
If you have 5V, you don't use the barrel jack. Go straight into vcc/5V.

Also, thanks for the snub. Maybe when someone else points out that MAX7219 will not turn on all LEDs for you, you'll listen.

Grumpy_Mike

#7
Apr 22, 2018, 11:06 pm Last Edit: Apr 22, 2018, 11:07 pm by Grumpy_Mike
1) no

2) yes

3) no

As you guessed the regulator on the Arduino can only take about 500mA so you can't pass the LED's current through it.

My advice I. Reply #2 still stands.

AllenPitts

Hello INTP,

No snub intended.
Just overlooked your handle.

Thanks for the feedback.

Allen

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