320 LEDs using Arduino & 74H595 Breakout

Community Projects
1

Hello Sparkfunners,

Had an idea for a linear LED display project that

would turn on 320 LEDs one at a time then

turn them all of at once and then relight

them one at a time using an Arduino Uno

as a controller.

Started with the Max7219 but the IC

is designed to operate an 8x8 matrix

or a seven segment display and the

breakout to a linear geometry was

a rat’s nest nightmare.

Then the SparkFun 74H595 Shift

Register Breakout board (BOB-10680)

was fgound which provided a more linear array.

The article that was the catalyst

http://bildr.org/2011/08/74hc595-breakout-arduino/

which talks about “1000 of these chained together”.

But the article was misunderstood.

The daisy-chained Sparkfun Breakout boards

can control hundreds of LEDs

but it can only turn on a dozen

or so at one time.

The specs on the 3mm LEDs say they draw

20mA per diode.

The spec sheet on the Arduino lists

the ampereage output for the 3 volt

supply at 50 mA but does not list amps

for the 5v supply. But some digging

around on the forum reveals that

a safe limit is about 450 mA.

The 3mm LEDs used in the project

draw 20mA. So if my math is correct

I can have 22 LEDs on at a time.

Hoping to decrease the amps

required to control the LEDs

and increase the number of diodes

that could be controlled the idea of having

the breakout board control transistors

that supply hight voltage/amperage

was envisioned and design for such

an araangement is displayed herewith.

If the datatsheets for the transistors

is read correctly the current needed

to turn on the transistor is

2N3904 .1 mA 1v

BC108 2 mA 5v

BC182 2 mA 5v

BC548 2 mA 5v

I beleievev that with any of these transistors R1

could be fairly low like 100 ohms.

Is there a transistor that takes

less current to be come saturated?

Assuming these ideas hold true then

the design of the circuit controlled by the

breakout boards is the focus.

Again the LEDs draw 20mA per unit.

If the the display is 320 LEDS that

is 6400 mA or 6.4 amps. Is that right?

I see some power supplies up around 5 amps

but not any at 6 amps.

What is the method for calculating R2

and the voltage and amperage of the

LED power supply?

Thanks.

Allen Pitts, Dallas Texas

2

An LED will light with reasonable brightness with less than 20mA of current. You can increase the resistor value to reduce to reduce the current to 10 mA and it’ll probably still be bright enough. If you’re worried about the current required to saturate the transistor, switching from a BJT to a MOSFET would probably be wise. BJTs are current controlled, while MOSFETs are voltage controlled.