Giant score board

Using Arduino Project Guidance
1

Hi Guys,

I'm making a giant score board made out of LED strips, for our rugby club. it has 4, 7-segment displays, with every segment made out of a peice of LED strip, drawing 300ma on 12V.
The arduino is connected to 2 SAA1064 chips by i2c. The SAA1064 is a dual 7segement controller. However, it is a current-sink IC.
As every segment pulls 300ma, there has to be a transistor between the segment and the IC. Can someone point me to a transistor that can switch this load, and with matching resistors? I'm not that good with electronics so a little help would be awesome.

I've found these pnp transistors, is this the right way to go?

50 x BC327 BC327-25 Transistor PNP 45V 0.5A

Manufacturer: PHILIPS/NXP
Manufacturer Part No: BC327-25
Package / Case: TO-92
RoHS: Yes
Datasheet: Click Here

Specifications
Current Rating: 0.5A
Voltage Rated: 45V
Transistor Type: PNP
Features: General Purpose

Thanks for helping out!

Best regards

Mike

2

You want to control the +supply to the strips?

I would go an easier route - use 4 standard shift registers, with the outputs controlling current sink transistors, either N-channel MOSFETs or NPN transistors, and switch the - of the LED strips to ground. Control the segments that turn on from software, easy to do.
I'd go with something like this:
N-channel, logic level, low Rds, low input capacitance, low cost, capable of desired current

Search your favorite suppliers.

Logic Level, Low Rds, N-channel. Can control from 0/+5 level, will not get hot, sink current to ground.

3

Hi Crossroads,

Thanks for replying.

Is it possible you draw a little scheme as example? I thought it was easier to use the Saa1064 so i dont have to handle the 7-segment part on the arduino.
I do have a bunch of IRF830 NPN transistors, would it be possible to use these?
Here is the datasheet":

http://www.datasheetcatalog.org/datasheet/fairchild/IRF830.pdf

thanks again
Mike

4

No, those are not logic level parts - you would need an NPN in front of them with a pullup resistor to +12V. The resistor pulls the Gate up to turn on, the NPN collector pulls the gate low to turn the mosfet off.
Get the right parts & do it like this.
SPI.transfer() to load up the high/lows you want.
Control OE/ also, do not bring it low until your sketch has had a chance to latch on 0's to start, otherwise you may have unpredictable results when power is first applied.

Shift-MOSFET_LED_strips.jpg
Shift-MOSFET_LED_strips.jpg960×720 46.1 KB

5

Wow great scheme, thanks! So what kind of shift register should i use. And it is possible to control 4 segment displays?

6

74HC595, run Dout to next Din, SCK goes to all 4 parts, Latch goes to all 4 parts, OE/ goes to all 4 parts. All 4 update when Latch goes from low to high.
To load data:

digitalWrite(Latch, LOW);
SPI.transfer(byte0);
SPI.transfer(byte1);
SPI.transfer(byte2);
SPI.transfer(byte3);
digitalWrite(Latch, HIGH);

byteX could also be a callout from an array:

numberArray[w];
numberArray[x];
numberArray[y];
numberArray[z];
7

Okay great, i'll be sourcing the parts for this. I also found a great example on the arduino site about shift registers, thats a great help
Thanks so far for pointing me out in the good direction, i thought the saa1064 direction was the way to go....