Help needed with MAX7219/7221 LED driver IC

[HexTheKiwi]

I am looking for some guidance on a project that I currently have underway where, due to my somewhat basic understanding of electronics (I should have listened more to my late father more when he was explaining it all to me) I have hit a bit of a barrier.

First a little background on the project... I am constructing a 10 lane LED based scoreboard for my kids' swim club. Each of the segments in the seven digits segment digits consists of 19 LEDs (2 x 5 LEDs in series and 1 x 4 LEDs in series) theoretically drawing 100mA. Decimal places draw 25mA (4 LEDs in series) and Colons draw 50mA (2 x 4 LEDs in series). I also have two segments being controlled from a single line which is drawing 200mA.

For each lane there are a lane number (a value from 1 to 10) a placing (a value from 1 to 10) and a time (mm:ss.dd). Note that this is a total of 10 digits a colon and a decimal place per lane. (I have attached a drawing of a lane's digits.)

Anyway, I came across the specs for the MAX7219 and MAX7221 driver ICs and the fact that you could chain upto 16 of those together seemed perfect for this application. One limitation was that I needed 10 digits per channel and the driver ICs support a maximum of 8. to get around this, instead of using an entire digit for the tens digit on the lane and placing numbers I plan on using the unused decimal point of the associated ones digits to control the tens digits, reducing my requirement back to 8 digits per lane. (This is where the two segments drawing a total of 200mA comes in.)

Here is where my problem lies - I have been trying to work through the Maxim
Application Note 1196 - (**broken link removed**). Since I am using 12V to power the display and the current requirement is between 25mA and 200mA per segment (most exceeding the maximum of 40mA per segment provided by the Maxim driver ICs) I need to use the circuit shown in figure 2 (see below for a copy) of the application notes for driving the digits and the circuit in figure 4 (also below) off the application notes for driving the segments.

The figure 2 schematic shows a transistor (Q3) and a PFET (Q2) but does not provide examples of the parts that should/could be used there. Figure 4 shows a transistor (Q1) which also has no part number specified. I have been searching the web for a couple of weeks now but can find no examples of this application note being put into practice. Searching through the specifications of P channel MOSFETs has not helped, primarily because I have no real understanding on how they work. I'm also afraid that if I make a mistake I may end up destroying my MAX7219/MAX7221.

So, is there anyone that could please help me work out what values of transistors and PFET I could/should be using here?

Thanks,
David

 

[HexTheKiwi]

I was meaning to include a couple images of the prototype digits with my original post. They are about 10" (250mm) tall and can be easily read from 100m away with a viewing angle of about 120 degrees.

 

[meikjohnson]

David, I'm working on a project using the MAX7219 chip also. I'm going to use P-FET FQP17P06 for Q2 and a 2N2222 for Q3.
How did your project work out and what were the final Q3/Q2 specs you came up with? Also are your LED's high brightness and specs on those. I'm going to be using 5mm High Flux LEDs from SuperBrightLEDs. IF=40ma, MCD=5500, Pulse Current at 1khz = 100ma.
good luck
Meik

 

[HexTheKiwi]

Hi Meik,

The project has worked out well. The prototype is completed and we are now waiting for a grant for funding of the whole scoreboard system.

I ended up going with a 2N2222A for Q3 and an IRFU9214PBF for Q2.

The LEDs I'm using are not super bright, but bright enough over a wide angle (100 degrees) to be seen clearly at 75 metres around a swimming pool complex. I have sourced them from BestHongKong.com where they are item number BURLC433W100BC07. I need 18,000 of them, and have been quoted a reasonable rate for them in those quantities.

I hope this helps.

David.

 

[meikjohnson]

David
Thanks for the reply. Sounds like we both were on track with Q3 (2N2222). I might modify Q2 specs a little after the 1st proto. Probably size it down to around 5A. The sign I'm working on will be in direct sunlight so I need the HB leds. the besthongkong.com link was awesome. Lots of good prices for HB Leds. thanks for the link. Keep me posted and let me know how your scoreboard works out. Maybe a photo of the finished design. Good Luck.
Meik