MAX7221 questions

Forum 2005-2010 (read only) Hardware Interfacing
1

my idea is to build a large clock with 7 segment displays, consisting of 4 4" displays and 2 2.3" displays, but now i have some questions about the max7221 i'm going to use, i recieved a few by sampling them from maxim.

on the playground page there is an table with what resistor to use with different voltages, but it won't go higher than 3.5v and i want to use these displays;

their forward voltage is 9.25v , is there a possibility of using these displays?

and another question, do you need only 1 max7221 to drive 6 displays or 6? i've got a bit confused because the pics on the playground page and different images on the internets.

2

their forward voltage is 9.25v , is there a possibility of using these displays?

Not gonna work. The MAX7221 has to power the display and it's a 5V device.

do you need only 1 max7221 to drive 6 displays or 6

The above aside, you would only need one MAX7221. According to the datasheet it can drive 8 seven-segment displays.

3

Yep the MAX7219/7221 is designed to run 6x 7-segment displays.

Here's my arduino clock running 6x 1" LED displays from futurlec

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1206916512/0

4

ok, thanks but is it possible to drive higher powered displays with these chips?

5

Not without starting to get complicated and use the outputs to drive buffers and transistors.

By that stage you may as well use a controller which is designed for higher voltages

6

i found a few displays on ebay from sure electronics based on some 74HC595 shift registers.
so, i think i'l choose them and learn a few things about shift registers in the process