this is a really helpful thread, although im struggling to get my head around what the different chip are that have been used. like you i have used a max 7219 chip and have got the prototyping board all sorted out using 2 or 3 smaller seven segments.
Now im trying to move onto the larger displays and they are not bright enough. What were the chips that you have used to allow for a greater voltage to power the display?
any help would be greatly appreciated. cheers
OK, let me try to explain how it works...
Since you've been using the MAX7219 already, I don't need to explain that one. So I'll concentrate on the drivers only.
Attached is the schematics again with a few additions.
On the anode side of the LED displays I used a UDN2981. This is a 8 channel source driver. Basically a darlington array that switches the output to the voltage applied to the chip when the input is high. On the attached schematic I have "zoomed in" to one of the drivers inside the chip.
On the cathode side I used a ULN2803. This is a 8 channel sink driver. It's very similar in function to the UDN2981 but it switches to GND when the input is high. Because of that it does not need a voltage connected to it, just GND .
But because it switches to ground when the input is high, it is inverting the input signal. That's why I have just put another one in front to invert the signal again. I could have just used a TTL level inverter chip instead of the first ULN2803 but I didn't find an 8 channel one and the ULN2803 has a pin-out that is very convenient to use on strip board PCBs.
The resistors between the 2 ULN2803 are pull-ups for the inputs of the second one. The first ULN2803 only switches to ground so we need pull-ups here to provide a high state to the input of the second ULN when the output of the first one is not low.
You would need to calculate the input voltage on the UDN2981 depending on your displays. There is a combined voltage drop over the ULN and UDN of about 2V. Add that to the voltage of one segment of the display and add another 1-2Volt that will drop over the current limiting resistors. Then calculate the resistors from that. Bear in mind that on most displays the decimal point needs a much lover voltage than the other segments. Either use a higher value resistor for the point or drop the excess voltage over a reverse polarity Zener diode in line with the decimal point segment line.
Well, I hope that helps. Let me know if you have more questions.
Oh, by the way, the intensity setting inside the MAZ7219 still works with this circuit.