usenetfan:
I am thinking of how to put together a large countdown timer. I have it working fine with one of those four digits in one block type displays, using a 74HC595 chip.
A 74HC595 is not a display driver. It is inappropriate for the purpose and most of such "displays" are using the chip well beyond its safety ratings.
usenetfan:
Now the challenge is that the big ones take 9 volts and thus cannot be powered by the Arduino. See:
Digits on eBay
Nice display but a trifle expensive. Four LEDs per segment, red, will require a TPIC6B595 to drive each digit with the anode connected to 12 V and a current limiting resistor in each of the segment lines from the TPIC6B595. The TPIC6B595s are cascaded in exactly the same fashion as the 74HC595, three pins of the Arduino will control them all.
Nothing - apart from a few LEDs at 20 mA each - is ever "powered" by the Arduino.
usenetfan:
So, I am slightly baffled on where do I supply the 9V?
12 V - to the anodes of all the displays.
Let me check out the resistor value for you ... 20 ma over 4 V drop, 220 Ohms for each segment, 470 Ohms for the decimal point.
usenetfan:
Should I plug it in the Arduino and branch out voltage and ground wires before the plug to gain common ground?
All the grounds of the TPIC6B595s are connected together to the 12 V power supply ground and the Arduino ground. You will need a 5 V DC regulated supply for the Arduino - do not attempt to use the "Vin" or "barrel jack" to power it from the 12 V; a separate switchmode "buck" regulator would be appropriate to power it all from 12 V.
usenetfan:
Attached is a copy of the code.
Please read the forum instructions to see how to post code here.