Did I get this right?

Hi Meden,

Unless you are using your other Arduino pins for something else, you definately don't need the shift register to drive the transistors. A 270R resistor from base to three output pins on the Arduin will work just fine.

You don't need the other shift register either, unless your LEDs are high current, you can just use eight outputs from the Arduino through the series resistors, as long as you are not taking more than 40mA per segment.

For example software for driving 4 digits in this way, download the sketches from http://www.arduinoevilgenius.com and look at project 30.

The completed project will be a climate control for a car. Consisting of 10 buttons to control acuators and a servo. The  system will read inside and outside temp. and adjust acordingly. I was  thinking of using another shift register for the buttons. I'd like to  keep the pin usage as minimal as possible and code lean and tight.

Hi,

10 butons - 7 pins (buttons in 4 x 3 array - there is an Arduino library for key matrix)

Actuator and servo - 2 pins

LED segments - 8 pins

LED common transistors - 3 pins

Makes 20 pins

Only 18 available on the Arduino - you can use the Analog Ins as outputs too.

So maybe keep the shift register for the segments.

The more hardware, the more chance of something failing. Once the software is right, it stays right!

Or go with a LCD in 4 bit mode to save some pins. Lower current as well and you can get nice backlit versions. Then of course you have actual text messages instead of numbers to make it more friendly. The LCD is really easy to use with the library. My son managed to talk to the LCD in only a few nights and he had never done any controller work in the past. Give it a try.

Another possible thing is to multiplex the 8 lines for the LED segments with button scanning. You can scan the buttons so quickly the eyes will never detect you actually turned off the LED's and were using the bits to scan buttons! I've done this type of thing in the past but it adds complexity as you need to add a 8 input/16 output/1 control line mux to the wiring. Use two of these:

For the LCD, there is also the option of using something like Sparkfun's LCD backpack (http://www.sparkfun.com/products/258). Reduces the number of pins down to one by sending all the data over the TX line. It is essentially just a pic with their software on so not too hard to do yourself if you feel inclinded to do so. Where the number of pins is an issue, it is certainly useful!

The car this is going in is a Lamborghini Countach 25th Anniversary which had an LED display that I'm attempting to replicate. The second version will use a LCD display and be used in a Diablo Replica. I'm looking into possibly using the analog pins as digital pins using this ...

http://arduino.cc/en/Tutorial/AnalogInputPins

Thanks for your help simon, jnissen, and John

Yeah....  I'm gonna need pics of the car/installation to confirm that you're doing this the right way.