Driving 12 Seven Segment Displays from an Arduino

Question

I will be using the 14 input/output Arduino. My goal is to create a plane radio console for use with a flight simulator. I am very competent at writing code, but have little knowledge of electrical engineering. There are 3 readouts on the panel I am making, which will display the different frequencies. I need to be able to directly control each 7 segment display of the 12 total digits needed.

Which shift registers would I need in order to properly wire this? I want to use as little of the pins on Arduino as possible as i need to also wire up 3 rotary encoders and two On/Off switches.

Is this possible? If so, which Shift Registers do I need, and what Make/Model?

Answer 1

Twelve 74HC595s. Daisy chain them serial out (Q7S) to serial in (DS), then you can clock in 96 bits for the segments and push them all out at once.

Answer 2

Three SAA1064 would solve this problem. These are connected through I²C. The individual address of each chip is set through the address select pin. There are four possible different addresses.

On the web there are tutorials describing how to connect SAA1064 to Arduino (this, for example).

Answer 3

Assuming the LED displays are of common-cathode type, Maxim offers a couple of excellent serially interfaced 7-segment multi-digit LED drivers:

• The MAX7219 (Maxim's legacy SPI variant) and MAX7221 (fully SPI compliant) are well-known go-to parts that have been around for ever: 8 digit serially interfaced common cathode drivers. Use two of them to go all the way up to 16 digits if needed.
  • The Arduino Playground has a tutorial with wiring instructions and ready code for using the MAX7219 / MAX7221
• The Maxim MAX6958 / MAX6959 provide for 9 digits each, via an I²C serial interface, if the requirement might conceivably expand to 18 digits at some point.

The main advantage of using these dedicated LED driver ICs, over using shift registers, is that LED current setting is done via a single resistor connected to the IC, with the IC then taking care of current regulation of each LED. No more per-LED resistors!

An added advantage if you need it, is that these drivers incorporate BCD (Binary Coded Decimal) decoding, should your application require it.

A second, possibly less expensive DIY alternative is to use a dedicated microcontroller just to drive the digits, such as has been described in the excellent post by Roman Black: The No-parts LED driver!. He specifically addresses 12 digit 7-segment displays, and provides schematics and a PCB layout.