To multiplex 7 segment displays, you will need the following: 1. 7 segment displays
-- I'm using 3 x 4.0 Inch Super Red 7 Segments from Kingbright (SA40-19SRWA)
I strongly suggest you purchase COMMON ANODE
displays. Common anode means all the anodes (+) pins are connected. You apply + voltage to the anode and use shift registers to ground the segments and form a complete circuit. Very simple!
However, with common cathode, all the ground (-) pins are connected. You then use shift registers to divert power to the anodes of the segments. However, the problem with this setup, as I've learned the hard way, is that you need to worry about sourcing
current. Most uControllers and shift registers cannot source nor sink a lot of current. Otherwise, you'd burn it out. If you require more voltage or current, you'll then need to worry about transistors or darlington arrays (external drivers) since you're using shift registers to tell them which segments need power (high voltage or current) and when to ground it. In other words, the hardware and code get more complicated and drives up cost. 2. Microcontroller
I strongly suggest getting an Arduino. The environment is much more intuitive and there is a huge pool of resources out there if you get stuck. If you're prone to making mistakes, get the Ruggeduino. It's only $10 more than Arduino Uno and protects you and your precious uController from stupid mistakes. 3. Serial-In Parallel-Out Shift Register
If you have the money, buy from the TPIC6x595 or TPIC6x596 family of shift registers by Texas Instruments. I use the TPIC6B
596 in this instructable. The difference between its siblings (A, B, and C series) is the current handling capacity. In addition, the 596 family provides better reliability in cascading applications. When choosing shift registers, always make sure you do not exceed their current-handling limits. Side notes:
- I would avoid the popular 74HC series as it can only source/sink a max of 70mA through the chip and cannot handle high voltages.
- I would also avoid using the common cathode / MAX7219/7221 setup with high voltage displays. Trust me... It's not worth it! You don't want to know the trouble I've been through with this setup. Even though there's a good library out there, it's best to understand and have control of the underlying mechanism behind shift registers and multiplexing. 4. Regulated DC Power Adapter
If you're planning to drive high-power displays, consider buying an regulated dc adapter that is greater than the forward voltage of your display. A regulated DC adapter provides stable voltage under any load. Just make sure its current rating is greater than what's required. In most cases, ratings higher than 500mA are enough (higher is better). 5. Resistors
You always need resistors to turn down current going through the LEDs.
The formula for calculating required resistor is:
(Supply Voltage - Minimum or Typical Forward Voltage per segment) / Desired forward current in Amps
Note that the desired current should always be a bit less than the absolute maximum forward current stated in the datasheet in order to extend the life of the LEDs. Don't forget that you need to do a separate calculation for the decimal point which may have a lower forward voltage requirement.
Also, be careful with datasheets of large displays
. If they show low forward voltages (< 5V), that rating may be for the individual LED
in a segment rather than the entire segment. So, for example, if there are 5 LEDs in series per segment and the datasheet shows a forward voltage of 2.6V for a 5 inch display, you probably need to multiply it by 5 to get the correct forward voltage for the entire segment. Here
is one such example. It's more complicated if it's series/parallel arrangement which is beyond the scope of this instructable. 6. Breadboard and jumper wires
I recommend buying large solderless breadboards and a lot of jumper wires of various configurations (Male to Male, Male to Female). They are also known as dupont cables.