7 Segment Displays

In the figure is a 7-segment display.

The segments are labeled as follows:



(Note that the diagram actually has two decimal points as well, so there are technically nine segments. One or both decimal points may be included.) These segments are individual LEDs, labeled a to g, and can be turned on individually, or all controlled by a BCD to 7 segment decoder.



The decoder takes a 4 bit BCD input, and has 7 outputs corresponding to the display segments. It will assert the correct signals for the segments based on the input.

There are two types of displays; common anode and common cathode. The distinction is whether the inputs need a "high" or a "low" to turn on.

• If the device is common anode, there will be an anode pin which must be pulled to V_cc, and then any segment to be turned on must be pulled "low".
  
  
  
  
• If the device is common cathode, there will be an cathode pin which must be grounded, and then any segment to be turned on must be pulled "high".
  
  
  
  

In either case, since each segment of the display is an LED, you need to use resistors on the segments just as you would for discrete LEDs. Otherwise you will probably destroy the device when you try to use it.

Here is a reminder of how the pins on an LED are identified:



The resistor can be put either before or after an LED, as long as the current flows in the right direction.


Since all of the LED segments are connected to the common pin, you could use a single resistor between the common pin and power or ground, (depending on whether the display is common anode or common cathode), to limit the current to all of the LEDs. The only drawback with this is that you might notice the display is dimmer when more segments are on.

(Hint: If you choose to use individual resistors, you can use the resistors to connect the decoder to the display, instead of having wires to connect the decoder to the resistors and wires to connect the resistors to the display then you will save a lot of wiring.)