Basics Of 7 Segment Displays

Even though LCD displays are sometimes preferred because of their low power requirements, lower costs, and straightforwardness of designing into a circuit, the standard 7 segment LED display is still usually utilized in a large number of applications because of their higher temperature range, visibility, bright colors, and wide viewing angle. The seven segments in a basic LED display are customarily labeled as: the letters a to g and dp, where dp is the decimal point. So has a grand total of eight control lines are wanted to turn each one of these segments off or on individually by connecting to one end of each of these segments and the decimal point.

There are two variations of 7 segment displays available, the common-anode and the common cathode. The common-anode display configuration has all anodes for each segment wired in common which is then usually connects to the positive voltage supply. The cathode ends of each LED are individually hooked up to the control lines which need to be current sinks that are switched off and on thru transistors or an LED driver chip. In a “common cathode”, the common cathode is typically connected to ground, and the control lines for each segment are current sources which are used to switch on and off individual segments.

So a 7 segment and decimal point package will only require 9 pins, but they typically come in 10-pin packages because the common pin is wired into two pins. Since each of the segments acts as a forward-biased diode, a current limiting resistor is needed between the driver transistor and the LED segment to restrict the maximum current flow to a safe value.

An IC driver chip such as the 74LS47 can be used for common anode LED types, while a 74LS48 is can be used for common cathode LED types. Most driver chips, such as the MC14511 have a decoder built in, so a binary number input is converted to the 7 segment control format.

Some driver chips, for example the CAT4008 or CAT4016 from Catalyst Semiconductor, Inc, update the 7 segment control format through a latch attached to a shift register, where the binary data is clocked in serially using a single pin.

A single LED driver chip can drive more than one seven segment display by a method called “multiplexing.” In this method, the extra displays are connected in parallel, while the “common” line is taken out separately from each display and hooked up to a switching circuit which turns each line (and hence each display) off and on for 1 or 2 milliseconds in sequence for each display. This lowers the pin count, but has some downsides, such as lower brightness and flickering displays.

Embedded Adventures is a great website to get information about microcontrollers as well as useful modules such as a 7 segment LED display, or an alphanumeric LED display.