What does it do?
| The 7-segment display is used to display numbers. |
 |
How does it operate?
|
7 Segment Display Circuit
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
A 7 segment display consists of seven LEDs (called ‘segments’), arranged so that when they are turned on and off they display the digits 0, 1, 2, …, 9. Many displays have an extra LED for the ‘dot’.
Usually 7 segment displays are driven by a PIC or by a 4026 counter.
The seven resistors are chosen to limit the current into the 7 segment display to a safe level. For use with a PIC a value of 270R is suitable. In the case of a 4026 counter the current should be limited to 3.2mA and 1k resistors are needed.
In the circuit diagram shown on the left the 7 segment display used is called a ‘common cathode’ display. This means that all the seven LEDs have their cathode (negative terminal) connected to 0V. So the current to illuminate a segment flows from a high signal on the output pin of the driver, through the current-limiting resistor, through the LED and down to 0V.
‘Common anode’ 7 segment displays are available but are more difficult to work with.
|
|
7 Segment Display with Decoder-Drvier circuit
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
Most counter ICs e.g. the 4029B, produce a binary output pattern. To show the corresponding digit on a 7 segment display a decoder-driver IC needs to be added.
One suitable decoder driver is the 4511B, which is suitable for driving ‘common cathode’ 7 segment displays. Unlike typical CMOS ICs it can provide up to 20mA to drive the 7-segment display.
A PIC can drive a 7 segment display without a decoder driver. Used in this way, the display can also show some letters as well as numbers:
However, the programming is quite complicated.
To display two digits using a PIC a pair of 4026B counters and 7 segment displays can be used. An alternative is to use eight pins on the PIC (forming a single ‘port’) and to send a binary coded decimal number to this port. A pair of 4511B decoder drivers can then be used to display the number on two 7 segment displays.
|
|
|
To identify each of the seven LEDs that make up the digits, they are given letters. |
|
How the seven LEDs are lettered
|
So, for example, to display a ‘1’, segments ‘b’ and ‘c’ are on; the others are off. |
Possible applications
- Showing the number of objects that have been counted
- Showing the time in seconds
Making
|
How part of the PCB might look
|
Pin connections of the Rapid 57-0127
common cathode seven segment display
|
The diagram above shows the relationship between the segments and the IC pins for the 7 segment display (Rapid order code 57-0127) shown in the PCB diagram. The Decimal Point (DP) is not usually used.
Note that different 7 segment displays have different pin arrangements.
Build and test the unit that will provide the input signals before building the 7 segment display.
 |
It is not usually possible to use a Dual In Line (DIL) socket for the display because of its shape and size. Use strips of Single In Line (SIL) sockets, snapped to the correct length. |
Before inserting the IC, connect the power supply and use a voltmeter to check that:
- the voltage on pin 8 is low (0V);
- the voltage on the input pins (the blue PCB tracks) go high and low in response to the unit that provides the input signals.
Before inserting the 7 segment display, make sure that you have identified pin 1.
Testing
Make sure that the display shows all the digits from 0 to 9 correctly.
Fault finding
If there is a fault, investigate if it is isolated to one or two segments; if so, check all the tracks and solder joints for the faulty segment(s).
Alternatives
- Liquid crystal displays (LCDs) can be used to display digits and also letters, but they are more expensive.
- Bar graph displays can be used to give a visual indication of a how something varies, but they do not give a numerical reading.
Web links
Return to list of datasheets
