A Seven Segment Display (SSD) or Seven Segment Indicator is an electronic display device1 often used to display a single digit number in decimal or hexadecimal form. This post briefly talks about it.
This is what it looks like :
A decimal system uses 10 symbols to represent all numbers :
0 1 2 3 4 5 6 7 8 9
The seven elements of the display can be lit in different combinations to represent the Arabic numerals2.
0 1 2 3 4 5 6 7 8 9
- Often we arrange them in an oblique (slanted) way, which aids readability.
0 1 2 3 4 5 6 7 8 9
By slanting digits, we’re making sure that we don’t mistake 7 for a 1 and vice-versa.
Hexadecimal number system uses 16 symbols to represent all numbers :
0 1 2 3 4 5 6 7 8 9 A B C D E F.
A = 10,
B = 11,
C = 12,
D = 13,
E = 14,
F = 15.
To display hexadecimal numbers :
0 1 2 3 4 5 6 7 8 9 A B C D E Fwith a SSD, we needed to think of a way to display letters with our 7 segments without creating an ambigous or non-unique shape.
Today, we use a combination of uppercase and lowercase letters to display
A b (c C) d E F. Otherwise, a capital ‘D’ would look identical to a ‘0’ and a capital ‘B’ would look identical to an ‘8’. However, this modern scheme wasn’t followed in the past.
Also the digit ‘6’ must be displayed with the top bar lit to avoid ambiguity with the letter ‘b’.
- A 7 segment display has segments which we label as :
a b c d e f gfor our ease.
- In the device, corresponding to each segment there’s a pin which sends ON or OFF signal to that segment.
- To display an ‘8’ all segments must be switched ON.
- To display a ‘2’,
a b g e dmust be on.
- Similarly, we can eye-ball rest of the conditions and make a list or a truth table.
Common Cathode & Common Anode SSDs
- When we say Common Cathode or Common Anode in relation to Segment Displays, we assume that all the segments in our SSD are LEDs. A Light Emitting Diode looks like this :
Much like their
diodecousins, LEDs are polarized. There are a handful of identifiers for finding the positive and negative pins on an LED. You can try to find the longer leg, which should indicate the positive, anode pin.
Or, if someone’s trimmed the legs, try finding the flat edge on the LED’s outer casing. The pin nearest the flat edge will be the negative, cathode pin.
But, what may be the meanings of Common Anode SSDs & Common Cathode SSDs ? The difference between the two displays, as their name suggests, is that the common cathode has all the cathodes of the 7-segments connected directly together and the common anode has all the anodes of the 7-segments connected together and are illuminated as follows:
In CC SSDs, all cathode (negative) pins are connected together to logic
0, which means segments are illuminated by supplying logic
1to respective pins.
In CA SSDs, all anode (positive) pins are connected together to logic
1, which means segments are illuminated by supplying logic
GNDto respective pins.
A display device is an output device for presentation of information in visual or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information that is supplied has an electrical signal, the display is called an electronic display. ↩
Arabic or Hindu–Arabic numerals, are the ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, based on the Hindu–Arabic numeral system, the most common system for the symbolic representation of numbers in the world today. They are descended from the Hindu-Arabic numeral system developed by Indian mathematicians around AD 500. ↩
LCD or liquid-crystal display is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals. Liquid crystals do not emit light directly, instead they use a backlight or reflector to produce images in colour or monochrome. ↩
LED or light-emitting diode is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated. When a suitable current is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. ↩