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All LED displays have the following properties ...
- High current consumption.
- Bright and easy to see in the dark and low light.
- Harder to see in bright light or sunshine.
- Fast response to data changes.
- Structurally robust so they withstand knocks and vibration.
Seven Segment Displays
- Able to display digits (0 to 9) and a very limited range of of text characters.
LED Matrix Displays
- Using multiplexing techniques, up to 64 LEDs can be connected to two 8 bit microcontroller ports.
- Most text characters can be displayed clearly and crude graphics too.
Here is a display with seven rows and five columns. The red LED is lit if C3 is LOW and R2 is HIGH.
- The microcontroller scans the columns taking each column low in turn.
- While each column is low, the microcontroller sends suitable data to the row lines.
- This happens so fast the the eye can not see the flickering.
- When viewed from a distance, you might notice the display doing odd things.
- This is caused by eyeball vibrations making the multiplexing noticeable.
- The letters seem to bob up and down.
Microcontroller Multiplexing Connections
- Each bipolar transistor is wired up in a simple switch circuit.
- The resistors limit the base current to a safe low value.
- To pull a column low, the transistor is switched on. A high PORTB output will cause this.
- Can display graphics.
- Slow to respond to data changes.
- Very low power consumption (a battery can last years).
- Easy to see in bright light or sunshine.
- Rather fragile and easily damaged.
- Can't be seen in the dark unless there is a back-light.
- Does not work at extreme temperatures below -5C and above about 60C.
- The liquid crystals rotate the polarisation of light when a potential difference is applied. This alters the blackness of the pixel.
- These displays are difficult to interface without purpose designed microcontrollers.
- The PICAXE displays are very easy to interface because the controller chips are built-in.
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