Displays

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LED Displays

All LED displays have the following properties ...

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.

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.