System Description (D)

The Decoder is used as our row control unit. We connected the select input pins to our micro controller. With this we are able to select which row should be active. The decoder is switched through the six rows at very high speeds. This allows the display to appear as though all 6 rows are active at once.

We use six of the output pins Y0 - Y5. Each pin is connected to an inverter connected to the base of a ZTX458 transistor. The selected output pin provides a low. This is inverted to a high to turn on the connected transistor. While the other 5 outputs are inverted and provide a low and 'cut off' state - turning off their corresponding transistors and the path to ground. The transistors are each connected from their collector to the cathode end of 24 parallel LEDs in the display. The emitter of the transistor is grounded. Thus when a row is selected the LEDs in that row are provided with a LOW/ground at their cathode end.