Basic (LED) Display

The purpose of this project is to learn more about the operation of a common-cathode, seven-segment, LED readout.

The LED manufacturer could construct the seven-segment readout with each segment having its cathode and anode leads brought out to separate pins, but this would require an excessive number of pins. Instead, seven-segment readouts are usually made with either the anodes or cathodes connected to one common pin.

Common cathode types require a common connection for the negative supply and common anode types require a common connection for the positive supply. This choice allows the circuit designer some freedom in circuit designs.

LED display segments are very small. Therefore, to obtain a line of light, a number of segment Diodes must be lined up next to each other to give the appearance of a continuous line. Some models of seven-segment readouts have a frosted lens so the individual segments can not be distinguished. Can you see the segments in this readout?

LED operation is extremely fast. An LED can be turned ON and OFF hundreds of times each second, so fast you can't see it blink.

Unlike an incandescent lamp, there is no warm up time and no great amount of heat produced by LED's. Demonstrate the fast LED action to yourself as follows.

1. Hook up the circuit but do not close the Key.

2. Decrease the surrounding ambient light to a very low level so that any LED light emission can be easily seen.

3. Close the Key for only a fraction of a second.

Notice that the display goes quickly ON and OFF. Now hold the platform steady but glance quickly across the LED display as you very briefly tap the Key. The display should appear to go abruptly ON and OFF. Actually, the persistence of the human eye is much longer than the LED's ON time, but without special instruments this gets the point across.