Controller Circuit

A seven-segment display is an integrated circuit package with seven light emitting diodes (LEDs) in it. The seven segments (LEDs) are arranged in a figure eight and given the standard letter designations a-g as shown in the figure below. The way to remember the letters is that they start at the top, increase clockwise ,and the crossbar is last.

Many seven-segment displays also have an eighth LED for a decimal point. This LED is often labeled RHP (for right-hand point) or LHP (for left-hand point) on pin-out diagrams. On RHP displays the decimal point is to the right of the figure 20 and on LHP displays to the left. Diodes (including light-emitting diodes) have two terminals called the anode and the cathode. The LED lights when the diode is forward biased, which occurs when the anode is at higher voltage than the cathode. The LED does not light when the anode and cathode are at the same voltage or if the LED is reversed biased (anode at lower voltage than cathode).

Since the seven-segment display contains seven diodes we would expect that there would be 14 pins corresponding to seven anodes and seven cathodes (16pins if there is a decimal point). However actual seven-segment displays almost always have a common terminal. A common-anode display has the anode terminals of every LED tied together internally. A common-cathode display has the cathode of every LED tied together internally. All of the LEDs in the project are common anode. Common-anode LEDs are more convenient to use since they can be driven by open-collector devices whereas common-cathode LEDs can not. A diagram of the pin-out of a common-anode seven segment display is shown in the figure below. Notice that there are often two internally-connected common pins that help with current-carrying capacity. There would be nine pins on this package (2 commons and the 7 pins for the a-g cathodes).

To make the 7-segment display active, we need to connect the common-anode pins to high voltage. The individual segments may now be lit by connecting the individual cathodes to ground or not lit by connecting the cathode to either the high voltage of by letting it float. One problem is that the LEDs will use to much power and possibly burn out if connected directly across a 5V power supply. To reduce power consumption and protect the diodes from damage, we use a current-limiting resistor. A 330Ω resistor will work well in this lab (220Ω and 470Ω are also reasonable). This will keep the current in each LED down to about 10-15 milliamps (mA). The figure below shows the use of seven current-limiting resistors with a common-anode display which is always enabled. The pin-out for the 7-segment display is normally as shown below. Notice that there are places where you would expect pins but there are no pins on the display package. Also, depending on which display is used, either RHP, LHP, or both may not be implemented.
Tags:- seven segment data sheet,seven segment display datasheet,seven segment display interfacing,seven segment display decoder,seven segment display circuits,seven segment datasheet,seven segment led digit delphi,seven segment digital clock