- BCD to Seven Segment Decoder Definition: A BCD to seven-segment decoder is defined as a circuit that converts BCD input into signals to drive a seven-segment display.
- Types of Seven Segment Displays: There are two types: common cathode and common anode displays, each requiring different driving signals.
- Common Cathode Display: In a common cathode display, all cathodes are tied together and grounded, so segments light up with a high input.
- Common Anode Display: In a common anode display, all anodes are tied together and powered, so segments light up with a low input.
- Truth Table: The truth table for the decoder indicates which segments to drive high or low to display the correct digit on the seven-segment display.
8421 BCD (Binary Coded Decimal) assigns one four-bit code to each decimal digit: 0000 through 1001 represent 0 through 9. Codes 1010 through 1111 are not valid decimal-digit BCD codes. A Seven segment displays uses segments a through g to form digits. LED versions use seven Light Emitting Diodes; LCD versions need a different alternating drive method.
An LED display is commonly wired as common cathode or common anode. The decoder or driver must match that polarity, supply and required segment current.
In Figure 2a, all cathodes of the LED segments share a return connection. A segment lights when its anode is driven sufficiently positive through a current-limiting path. In Figure 2b, all anodes share the positive supply voltage. A segment lights when its cathode driver sinks current. Logic level alone is not enough; output-current ratings and a resistor or other current control must also be correct.
A BCD to seven-segment decoder maps four BCD inputs A, B, C and D to segment outputs a through g. Table I gives an ideal active-high pattern for a common-cathode display. A physical device may also have latch, blanking and lamp-test inputs, so its truth table and data sheet take priority.
Table I: ideal active-high BCD to seven segment decoder pattern
A high in this table selects a segment of the common-cathode seven segment display. An ideal common-anode pattern uses the complementary selected level, but the circuit cannot be chosen by inverting this table alone. For example, an SN74LS47 uses active-low open-collector outputs, while a CD4511 uses active-high outputs for common-cathode displays. The device truth table also defines blanking, lamp test and the result of invalid BCD inputs 1010 through 1111.
