13 Jun 2026
BCD-to-Seven-Segment Display Using IC 7447
Aim
To study the operation of a BCD-to-seven-segment decoder and display decimal digits from 0 to 9.
Apparatus
Digital trainer, BCD switches, decoder IC 7447 or equivalent, common-anode seven-segment display, resistors, and regulated supply.
Experimental arrangement
Theory
A decimal digit has ten possible values, whereas four binary bits have sixteen possible combinations. Binary-coded decimal (BCD) uses only $0000$ through $1001$ to represent decimal 0 through 9. A seven-segment display contains LED segments labelled $a$ to $g$; a numeral is formed by illuminating a particular combination, for example $a,b,c,d,e,f$ for 0 and $b,c$ for 1.
The IC 7447 is a combinational BCD-to-seven-segment decoder/driver. Its four inputs have weights 1, 2, 4, and 8, and its seven outputs are determined solely by the present BCD word. It is designed to drive a common-anode display. All LED anodes are connected to $+V_{CC}$, while each 7447 output sinks current through a selected segment. Consequently, its segment outputs are active LOW: output 0 turns a segment on and output 1 turns it off.
The 7447 uses open-collector outputs, so each segment requires an external current-limiting resistor. If the supply is $V_{CC}$, the LED drop is $V_F$, and the desired current is $I_s$, a suitable value is approximately
\[R_s=\frac{V_{CC}-V_F-V_{OL}}{I_s}.\]Inputs $1010$ to $1111$ are not valid BCD digits and need not produce decimal symbols. The lamp-test and blanking pins must be held at their inactive logic levels during the normal truth-table test.
Observations
| Decimal digit | BCD input | Illuminated segments |
|---|---|---|
| 0 | 0000 | abcdef |
| 1 | 0001 | bc |
| 2 | 0010 | abdeg |
| 5 | 0101 | acdfg |
| 9 | 1001 | abcdfg |
Calculation
For the observed digit 5, the BCD input is
\[0101_2=0(2^3)+1(2^2)+0(2^1)+1(2^0)=5_{10}.\]The decoder therefore activates segments $a,c,d,f,g$, which gives the numeral 5. Because the display is common-anode and the 7447 outputs are active low, these five segment outputs are LOW while the unused outputs remain HIGH. The codes $1010_2$ to $1111_2$ are not used for decimal digits.
Result
The decoder displays the correct decimal digit for each valid BCD input.
Viva Questions
- Why are resistors connected to the segments? To limit LED segment current.
- How many segments are used? Seven, labelled $a$ through $g$.
- What is an invalid BCD code? Any four-bit code from 1010 to 1111.
Discussion