Project # 17 – BCD Counter
Project # 17 – The BCD Counter
The problem with the binary counter you built in the last project is that it counts from 0000 to 1111 (that is, from 0 to 15, in decimal). To make the counter count in decimal it is necessary to have the counter count only the binary numbers 0000 to 1001 (from 0 to 9). The BCD (Binary Coded Decimal) counter accomplishes this result. In this project you will convert the binary counter into a BCD counter.
|[pic] | |
| |Parts List |
| |R4 = 2.2 K |
| |R5 = R6 = 4.7 K |
| | |
| |4520 Pinout |
| |pin 3 = Q0 (LSB) |
| |pin 4 = Q1 |
| |pin 5 = Q2 |
| |pin 6 = Q3 (MSB) |
Procedure:
1. COPY the schematic diagram onto your report.
2. Remove S1 and R4 from the binary counter you built in the last project.
3. Build the above circuit. Place the 4011 a little to the right of the binary counter. Connect Q1 and Q3 from the binary counter into the above circuit, and connect pin 4 of the above circuit into the reset of the 4520 chip.
4. At this point obtain teacher’s initials.
5. NOTE: in theory, R5 and R6 are not needed, and you will see later that they will be eliminated, however, in this circuit they are necessary because the LED’s draw so much current from the 4520 (which is not built to handle a lot of current) that when the outputs Q1 and Q3 are high the actual voltage output of Q1 and Q3 is actually below the range the 4011 chip accepts as a logic HIGH.
6. Make the counter count from 0000 to 0101 (from 0 to 5), then change it back to a 0 to 9 counter.
Conclusion:
1. In general terms, how did the above circuit convert the binary counter into a BCD counter? Why were Q1 and Q3 chosen to go into the 4011 and not the other outputs?
2. If you had made a mistake and connected Q0 and Q2 to the 4011 instead, how would you be able to tell? How high would the counter go before resetting?
3. How did you make the counter count from 0 to 5 (0000 to 0101) in step 6?
4. Look at the S1, R4 section of the above circuit leading to pin 5 of the 4011. When the switch is pressed, what will the output of the 4011 chip be? Explain using a NAND truth table.
5. When the switch is left open, the voltage at pin 5 of the 4011 is +9V (a HIGH, or a 1). What does a NAND gate become if one of its inputs is already a 1? How does this explain the fact that we were able to use NAND’s to reset the counter instead of an AND gate as shown in class?
6. Why is R4 needed?
7. Explain, in your own words, why R5 and R6 are needed.
Do not take the circuit apart. Keep this sheet and your report.
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