PH337 Electronics
Experiment #9

BINARY CODED DECIMAL (BCD) Counter with Display LAB

OBJECTIVES

In this exercise you will build a "single digit BCD Counter". You'll also cascade your counter with others in the class to make a multi-digit counter.

DISCUSSION


BCD Counter
The basic elements of the decade counter are all incorporated into a single DIP package the 7490. This high-speed monolithic decade counter consists of four master-slave JK flip flops that are internally connected to provide a ctivide by two and a divide by five counter. Attached are the manufacturer's spec sheets that show the external connections, the internal functional structure, and the truth table for this series of BCD counter. You have the N type package.

Decoder Driver
After converting the number of pulses to a binary coded decimal, it is necessary to convert these four binary digits to a readable form. This is done with a "BCD to seven segment decoder/driver". You'll use the 7447 to logically combine the BCD bits to seven outputs (a,b,c,d,e,f, and g). The outputs of this decoder driver are "open collector" and are used to drive a common anode LED display. This LED display indicates the digits O to 9 by turning on the the appropriate LED segments,

PROCEDURE


Assembly:

1. Refer to figure one to assemble your circuit. Figure one shows a two digit counter.


You will assemble just a single digit counter; i.e, just the top half of the schematic.

2. Remember that these are 'TTL levels, so do not exceed 5.2 v.

3. You'll need to refer to figure 2 to see how to connect to the common anode LED displav, This puts the voltage across the correct LED segments.

3. After assembly, put in a 5 v square wave on pin 14 of the 7490. If the tiequency is greater than 5 or 10 Hz, it will appear as though ail display segments are illuminated, i.e, it'll look like an "8". By interrupting the input you should be able to get other digits to display.

Decade Counter Functioning.

5. After you have connected your circuit and it appears to work then investigate the functioning of the 7490 decade counter. Specifically, measure the voltages at the 4 output lines, A (pin 12), B (pin 9), C (pin 8), and D (pin 11) to verify the the truth table, BCD count sequence that's shown on the manufacturer's specification sheet.

Do this by connecting the oscilloscope to externally nigger on pin Il (output D). Put the input signal on channel 1. Connect, in turn, channel 2 to outputs A, B, C, and D and note the relationship between channel 1 and channel 2. With this information, verify the truth table on the mfgr's spec sheet.



The oscilloscope probes with the alligator clips "load" the counter output, You can tell when this occurs by watching the LED display for an erroneous count. We have a limited number of higher impedance probes that don't load the circuit. Call for them when you ate ready

Two or more digit counter

6. Cooperate with another set of students to do this next section. Make a two digit BCD counter as shown in t'tgure 1.
That is, make an additional connection from pin ii to the input of a second 7490 on its pin 14. [Use one common power supply for both sets of circuits!!!l Note which is the least significant digit when you connect a square wave input and count the pulses. Explain what happens by interpreting the output truth table.

Question: Draw a schematic of a logic circuit that you could use to start and stop this BCD counter, Assume that the signal you want to count is oscillating countinuously. You want to "gate" the input to pin 1·1 with a logic element to start.and stop the counter. Do not use a mechanical switch