ENGR 12LExperiment 16 DIGITAL ELECTRONICS – CLOCKS and COUNTERS

Step 1: Wire the 555 timer circuit with an LED as shown. It should flash about 5 times per second.

Step 2: Send the Clock Signal to the 74HC192 wire the outputs thru the resistor chip and then to the LED bar

Step 3: Displaying Count as a Numeral:

Wire in the 7 segment Decoder and the Display

REFERENCE





DIP Switch: schematic
555 Timer:
Resistors:
LED Bar:
74HC192E:

7490 Pinout

Let's look at the 7490 briefly to see how it works. Here is the pinout:

The 7490 is a decade counter, meaning it is able to count from 0 to 9 cyclically, and that is its natural mode. That is, QA, QB, QC and QD are 4 bits in a binary number, and these pins cycle through 0 to 9, like this:

QD / QC / QB / QA
0 / 0 / 0 / 0
0 / 0 / 0 / 1
0 / 0 / 1 / 0
0 / 0 / 1 / 1
0 / 1 / 0 / 0
0 / 1 / 0 / 1
0 / 1 / 1 / 0
0 / 1 / 1 / 1
1 / 0 / 0 / 0
1 / 0 / 0 / 1

You can also set the chip up to count up to other maximum numbers and then return to zero. You "set it up" by changing the wiring of the R01, R02, R91 and R92 lines. If both R01 and R02 are 1 (5 volts) and either R91 or R92 are 0 (ground), then the chip will reset QA, QB, QC and QD to 0. If both R91 and R92 are 1 (5 volts), then the count on QA, QB, QC and QD goes to 1001 (5). So:

  • To create a divide-by-10 counter, you first connect pin 5 to +5 volts and pin 10 to ground to power the chip. Then you connect pin 12 to pin 1 and ground pins 2, 3, 6, and 7. You run the input clock signal (from the timebase or a previous counter) in on pin 14. The output appears on QA, QB, QC and QD. Use the output on pin 11 to connect to the next stage.
  • To create a divide-by-6 counter, you first connect pin 5 to +5 volts and pin 10 to ground to power the chip. Then you connect pin 12 to pin 1 and ground pins 6 and 7. Connect pin 2 to pin 9 and pin 3 to pin 8. Run the input clock signal (from the timebase or a previous counter) in on pin 14. The output appears on QA, QB and QC. Use pin 8 to connect to the next stage.

Displaying the Time as Numerals

If you want to display the time as numerals, you need to use the 7447s. Here is the pinout of a 7447, as well as the segment labeling for a 7-segment LED:

You connect a 7447 to a 7490 like this:

  • Provide +5 volts on pin 16 and ground on pin 8 to power the 7447 chip.
  • Connect QA, QB, QC and QD from a 7490 to pins 7, 1, 2 and 6 of the 7447, respectively.
  • Connect 330-ohm resistors to pins 13, 12, 11, 10, 9, 15 and 14 of the 7447, and connect those resistors to the a, b, c, d, e, f, and g segments of the 7-segment LED.
  • Connect the common anode of the 7-segment LED to +5 volts.

You will need to have the pinout for the specific LED display that you use so that you know how to wire the outputs of the 7447 to the LEDs in the 7-segment device. (Also, note that the 7448 is equivalent to the 7447 except that it drives common-cathode displays. Ground the common cathode of the LED in that case.)

You can see that by extending the circuit, we can easily create a complete clock. To create the "minute hand" section of the clock, all that you need to do is duplicate the "second hand" portion. To create the "hour hand" portion, you are going to want to be creative. Probably the easiest solution is to create a clock that displays military time. Then you will want to use an AND gate (or the R inputs of the 7490) to recognize the binary number 24 and use the output of the recognizer to reset the hour counters to zero.
7447 and 7448 Pinouts

7490 and 7492 Pinouts

Experiments

The LED 7-segment display is the easiest to work with, and is a good introduction to 7-segment displays in general. The pinout of a typical display is shown at the right. The segments are usually labelled a-g as shown. The pins are the same as for a 14-pin DIP, and it can be oriented by the missing pins. This represents a common-anode display, in which the anodes of all eight internal LED's are connected together and brought out at the pins labelled "A." One or both A pins can be used. The light from each LED is spread optically to illuminate its segment. There is usually only one decimal point, to the right or the left of the digit. The pin for the missing decimal point is not connected.

To test the display, connect an anode pin to +5 V, and then connect the pin corresponding to any segment to ground through a 470Ω resistor, which will give a current of about 7 mA. This should give satisfactory illumination. The smallest current that is satisfactory should be used, since the displays use a lot of current. Don't forget that something has to limit the current through the diode! In a display connected to a computer, the 7-segment displays are usually scanned for economy. Corresponding segments are all connected together and activated simultaneously, but 5 V is only applied to one anode, so that only that display lights. The displays are activated one by one in sequence, each one displaying its segments for a short time. If 7 mA gives sufficient illumination, and there are 4 displays, then the cathode resistors are chosen to give a current of 4 x 7 = 28 mA. Up to 60 mA is permissible, if it is applied intermittently. To the eye, all the displays will appear to be illuminated.

7-segment displays are often used with BCD (binary-coded decimal) decoder chips that take a 4-bit input and have 7 outputs, one for each segment. If 0110 is input, for example, the segments to display a "6" are illuminated. The pinout of a popular BCD-to-7-segment decoder, the 74LS47A, is shown at the left. The BCD input is DCBA, and the corresponding output goes low. To use this chip, simply connect each output with the corresponding segment of the display through a 470Ω resistor, with +5 V on the common anode. The decimal point is lighted by separate circuitry, which can simply be a resistor connected to ground to light it, or leaving the pin unconnected to hide it. The chip will work as you want it to with the three control inputs left unconnected. If you ground "test" all 7 segments will light. If you ground BI, all the outputs will be high, and the display will be blank. If you ground RBI, a zero will be blanked (this is for blanking leading zeros). RBI and RBO are used for a series of digits. We will not use these functions here, but they are there if you need them.

The decoder chip steals some flexibility from you. In particular, inputs greater than hex 9 give funny outputs. They could as well have displayed A - F, as hex digits, but they do not. You can find out what happens by experiment, or in the data sheet for the chip. If you control each segment separately and explicitly, you can display anything, and I think it is best to do this if you have computer control. Without a computer, the decoder chip makes things a little easier, however. Remember that it is not necessary to use a BCD to 7-segment decoder with the display.