Switch on the power supply and touch the free end of the positive wire to each row of holes serving the display on its left and right sides. You should see each segment light up, as shown in Figure 4-31.
Figure 4-31. After putting a 1K resistor between the common cathode of the display and the negative supply voltage, you can use the positive supply voltage to illuminate each segment in turn.
Each numeral from 0 to 9 is represented by a group of these segments. The segments are always identified with lowercase letters a through g, as shown in Figure 4-32. In addition, there is often a decimal point, and although we won’t be using it, I’ve identified it with the letter h.
Figure 4-32. The most basic and common digital numeral consists of seven LED segments identified by letters, as shown here, plus an optional decimal point.
Check Figure 4-33 showing the Kingbright display, and you’ll see I have annotated each pin with its function. You can step down the display with the positive wire from your power supply, making sure that each pin lights an appropriate segment.
Figure 4-33. This Kingbright unit incorporates three seven-segment numeric displays in one package, and can be driven by three chained 4026 decade counters. The pin numbers are shown close to the chip. Segments a through g of numeral 1 are identified as 1a through 1g. Segments a through g of numeral 2 are identified as 2a through 2g. Segments a through g of numeral 3 are identified as 3a through 3g.
Incidentally, this display has two pins, numbered 3 and 26, both labeled to receive negative voltage for the first of the digits. Why two pins instead of one? I don’t know. You need to use only one, and as this is a passive chip, it doesn’t matter if you leave the unused one unconnected. Just take care not to apply positive voltage to it, which would create a short circuit.
A numeric display has no power or intelligence of its own. It’s just a bunch of light-emitting diodes. It’s not much use, really, until we can figure out a way to illuminate the LEDs in appropriate groups—which will be the next step.
Step 2: Counting
Fortunately, we have a chip known as the 4026, which receives pulses, counts them, and creates an output designed to work with a seven-segment display so that it shows numbers 0–9. The only problem is that this is a rather old-fashioned CMOS chip (meaning, Complementary Metal Oxide Semiconductor) and is thus sensitive to static electricity. Check the caution on page 172 before continuing.
Switch off your power supply and connect its wires to the top of the breadboard, noting that for this experiment, we’re going to need positive and negative power on both sides. See Figure 4-34 for details. If your breadboard does not already have the columns of holes color-coded, I suggest you use Sharpie markers to identify them, to avoid polarity errors that can fry your components.
Figure 4-34. When building circuits around chips, it’s convenient to have a positive and negative power supply down each side of your breadboard. For the reaction timer circuit, a 9V supply with a 100 µF smoothing capacitor can be set up like this. If your breadboard doesn’t color-code the columns of holes on the left and right sides, I suggest you do that yourself with a permanent marker.
The 4026 counter chip is barely powerful enough to drive the LEDs in our display when powered by 9 volts. Make sure you have the chip the right way up, and insert it into the breadboard immediately above your three-digit display, leaving just one row of holes between them empty.
The schematic in Figure 4-35 shows how the pins of the 4026 chip should be connected. The arrows tell you which pins on the display should be connected with pins on the counter.
Figure 4-36 shows the “pinouts” (i.e., the functions of each pin) of a 4026 counter chip. You should compare this with the schematic in Figure 4-35.
Include a tactile switch between the positive supply and pin 1 of the 4026 counter, with a 10K resistor to keep the input to the 4026 counter negative until the button is pressed. Make sure all your positives and negatives are correct, and turn on the power. You should find that when you tap the tactile switch lightly, the counter advances the numeric display from 0 through 9 and then begins all over again from 0. You may also find that the chip sometimes misinterprets your button-presses, and counts two or even three digits at a time. I’ll deal with this problem a little further on.
The LED segments will not be glowing very brightly, because the 1K series resistors deprive of them of the power they would really like to receive. Those resistors are necessary to avoid overloading the outputs from the counter.
Grounding Yourself
To avoid the frustration that occurs when you power up a circuit and nothing happens, be sure to take these precautions when you use the older generation of CMOS chips (which often have part numbers from 4000 upward, such as 4002, 4020, and so on):
Chips are often shipped with their legs embedded in black foam. This is electrically conductive foam, and you should keep the chips embedded in it until you are ready to use them.
If the chips are supplied to you in plastic tubes, you can take them out and poke their legs into pieces of conductive foam or, if you don’t have any, use aluminum foil. The idea is to avoid one pin on a chip acquiring an electric potential that is much higher than another pin.
While handling CMOS components, grounding yourself is important. I find that in dry weather, I accumulate a static charge merely by walking across a plastic floor-protecting mat in socks that contain some synthetic fibers. You can buy a wrist strap to keep yourself grounded, or simply touch a large metal object, such as a file cabinet, before you touch your circuit board. I am in the habit of working with my socked foot touching a file cabinet, which takes care of the problem.
Never solder a CMOS chip while there is power applied to it.
Grounding the tip of your soldering iron is a good idea.
Better still, don’t solder CMOS chips at all. When you’re ready to immortalize a project by moving it from a breadboard into perforated board, solder a socket into your perforated board, then push the chip into the socket. If there’s a problem in the future, you can unplug the chip and plug in another.
Use a grounded, conductive surface on your workbench. The cheapest way to do this is to unroll some aluminum foil and ground it (with an alligator clip and a length of wire) to a radiator, a water pipe, or a large steel object. I like to use an area of conductive foam to cover my workbench—the same type of foam that is used for packaging chips. However, this foam is quite expensive.
Figure 4-35. IC3 is a 4026 counter. IC4 is a triple seven-segment display chip. The arrows tell you which pins on the LED display should be connected to the pins on the counter.
Figure 4-36. The 4026 decade counter is a CMOS chip that accepts clock pulses on pin 1, maintains a running total from 0 to 9, and outputs this total via pins designed to interface with a seven-segment LED numeric display.
Fundamentals
Counters and seven-segment displays
Most counters accept a stream of pulses and distribute them to a series of pins in sequence. The 4026 decade counter is unusual in that it applies power to its output pins in a pattern that is just right to illuminate the segments of a 7-segment numeric display.
Some counters create positive outputs (they “source” current) while others create negative outputs (they “sink” current). Some seven-segment displays require positive input to light up the numbers. These are known as “common cathode” displays. Others require negative input and are known as “common anode” displays. The 4026 delivers positive outputs and requires a common cathode display.
Check the data sheet for any counter chip to find out how much power it requires, and how much it can deliver. CMOS chips are becoming dated, but they are very useful to hobbyists, because they will tolerate a wide range of supply voltage—from 5 to 15 volts in the case of the 4026. Other types of chip
s are much more limited.
Most counters can source or sink only a few milliamps of output power. When the 4026 is running on a 9-volt power supply, it can source about 4mA of power from each pin. This is barely enough to drive a seven-segment display.
You can insert a series resistor between each output pin of the counter and each input pin of the numeric display, but a simpler, quicker option is to use just one series resistor for each numeral, between the negative-power pin and ground. The experiment that I’m describing uses this shortcut. Its disadvantage is that digits that require only a couple of segments (such as numeral 1) will appear brighter than those that use many segments (such as numeral 8).
If you want your display to look bright and professional, you really need a transistor to drive each segment of each numeral. An alternative is to use a chip containing multiple “op amps” to amplify the current.
When a decade counter reaches 9 and rolls over to 0, it emits a pulse from its “carry” pin. This can drive another counter that will keep track of tens. The carry pin on that counter can be chained to a third counter that keeps track of hundreds, and so on. In addition to decade counters, there are hexadecimal counters (which count in 16s), octal counters (in 8s), and so on.
Why would you need to count in anything other than tens? Consider that the four numerals on a digital clock each count differently. The rightmost digit rolls over when it reaches 10. The next digit to the left counts in sixes. The first hours digit counts to 10, gives a carry signal, counts to 2, and gives another carry signal. The leftmost hours digit is either blank or 1, when displaying time in 12-hour format. Naturally there are counters specifically designed to do all this.
Counters have control pins such as “clock disable,” which tells the counter to ignore its input pulses and freeze the display, “enable display,” which enables the output from the chip, and “reset,” which resets the count to zero.
The 4026 requires a positive input to activate each control pin. When the pins are grounded, their features are suppressed.
To make the 4026 count and display its running total you must ground the “clock disable” and “reset” pins (to suppress their function) and apply positive voltage to the “enable display” pin (to activate the output). See Figure 4-36 to see these pins identified.
Assuming that you succeed in getting your counter to drive the numeric display, you’re ready to add two more counters, which will control the remaining two numerals. The first counter will count in ones, the second in tens, and the third in hundreds.
In Figure 4-37, I’ve continued to use arrows and numbers to tell you which pins of the counters should be connected to which pins of the numeric display. Otherwise, the schematic would be a confusing tangle of wires crossing each other.
At this point, you can give up in dismay at the number of connections—but really, using a breadboard, it shouldn’t take you more than half an hour to complete this phase of the project. I suggest you give it a try, because there’s
something magical about seeing a display count from 000 through 999 “all by itself,” and I chose this project because it also has a lot of instructional value.
Figure 4-37. This test circuit, laid out as you would be likely to place it on a breadboard, allows you to trigger a counter manually to verify that the display increments from 000 upward to 999.
Component values:
All resistors are 1K.
S1, S2, S3: SPST tactile switches, normally open
IC1, IC2, IC3: 4026 decade counter chips
IC4: Kingbright 3-digit common-cathode display
C1: 100 µF (minimum) smoothing capacitor
Wire the output pins on IC1, IC2, and IC3 to the pins on IC4, according to the numbers preceded by arrows. The actual wires have been omitted for clarity. Check for the pinouts of IC4.
S1 is attached to the “clock disable” pin of IC1, so that when you hold down this button, it should stop that counter from counting. Because IC1 controls IC2, and IC2 controls IC3, if you freeze IC1, the other two will have to wait for it to resume. Therefore you won’t need to make use of their “clock disable” features.
S2 is connected to the “reset” pins of all three counters, so that when you hold down this button, it should set them all to zero.
S3 sends positive pulses manually to the “clock input” pin of the first counter.
S1, S2, and S3 are all wired in parallel with 1K resistors connected to the negative side of the power supply. The idea is that when the buttons are not being pressed, the “pull-down” resistors keep the pins near ground (zero) voltage. When you press one of the buttons, it connects positive voltage directly to the chip, and easily overwhelms the negative voltage. This way, the pins remain either in a definitely positive or definitely negative state. If you disconnect one of these pull-down resistors you are likely to see the numeric display “flutter” erratically. (The numeric display chip has some unconnected pins, but this won’t cause any problem, because it is a passive chip that is just a collection of LED segments.)
Always connect input pins of a CMOS chip so that they are either positive or negative. See the “No Floating Pins” warning on the next page.
I suggest that you connect all the wires shown in the schematic first. Then cut lengths of 22-gauge wire to join the remaining pins of the sockets from IC1, IC2, and IC3 to IC4.
Switch on the power and press S2. You’ll see three zeros in your numeric display.
Each time you press S3, the count should advance by 1. If you press S2, the count should reset to three zeros. If you hold down S1 while you press S3 repeatedly, the counters should remain frozen, ignoring the pulses from S3.
Fundamentals
Switch bounce
When you hit S3, I think you’ll find that the count sometimes increases by more than 1. This does not mean that there’s something wrong with your circuit or your components; you are just observing a phenomenon known as “switch bounce.”
On a microscopic level, the contacts inside a pushbutton switch do not close smoothly, firmly, and decisively. They vibrate for a few microseconds before settling; the counter chip detects this vibration as a series of pulses, not just one.
Various circuits are available to “debounce” a switch. The simplest option is to put a small capacitor in parallel with the switch, to absorb the fluctuations; but this is less than ideal. I’ll come back to the topic of debouncing later in the book. Switch bounce is not a concern in this circuit, because we’re about to get rid of S3 and substitute a 555 timer that generates nice clean bounceless pulses.
Pulse Generation
A 555 timer is ideal for driving a counter chip. You’ve already seen how to wire a 555 to create a stream of pulses that made noise through a loudspeaker. I’m reproducing the same circuit in Figure 4-38 in simplified form, using the positive and negative supply configuration in the current project. Also I’m showing the connection between pins 2 and 6 in the way that you’re most likely to make it, via a wire that loops over the top of the chip.
For the current experiment, I’m suggesting initial component values that will generate only four pulses per second. Any faster than that, and you won’t be able to verify that your counters are counting properly.
Install IC5 and its associated components on your breadboard immediately above IC1. Don’t leave any gap between the chips. Disconnect S3 and R3 and connect a wire directly between pin 3 (output) of IC5 and pin 1 (clock) of IC1, the topmost counter. Power up again, and you should see the digits advancing rapidly in a smooth, regular fashion. Press S1, and while you hold it, the count should freeze. Release S1 and the count will resume. Press S2 and the counter should reset, even if you are pressing S1 at the same time.
Figure 4-38. A basic astable circuit to drive the decade counter in the previous schematic. Output is approximately 4 pulses
per second.
R7: 1K
R8: 2K2
C2: 68 µF
C3: 0.1 µF
IC5: 555 timer
No Floating Pins!
A CMOS chip is hypersensitive. Any pin that is not wired either to the supply voltage or to ground is said to be “floating” and may act like an antenna, sensitive to the smallest fluctuations in the world around it.
The 4026 counter chip has a pin labeled “clock disable.” The manufacturer’s data sheet helpfully tells you that if you give this pin a positive voltage, the chip stops counting and freezes its display. As you don’t want to do that, you may just ignore that pin and leave it unconnected, at least while you test the chip. This is a very bad idea!
What the data sheet doesn’t bother to tell you (presumably because “everyone knows” such things) is that if you want the clock to run normally, the clock-disable feature itself has to be disabled, by wiring it to negative (ground) voltage. If you leave the pin floating (and I speak from experience), the chip will behave erratically and uselessly.
All input pins must be either positively or negatively wired, unless otherwise specified.
Refinements
Now it’s time to remember that what we really want this circuit to do is test a person’s reflexes. When the user starts it, we want an initial delay, followed by a signal—probably an LED that comes on. The user responds to the signal by pressing a button as quickly as possible. During the time it takes for the person to respond, the counter will count milliseconds. When the person presses the button, the counter will stop. The display then remains frozen indefinitely, displaying the number of pulses that were counted before the person was able to react.
Make: Electronics Page 24
