13 Experiments using the Logic Designer

Most of these experiments are carried out on the LOGIC DESIGNER Board. As they become more complex, you will need extra components and a piece of bread-board.
At the start, you will need a set of jumper leads made from single-strand hook-up wire similar to telephone wire.
For some of the more complex experiments you will have to delve into your parts-box.
For these experiments, the power supply can be set to 5v or 9v. It does not matter which voltage is selected.
These experiments will re-enforce your understanding of building blocks and reveal the capabilities of the LOGIC DESIGNER.
We will assume you have built up the board and everything is operating as specified.

CHANGING THE VOLTAGE

The in-built power supply has 3 ranges: 5v, 9v and 12v. Before changing the range, the power to the LOGIC DESIGNER must be turned off. If you don’t turn the power off, the output voltage will rise to the full input voltage of about 18v when the jumper clip is unplugged. This may damage some of the chips.
The 9v and 12v ranges are only approximate. If you require accurate voltages, you will have to trim the two resistors in the voltage dividing network. You can do this by either replacing the resistors with others from a 5% series, or soldering other resistors in parallel until the exact value of output voltage is obtained.

Wiring the blocks is carried out with jumper leads inserted into the Machine pins on the PC board.
The block diagrams in the following experiments are connected with a line. This represents a signal path and you will have to study the block diagrams to understand which machine pins are to be connected together.
We are not coming down to the stage of saying “connect point 34 to point 67” so you will have to do some thinking!!!

EXPERIMENT 1

A COUNT-OF-ONE FOR THE 7 SEGMENT DISPLAY

The aim of this experiment is to show how much noise is generated with a push-button switch. Connect any push-button you can find in your parts-box between the positive rail and the clock input of the 4026 IC.
Reset the chip with a jumper lead between HIGH (the positive rail) and the reset pin. Push the button once and see what number registers on the readout. It will possibly be any number except 1! Repeat this, a number of times to show that it is futile trying to make an ordinary push-button clock a digital circuit. You just can’t do it successfully.
The push-button must be de-bounced. This is what our one-shot circuit does. It eliminates the switch noise and provides a clean pulse via the 555 (wired as a monostable multivibrator) to drive the digital counting circuits.

ADDITIONAL EXPERIMENTS

1. Try a toggle switch and a micro switch in place of the push-switch to see if they produce less noise.
2. Connect the three switches in turn to the 4024 binary counter and note the effect of switch-noise on this chip. This will confirm the fact that mechanical switches are not compatible with digital circuits.

EXPERIMENT 2

HOW THE ONE-SHOT CIRCUIT WORKS

This is a slowed-down version of events

When the button is pressed, pin 2 of the 555 is brought LOW and starts the timing operation of the chip. It has no further effect until the time-delay cycle has been completed. Provided you push and release the button within this interval of time, you will produce 1 pulse. If you hold your finger on the button, a single clock pulse will be produced immediately after you release your finger.

The one-shot 555 can be connected to one of the 4 buffer transistors to detect the HIGH. To prove that the output is clean, you will need to use a digital chip as it is more responsive. For this experiment choose the 4024 as it will detect up to 128 noise pulses. It will readily show you if the clock pulse is free from noise.

EXPERIMENT 3

ONE-SHOT THE BINARY COUNTER

A BINARY COUNTER is very economical on parts. To obtain a readout from 0 to 128 we need only one chip and 7 LEDs. The only disadvantage is the readout. It has to be tallied to obtain the final value. This is where you have to learn the binary table. The value of each LED when lit is written on the PC overlay and once you understand the BINARY CODE it is only a matter of adding up the numbers.
Connect the one-shot 555 to the clock pin of the 4024 and bring a HIGH to the reset pin to zero the counter. No LEDs will be lit. Press the button once and the first LED will light. Press the button again and only the second LED will light up. The third pulse will light the first and second LEDs. The fourth pulse will light only the third LED and so on. Notice the pattern produced as the LEDs fill up and spill into the next row.
Manually count to 128 and confirm that the counter completes one cycle.

THE BINARY TABLE

This table can be used to check the values of the 128 readouts.

EXPERIMENT 4

ONE-SHOT THE NUMERIC COUNTER

Connect the 555 one-shot to the 4026 with a jumper lead. Reset the 4026 via a HIGH from the power line to give a zero readout on the display and remove the HIGH.
Press the button a number of times to get the feel of how it operates the 555. Press it quickly 10 times and see if the display is responding accurately to the count. Press the button slowly and see if any switch bounce is coming through. If the counter is being falsely triggered, the time delay can be increased by increasing the 47k to 150k or the 1mfd to 2u2. The one-shot is designed to give a maximum of 10 pulses per second and to produce a clean pulse from the switch on each of these cycles.
If the button is released before the time interval is complete, the pulse will be emitted some time after the button has opened. This is why the digital circuit does not have any direct connection with the push switch.

EXPERIMENT 5

CYCLING THE COUNTERS

The LOGIC DESIGNER has two counters, a decimal counter and a binary counter. These can be clocked by the 10Hz oscillator individually or both at the same time.
The object of this experiment is to watch the clocking of the two displays and determine the best to display the 10Hz.
Set the readout of each to zero by bringing a HIGH to the reset pins from one of the line-voltage pins marked on the top edge of the board. This will produce a zero on the displays. The reset must now be removed to enable the chips to clock. If either chip is reset when any figure is showing on the display, the readout will return to zero. If the HIGH is maintained while the chip is being clocked, the readout will still be zero when the HIGH is removed. In other words, the reset over-rides the clock signal and prevents the chip from performing any further counting operations.

EXPERIMENT 6

USING THE DECADE DIVIDER

The 4026 contains 2 sections. The first is a DECADE DIVIDER (divide by 10) and it is this section we are interested in.
The other section is a 7-segment decoder which is capable of driving a 7-segment display. This section is left operating but is not used.
The decade divider has two outputs. Refer to the circuit diagram and note pin 5 is the carry-out and pin 14 is un-gated ‘c’ out. Both of these pins go HIGH-TO-LOW every 10 cycles but they operate at different times.
Pin 5 is CARRY OUT and its signal is derived from the final section of a 5 stage walking ring counter. It is HIGH for counts 0 through 4 and LOW for counts 5 through 9. Therefore it will trigger a positive-edge (ground-to-positive transition) IC when 9 changes to zero.
Pin 14 is classified as ‘2-out’. It goes LOW only when the 2 is displayed as this is the only time the ‘c’ segment is not lit.
Either output can be used as a divider since they do not have to synchronize with the numeric display.
Take the CARRY OUT to Buffer A and note the time it is HIGH.
Take the UN-GATED ‘c’ output to Buffer A and note the HIGH time.

EXPERIMENT 7

CARRY-OUT TIMING AND ‘c’ TIMING

Connect the ONE-SHOT to the clock input of the 4026 and connect the CARRY-OUT to the clock input of the 4024. Reset the FND500 to get a zero by bringing a HIGH lead to the reset pin of the 4026 and reset the binary counter via the same HIGH lead. Remove this jumper so that both counters will cycle.
Count the pulses as you press the push button and note the timing of the count when the first LED comes on.
Timing is one of the most important design points in digital circuits. In a large computer all the sections must be designed to accept a count pulse at the correct moment and for this we use a master clock. In this experiment we are noting just when the change occurs.
Now use the un-gated ‘c’ segment of the 4026 and again note the timing of the change.

EXPERIMENT 8

PICKING UP THE 50 Hz AC

Normally the 50Hz AC is beyond reach as it is usually tied in with the 240v AC. But cunningly, we have made it available in this project at terminal B of the TRANSISTOR TESTER.
You are quite safe with this 50 cycle frequency as it comes via the isolating transformer and has a peak-to-peak of only about 20v.
This part of the experiment makes you aware of the speed of a 50Hz frequency and how it can be divided down with a binary counter.
The two LEDs in the transistor tester are operating on 50Hz but as this frequency is above your eyes Persistence of Vision, you cannot detect the off time and thus you think they are on continually. It’s only when you move the board around that you notice the flicker. By dividing the frequency down you will begin to see the flickering.
The B terminal (or any of the 3 TRANSISTOR TESTER terminals) has 20v P-P on it, with respect to the earth line of the LOGIC TESTER.
It is important to use a 47k to 100k limiting resistor in series with the jumper to limit the effect of over-voltage on the chips. IC’s do not like voltages above rail voltage or below zero volts. If these do occur, the protection diodes in the input gates come into operation.
If you connect terminal B of the TRANSISTOR TESTER via a 47k resistor to the input of the 4024, you will see the frequency being divided down to 25 Hz, 12½Hz etc. Connect any of the 7 outputs of the 4024 to the input of the 4026 and you will have a count-recording of the number of times each output goes HIGH. Obviously the slowest count will occur when the 7th output is connected to the numeric counter.
Work out just how long it will take for the FND500 to register 10 counts (after the circuit has passed its first cycle).

EXPERIMENT 9

USING THE TRANSISTOR TESTER

The transistor tester is designed to test both PNP and NPN transistors. To really prove its worth, you would need to have bought a large number of odd types at a special price. All we can really tell you is the polarity and collector, base, emitter leads. In the first instance you should try a good PNP type and NPN type to get an understanding of how it works. Then you can test an unknown type or one with the numbers rubbed off.

EXPERIMENT 10

DECODING THE 4024

We have seen how the 4024 is capable of halving or quartering a frequency to give a readout on a set of 7 LEDs. But suppose you require a divide-by-five? Can you see how this is possible with the binary counter?
If you gate the first output and the third output together with a pair of diodes as shown in the diagram, you will produce an AND gate and this will give a HIGH when both outputs are HIGH. This AND gate is connected to buffer ‘A’ to detect the divide-by-five.
At this stage we must manually reset the counter after the counting operation as the circuit will not cycle as a divide-by-five counter without a reset line (this will come later).
Now you can see the versatility of a binary counter.
A point to note with the identification of the 7 readouts is this:
The first LED is labeled ‘1’ and this is its value when summing up all the outputs during the first cycle. After the first cycle, the first output is a divide-by-two. The second output is a divide-by four and this continues to the last output which is a divide-by-128. These values only come into operation after the first complete cycle has been completed.

EXPERIMENT 11

MAKING A DIVIDE-BY-1280 COUNTER

Although a 1280 counter is not of any great importance, it does show the enormous divisions which are possible when counters are connected together. High divisions are very common in electronics. The simple example of your electronic watch shows the enormous amount of division it performs. A good electronic watch is capable of adjusting for 28 days in February every 4 years!
It would be excessive for you to single-step the count via the push-switch any more than one complete cycle, but carry out the operation just once. You will then have some idea of how long it takes to count 1,000 or so. You will then appreciate the 10Hz clock.
When you have single-stepped this sequence, connect the input of the 4026 via a 47k resistor to the B terminal of the TRANSISTOR TESTER and connect the CARRY-OUT to the 4024.
Turn the power on and wait for the 7th LED to come on. It will have a 50/50 ON-OFF time which will total about 1 minute. The OFF time will be about 30 seconds.

EXPERIMENT 12

MAKING AN ACCURATE “SECONDS” CLOCK

An accurate 1 second readout can be obtained by using 3 gating diodes to decode the 4024 at outputs 2,16 and 32, to obtain a divide-by-50. This is how the circuit works:
The diodes form an AND gate so that when outputs 2, 16 and 32 are HIGH, the 4026 will clock one count and at the same time reset the 4024.
Since we have LEDs in the outputs of the 4024, the voltage at these points will not rise enough (due to the loading effect of the LED) to clock the 4026 and reset the 4024. We must therefore provide a means of increasing this voltage to at least ½Vdd. This is achieved by placing 3 diodes in series between the two chips.
The voltage drop across the three diodes is 1.8v and this is added to the 4 volts produced by the 4024 (when the three outputs are HIGH).
This will produce about 6v to clock the 4024. The clock and reset line will see a voltage range from 2.4v to 6v and will interpret this as a LOW and a HIGH.
The diodes and resistor can be mounted on a bread-board or simply fitted into the machine pins and soldered together in a birds-nest above the Logic Designer.
This will give you a 0 - 9 seconds clock which is, in effect, a 10 seconds clock.
You will notice a number of arrows on the block diagram to help you follow the direction of the signal. Don’t go by the arrow symbols on the diodes, they don’t indicate the signal direction, just the cathode end of the diode.
The trickiest part is understanding a feed-back line such as the reset line. In these lines, the signal travels from right to left. All other signals flow from left to right as shown by the counting signal arrow.
The 47k limiting resistor in the clock line of the 4024 reduces the AC signal from the power transformer so that the chip can limit the negative portion of the waveform to allow counting to take place.
When you build the circuit and get it operating correctly, don’t leave it there. Try some more experimenting. One simple experiment it to produce a 20 seconds clock by dividing the 4024 by 100. Try a few other arrangements too. By now you should be making a few new discoveries yourself. Even if they don’t follow the strict rules of digital electronics, it doesn’t matter. You are here to learn and discover. This circuit, for instance, is not an allowed arrangement in the laws of digital design since we are creating a HIGH and LOW artificially on the input of the 4026 and this is not providing a full rail-voltage swing. Nor are we providing a clean square-wave input to the 4024. But at this stage, if It works, we will let it be. Later-on it can be refined.

EXPERIMENT 13

“FREEZE THE COUNT”

A simple game can be constructed using the binary counter and the 50Hz, with a 555 one-shot as the freeze control.
When the circuit is wired as shown, the LOW on the output of the 555 will not affect the counting of the 4024. By pushing the button, a HIGH is produced and this is transferred to the input of the 4024, to freeze the count, since it will prevent the input going LOW.
The object of the game is to decide which 2 or 3 LEDs are to be “frozen on” and then count the attempts to be successful.
If the diode is reversed as shown in the diagram, the count will be frozen until the switch is pressed. This will give you a reverse effect and you can try you skill at reaction-time by looking at the 7-segment display and selecting one of the numbers. When this number appears, you will then have to freeze the bottom set of binary readouts on some particular pattern as previously arranged.
Either way, the circuit will provide an interest as you match your skills against an opponent or improve your reaction-time, when playing solo.

CONCLUSION

This concludes our experiments using the LOGIC DESIGNER project. But I am sure you will agree this is only the beginning.
The more circuits you build, the more ideas you get.
The next stage is to combine the LOGIC DESIGNER with your own circuits. You can start with some circuits described in this e-magazine. Power them from the LOGIC DESIGNER. Use the buffers and one-shot to analyse how they work as you clock them through their sequences in slow motion. Some circuits are especially suited to slow clocking.
There are lots of circuits suitable for use with the DESIGNER project. It only takes experimenting to find them.