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Digital Inputs

Introduction

Logic inputs for experimental circuits are easy to produce. Just connect an input to either +5 volts or ground, and you have it. However, sometimes it is desirable to have the input signal sources gathered together in one place so they can be located and monitored easily, and checked as needed. When you're doing this on a breadboard socket, you can still use a set of jumper wires for the purpose. But on a formal experimental platform, you'll need a set of logic switches of one kind or another.

Some commercial platforms use standard slide switches in this application. However, even small slide switches take up a significant amount of space and still won't fit on a breadboard socket. I still don't like jumper wires for this particular application, so I use a little construct called a dip switch instead.



An 8-station dipswitch

The DIP Switch

Dip switches are small blocks that look very much like ICs in some ways. The difference, as shown to the right, is that the block contains a series of switches. (They may be either miniature slide switches or rocker switches.) This provides a simple and neat way to mount logic switches on the breadboard socket or experimental platform. The spacing between pin rows is 0.3", just like a standard IC. I've seen them in packages of two, three, four, five, eight, and ten switches, so you can get whatever suits your need.



Schematic diagram of a logic switch.

Schematic Diagram

The logic switch is an extremely simple circuit. We begin with a 1K pull-up resistor to +5 volts. This matches the 1K output resistor we will use in most of our experimental circuits to demonstrate the different logic families.

If the switch is open, the circuit applies a logic 1 to whatever input is connected to that switch. If the switch is closed, it overrides that by directly grounding the connection, thus forcing its output to logic 0. Thus, this circuit can be used to manually control the state of a digital signal.



Parts List

For this circuit, we'll use an 8-station DIP switch so we can be sure of having all the input signals we'll need for our experiments. The complete list of parts is:



Constructing the Circuit

You will construct this circuit on your breadboard socket just to the right of your LED indicators but to the left of the center. There's just enough room here to install your logic switches. You'll also be installing jumpers to supply power to the right-hand side of the breadboard socket.



Circuit Assembly

Start assembly procedure


























Getting Started

We will install the logic switches just to the right of the LED indicator circuits, which are partly shown in the assembly diagram to the right for reference. There is just enough room for this to the left of the center divider. This will leave the entire right-hand side of your breadboard socket available for experimental circuits.

To provide power for these experimental circuits, we will start by adding jumpers to connect the right side bus strips to the left side strips which are already powered.

Click the `Start' button below to begin.

0.3" Red Jumper

Prepare a 0.3" red jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Red Jumper

Prepare a 0.3" red jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

The four jumpers you have just installed provide power and ground connections to the right side of your breadboard socket. You will need these to perform the various experiments.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

Click on the jumper image to move on to the next step.

0.3" Black Jumper

Prepare a 0.3" black jumper as shown above, and install this jumper in the location indicated in the assembly diagram to the right.

By this time, you may be getting the feeling that breadboarding consists mostly of installing jumpers all over the place. Sometimes it can seem that way, but it is not really the case. In fact, at this point you only have one very short jumper left to install.

Click on the jumper image to move on to the next step.

0.1" Bare Jumper

If you have a leftover clipped resistor lead, this is a perfect opportunity to put it to use. If not, cut a ½" length of bare hookup wire. In either case, bend it in half as shown here, and install this short jumper across adjacent columns as shown in the assembly diagram.

Click on the image of the jumper you just installed to continue.

1K, ¼-Watt Resistor

Locate a 1K, ¼-watt resistor and form its leads to a spacing of 0.5" as shown above. Install this resistor in the location indicated to the right.

Click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate a second 1K, ¼-watt resistor and form its leads to a spacing of 0.5". Install this resistor in the location indicated to the right.

Click on the image of this resistor to continue.

1K, ¼-Watt Resistor

Locate a third 1K, ¼-watt resistor, form its leads to a spacing of 0.5", and install this resistor in the location indicated to the right.

Again, click on the image of this resistor to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor and form its leads to a spacing of 0.5" as shown above. Install this resistor in the location indicated to the right.

Click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor and form its leads to a spacing of 0.5" as shown above. Because there is no contact in the +5 volt bus strip for this column of contacts, you'll need to install this resistor on a slight diagonal as shown to the right.

Click on the image of this resistor to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor and form its leads to a spacing of 0.5". Again, you'll need to install this resistor on a slight diagonal as shown to the right.

Click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate another 1K, ¼-watt resistor and form its leads to a spacing of 0.5". Once more, you'll need to install this resistor on a slight diagonal as shown to the right.

Click on the image of this resistor to continue.

1K, ¼-Watt Resistor

Locate one more 1K, ¼-watt resistor and form its leads to a spacing of 0.5". As before, you'll need to install this resistor on a slight diagonal as shown to the right.

Click on the image of this resistor to continue.

The DIP Switch

Install the DIP switch across the center channel as shown to the right, just to the right of the indicator LEDs already in place. Note especially that the switch block is installed so that the switches will be on or closed when pulled towards you, and off or open when pushed away from you. When your assembly is completed, a switch will connect the corresponding input signal to ground (logic 0) when it is on, but will allow that signal to rise to +5 volts (logic 1) when it is off.

Be sure to place the switch so that its contact pins are aligned with the resistors and jumpers you just installed. When the DIP switch is positioned correctly, gently press it down so its pins enter their appropriate contact holes. Do not force it down. If the DIP switch does not seat easily, you may have a pin out of alignment. Check all pins again and then try once more to seat the switch block. Then, click on its image to continue.

Assembly Complete

This completes the assembly of your logic switches. In your experiments, you will be making various connections to these switches. To identify them accurately, we will designate them as S0 through S7, with S0 being on the right as shown in the assembly diagram.

Now, scroll on down the page to test the operation of your logic switches and verify that they all work correctly.

Restart assembly procedure


Testing the Logic Switches

You will test your logic switches by connecting each of them in turn to LED indicator L0 and then manipulating that switch. To accomplish this, cut a 3" length of orange hookup wire and remove ¼" of insulation from each end. In your experiments, you'll use several of these to make short connections to the inputs of your experimental circuits. You'll also make and use some 6" orange jumpers to make longer input connections. For now, however, you'll only need one short jumper.

Connect one end of your 3" orange jumper to the L0 input, and the other end to S0. Turn on power and observe L0. Turn S0 on and off as you continue to observe the LED. You may find it helpful to use a pen or pencil, or a similar object, to manipulate the switch. Determine which position of the switch turns the LED on, and which position turns it off.

Remove the orange jumper from S0 and connect it to S1 instead. Again, manipulate the switch as you observe L0. Does S1 behave in the same way as S0?

Repeat this action for each of the remaining switches in turn as you continue to observe L0. Do all switches behave in the same way?

Turn off power when you have checked all eight switches.



Discussion

You should have found that all switches behaved in exactly the same way. With the switch in the up (open) position, L0 turned on, indicating the presence of a logic 1 signal. With the switch in the down (closed) position, that input was grounded and L0 turned off.

If the direction of operation of your switches was reversed from this, you need to remove the DIP switch, reverse it, and then re-install it in the same location. If one switch was unable to turn L0 off, remove the DIP switch and check the pins. One of the pins for that switch may have been bent under the body of the DIP switch rather than making contact with the breadboard socket. This can easily happen if you forced the DIP switch into place.

If one of the DIP switches leaves L0 off under all circumstances, try removing and reinserting the jumper, and then the 1K resistor associated with that switch. It is possible that a speck of dirt is preventing a good electrical connection.

When you are sure that all eight switches are working correctly, go on to verify the layout of your breadboard socket prior to performing the upcoming experiments.


Prev: Logic Indicators Next: Verifying the Test Setup

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