How Electronic Gates Work

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How Electronic Gates Work
by Marshall Brain

If you have read the article on Boolean Logic, then you know that digital devices depend on Boolean gates. You also know from that article that one way to implement gates involves relays. However, you are probably aware that no modern computer uses relays - they all use "chips".

What if you want to experiment with Boolean gates and chips? What if you would like to build your own digital devices? It turns out that it is not that difficult. If you have ever asked yourself "How can I experiment with Boolean gates myself? How can I build my own digital devices?" then this edition of How Stuff Works will really help. In this edition we will see how you can experiment with all of the gates discussed in the Boolean Logic article. We will talk about where you can get parts, how you can wire them together, and how you can see what they are doing. In the process you will open the door on a whole universe of technology that is some of the most powerful and interesting in the world.

Setting the Stage
In the Boolean Logic article, we looked at 7 fundamental gates. These gates are the building blocks of all digital devices. We also saw how to combine these gates together into higher-level functions like full adders. If you would like to experiment with these gates so you can try things out yourself, the easiest way to do it is to purchase something called TTL chips and quickly wire circuits together on a device called a solderless breadboard. Let's talk a little bit about the technology and the process so you can actually try it out!

If you look back at the history of computer technology, you find that all computers are designed around Boolean gates. The technologies used to implement those gates, however, have changed dramatically over the years. The very first electronic gates were created using relays as discussed in the Boolean Logic page. These gates were slow and bulky. Vacuum tubes replaced relays. Tubes were much faster but they were just as bulky and they were also plagued by the problem that tubes burn out (like light bulbs). Once transistors were perfected (transistors were invented in 1947), computers started using gates made from discrete transistors. Transistors had many advantages: high reliability, low power consumption and small size compared to tubes or relays. These transistors were "discrete devices", meaning that each transistor was a separate device. Each one came in a little metal can about the size of a pea with three wires attached to it. It might take 3 or 4 transistors and several resistors and diodes to create a gate.

In the early 1960's, Integrated Circuits (ICs) were invented. Transistors, resistors and diodes could be manufactured together on silicon "chips". This discovery gave rise to SSI (Small Scale Integration) ICs (Integrated Circuits). An SSI IC typically consists of a 3mm-square chip of silicon on which perhaps 20 transistors and various other components have been etched. A typical chip might contain 4 or 6 individual gates. These chips shrank the size of computers by perhaps a factor of 100 and made them much easier to build.

As chip manufacturing techniques improved, more and more transistors could be etched onto a single chip. This led to MSI (Medium Scale Integration) chips containing simple components like full adders made up of multiple gates. Then LSI (Large Scale Integration) allowed designers to fit all of the components of a simple microprocessor onto a single chip. The 8080 processor, released by Intel in 1974, was the first commercially successful single-chip microprocessor. It was an LSI chip that contained 4,800 transistors. VLSI (Very Large Scale Integration) has increased the number of transistors steadily ever since. The first Pentium processor was released in 1993 with 3.2 million transistors, and current chips can contain up to 20 million transistors.

In order to experiment with gates, we are going to go back in time a bit and use SSI ICs. These chips are still widely available and are extremely reliable and inexpensive. With them you can build anything you want one gate at a time. The specific ICs we will use are of a family called TTL (Transistor Transistor Logic, named for the specific wiring of gates on the IC). The chips we will use are from the most common TTL series, called the "7400 (seventy four hundred) series". There are perhaps 100 different SSI and MSI chips in the series, ranging from simple AND gates up to complete ALUs (arithmetic logic units).

7400 series chips are housed in DIPs (Dual Inline Packages). As pictured on the right, a DIP is a small plastic package with 14, 16, 20 or 24 little metal leads protruding from it to provide connections to the gates inside. The easiest way to construct something from these gates is to place the chips on a solderless breadboard. The breadboard lets you wire things together simply by plugging pieces of wire into connection holes on the board. Nothing could be simpler.

A Solderless Breadboard

All electronic gates need a source of electrical power. TTL gates use 5 volts for their operation. The chips are fairly particular about this voltage, so we will want to use a clean, regulated 5 volt power supply whenever working with TTL chips. Certain other chip families, such as the 4000 series of CMOS chips, are far less particular about the voltages they use. CMOS chips have the additional advantage that they use much less power. However, they are very sensitive to static electricity and that makes them less reliable unless you have a static-free environment to work in. Therefore we will stick with TTL here.

Assembling Your Equipment
In order to play with TTL gates, you must have several pieces of equipment. Here's a list of what you will need to purchase:

A breadboard
A volt-ohm meter (also known as a multimeter)
A logic probe (optional)
A regulated 5 volt power supply
A collection of TTL chips to experiment with
Several LEDs (Light Emitting Diodes) to see outputs of the gates
Several resistors for the LEDs
Some wire (20 to 28 guage) to hook things together.

These parts together might cost between $40 and $60 or so, depending on where you get them.

Let's walk through a few details on these parts to make you more familiar with them:

As described and pictured above, a breadboard is a device that makes it easy to wire up your circuits.
A volt-ohm meter lets you measure voltage and current easily. We will use it to make sure that our power supply is producing the right voltage.
The logic probe is optional. It makes it easy to test the state (1 or 0) of a wire, but you can do the same thing with an LED.
Of all the parts described above, all are easy except the 5 volt power supply. No one seems to sell a simple, cheap 5 volt regulated power supply. You therefore have two choices. You can either buy a surplus power supply from Jameco for something like a video game and use the 5 volt supply from it, or you can use a little power-cube transformer and then build the regulator yourself. We will talk through both options below.
An LED (Light Emitting Diode) is a mini light bulb. You use them to see the output of a gate.
We will use the resistors to protect the LEDs. If you fail to use the resistors the LEDs will burn out immediately.

This equipment is not the sort of stuff you are going to find at Wal-Mart. However, it is not hard to obtain these parts. You have three choices when trying to purchase the components listed above:

Radio Shack - with thousands of stores across America, Radio Shack is a convenient place to go.
A local electronics parts store - Most major cities have electronics parts stores, and many cities are blessed with good surplus electronics stores. If you can find a good surplus store in your area that caters to people building their own stuff, then you have found a goldmine.
A mail-order house like Jameco - Jameco has been in business for decades, has a good inventory and good prices. Jameco also has a great web site, so ordering on the web is easy. Be sure to download their PDF catalog or get a paper catalog from them - it makes it much easier to traverse the web site.

The following table shows you what you need to buy, with both Radio Shack and Jameco part numbers listed. In the Jameco column, you can click on several of the links to go look at the part.

Part name	Radio Shack	Jameco
A breadboard	276-174	20722
A volt-ohm meter	22-163*	119212
A logic probe (optional)	22-303	149930*
A regulated 5 volt power supply	See below	See below
7400 (NAND gates)	276-2801	48979
7402 (NOR gates)	276-2802	49015
7404 (NOT gates)	276-2804	49040
7408 (AND gates)	276-2805	49146
7432 (OR gates)	276-2813	50235
7486 (XOR gates)	276-2819	50665
Several (5-10) LEDs	276-041**	94529**
Several (5-10) 330 ohm resistors	271-1315	30867
Some wire (20 to 28 guage).	278-1222	36-767

For the Power Supply, if desired
(See next section for details):

Part name	Radio Shack	Jameco
Transformer (7 to 12 volts, 300ma)	273-1455	149964
7805 5 volt voltage regulator (TO-220 case)	276-1770	51262
Two 470 microfarad Electrolytic Capacitors	272-958	93817

Notes:

* - In the case of the 2 items with a "*", the "*" indicates that the same part costs significantly less at one place or the other. The logic probe is half the price at Radio Shack ($17 vs. $35), while the volt-ohm meter is half the price at Jameco ($15 vs. $35). In all other cases Radio Shack is more expensive, but if there is a store right down the street you can get a part in a few minutes as opposed to a few days. That convenience is often worth a few bucks.
** - Both companies also have "assorted LEDs" (or grab bags) that are much cheaper on a per-LED basis. Look around and see what they have. This is one place where a surplus electronics shop will have much better prices.
Note that the gates from Jameco are standard 7400 series, while those at Radio Shack are 74HCT00 components. TTL ICs come in the following classes: 7400, 74C00, 74S00, 74LS00, 74HC00, 74HCT00, and probably a few others. The parts at Radio Shack are HCT, meaning CMOS. That means low power consumption but high sensitivity to static electricity. The 7400 or 74LS00 parts are the most abuse-tolerant, and you may prefer to get those from Jameco. (LS parts consume much less power and are just as rugged - use them if you have a choice)
If you are shopping at Jameco you may want to get 2 or 3 of each chip just in case - they only cost 30 cents each or so. You might also want to purchase an extra 7805 or 2 as well.
You will also need a pair of wire cutters and wire strippers. In a pinch you can use scissors and your fingernails, but having an official tool makes it easier. You can get wire cutters and wire strippers at Radio Shack, Jameco or Wal-mart. I also find that a small pair of needle nose pliers are helpful at times.

A resistor, an LED and an electrolytic capacitor

The Power Supply

Using a Volt-Ohm Meter
A volt-ohm meter (also known as a multimeter) measures voltage, current and resistance. It has two "leads" (wires), one black and one red. What we want to do with the meter right now is learn how to measure voltage. To do this find a AA, C or D battery to play with (not a dead one) and we will use it as a voltage source.
Every meter is different, but in general here's the steps to get ready to measure a battery's voltage:

Take your black test lead and insert it in the hole marked (depends on the meter) "Common", "Com", "Ground", "Gnd" or "-" (minus).
Take your red test lead and insert it in the hole marked (depends on the meter) "Volts", "V", "Pos" or "+" (plus). Some meters have multiple holes for the red lead - make sure you use the one for volts.
Turn the dial to the "DC Volts" section. There will usually be multiple voltage ranges available in this section - on my meter the ranges are 2.5 volts, 50 volts, 250 volts and 1000 volts (fancy auto-ranging meters may set the range for you automatically). Your meter will have similar ranges. The battery will have a voltage of 1.25 volts, so find the closest voltage greater than 1.25 volts. In my case that is 2.5 volts.
Now hold the black lead to the negative terminal of the battery and the red lead to the positive terminal. You should be able to read something close to 1.25 volts off the meter. It is important that you hook the black lead up to negative and the red lead up to positive and stay in the habit of doing that.
Now you can use the meter to test your power supply as well. Change the voltage range if necessary and then connect the black lead to ground and the red lead to what you presume to be the positive 5 volt wire. The meter should read 5 volts.

You will definitely need a regulated 5 volt power supply to work with TTL chips. As mentioned previously, neither Radio Shack nor Jameco seem to offer a standard, inexpensive 5 volt regulated power supply. One option you have: You can buy from Jameco something like part number 116089. This is a 5 volt power supply from an old Atari video game. If you look in the Jameco catalog you will find that they have about 20 different surplus power supplies like this producing all sorts of voltages and amperages. You need 5 volts at at least 0.3 amps (300 milliamps)(you need no more than 2 amps, so do not purchase more power supply than you need). What you can do is buy the power supply, then cut off the connector and get access to the 5 volt and ground wires. That will work fine, and is probably the easiest path. You can use your volt meter (see sidebar) to make sure the power supply produces the voltage you need.

Your alternative is to build a 5 volt supply from a little power-cube wall-transformer. What you need is a transformer that produces 7 to 12 DC volts at 100 milliamps or more. Note that the transformer MUST produce DC voltage. It MUST produce 7 to 12 volts. It MUST produce 100 milliamps (0.1 amps) or more. You may have an old one lying around that you can use - read the imprint on the cover and make sure it meets all three requirements. If not, you can purchase a transformer from Radio Shack or Jameco.

Radio Shack sells a 9 volt 300 milliamp transformer (part number 273-1455). Jameco has a 7.5 volt 300 milliamp model (part number 149964). Clip the connector off the transformer and separate the two wires. Strip about a centimeter of insulation off both wires. Now plug the transformer in (once it is plugged in, NEVER let the two wires from the transformer touch one another or you are likely to burn out the transformer and ruin it). Use your volt meter (see the sidebar) to measure the voltage. You want to make sure that the transformer is producing approximately the stated voltage (it may be high by as much as a factor of 2 - that is OK). Your transformer is acting like a battery for you, so you also want to determine which wire is the negative and which is the positive. Hook the black and red leads of the volt meter up to the transformer's wires randomly and see if the voltage measured is positive or negative. If it is negative, reverse the leads. Now you know that the wire to which the black lead is attached is the negative (ground) wire, while the other is the positive wire.

To build the regulator you need 3 parts:

A 7805 5-volt voltage regulator in a TO-220 case (Radio Shack part number 276-1770)
Two electrolytic capacitors (anywhere between 100 and 1,000 microfarads) (typical Radio Shack part number 272-958).

The 7805 takes in a voltage between 7 and 30 volts and regulates it down to exactly 5 volts. The first capacitor takes out any ripple coming from the transformer so that the 7805 is receiving a smooth input voltage, and the second capacitor acts as a load balancer to ensure consistent output from the 7805.

The 7805 has 3 leads. If you look at the 7805 from the front (the side with printing on it), the three leads are, from left to right, input voltage (7 to 30 volts), ground and output voltage (5 volts). To connect the regulator to the transformer you can use this configuration:

The two capacitors are represented by parallel lines. The "+" sign indicates that electrolytic capacitors are polarized: There is a positive and a negative terminal on an electrolytic (one of which will be marked). You need to make sure you get the polarity right when you install the capacitor.

You can build this regulator on your breadboard. To do this, you need to understand how a breadboard is internally wired. The following figure shows you the wiring:

On the outer edges of the breadboard are two lines of terminals running the length of the board. These terminals are internally all connected. Typically you run +5 volts down one of them and ground down the other. Down the center of the board is a channel. On either side of the channel are sets of 5 interconnected terminals. You can use your volt-ohm meter to see the interconnections. Set the meter's dial to its ohm setting, and then stick wires at different points in the breadboard (the test leads for the meter are likely too thick to fit in the breadboard's holes). In the ohm setting, the meter measures resistance. Resistance will be zero if there is an connection between 2 points (touch the leads together to see this), and infinite if there is no connection (hold the leads apart to see this). You will find that points on the board really are interconnected as shown in the diagram. Another way to see the connections is to pull back the sticker on the back of the breadboard a bit and see the metal connectors.

So connect together the parts for your regulator:

Connect the ground wire of the transformer to one of the long outer strips on the breadboard.
Plug the 7805 into 3 of the 5-hole rows.
Connect ground from the terminal strip to the middle lead of the 7805 with a wire - simply cut a short piece of wire, strip off both ends and plug them in.
Connect the positive wire from the transformer to the left lead (input) of the 7805.
Connect a capacitor from the left lead of the 7805 to ground, paying attention to the polarity.
Connect the 5-volt lead of 7805 to the other long outer terminal strip on the breadboard.
Connect the second capacitor between the 5-volt and ground strips.

You have created your regulator - It might look like this when you are done (2 views):

In both figures the lines from the transformer come in from the left.
You can see the ground line of the transformer connected directly into the ground strip
running the length of the board at the bottom. The top strip supplies +5 volts and is connected
directly to the +5 pin of the 7805. The left capacitor filters the transformer voltage,
while the right capacitor filters the +5 volts produced by the 7805. The LED connects
between the +5 and ground strips through the resistor and lets you know when the power
supply is "on".

Plug in the transformer and measure the input and output voltage of the 7805. You should see exactly 5 volts coming out of the 7805, and whatever voltage your transformer delivers going in. If you do not then immediately disconnect the transformer and check the following:

Pull out the capacitors. Plug the transformer back in for a moment and see if that changed anything.
Make sure the ground wire and positive wire from the transformer are not reversed (if they are, it is likely the 7805 is very hot, and possibly fried).
Make sure the transformer is producing any voltage at all by disconnecting it and checking it with your volt meter. See the sidebar above for how to do this.

Once you see 5 volts coming out of the regulator, you can test it further and see that it is on by connecting an LED to it. You need to connect a LED and a resistor in series - something that is easy to do on your breadboard. You must use the resistor, or the LED will burn out immediately. 330 ohms is a good value for the resistor, although anything between 200 and 500 ohms will work fine. LEDs, being diodes, have a polarity, so if your LED does not light try reversing the leads and see if that helps.

It might seem like we've had to go to a tremendous amount of trouble just to get the power supply wired up and working... But you've learned a couple of things in the process. Now we can experiment with Boolean gates!

Playing with Boolean Gates
If you used the table above to order your parts, you should have 6 different chips containing 6 different types of gates:

7400 - NAND (4 gates per chip)
7402 - NOR (4 gates per chip)
7404 - NOT (6 gates per chip)
7408 - AND (4 gates per chip)
7432 - OR (4 gates per chip)
7486 - XOR (4 gates per chip)

Inside the chips, things look like this:

Let's start with a 7408 AND chip. If you look at the chip, there will normally be a dot at pin 1, or an indentation at the pin 1 end of the chip, or some other marking to indicate pin 1. Push the chip into the breadboard so it straddles the center channel. You can see from the diagrams that on all chips pin 7 must connect to ground, and pin 14 must connect to +5 volts. So connect those 2 pins appropriately (if you connect them backwards you will burn the chip out, so don't connect them backwards. If you happen to burn a chip out accidentally, throw it away so you do not confuse it with your good parts.). Now connect an LED and resistor between pin 3 of the chip and ground. The LED should light. If not, reverse the LED so it lights. Your IC should look like this:

In this figure, the chip is receiving +5 volts
on pin 14 (red wire) and ground on pin 7 (black wire). The resistor
leaves pin 3 and connects to the LED, which is also connected to ground.
Connect wires from +5 and ground to the gate's A and B inputs to exercise the gate.

Here is what is happening. In TTL +5 represents a binary "1" and ground represents a binary "0". If an input pin to a gate is not connected to anything, it "floats high", meaning the gate makes an assumption that there is a "1" on the pin. Therefore the AND gate should be seeing 1s on both the A and B inputs, meaning that the output at pin 3 is delivering 5 volts. Therefore the LED lights. If you ground either pin 1 or 2 or both on the chip, the LED will extinguish. This is the standard behavior for an AND gate, as desribed in the Boolean logic article.

Try out the other gates by connecting them on your breadboard and see that they all behave according to the logic tables in the Boolean logic page. Then try wiring up something more complicated. For example, wire up the XOR gate shown in the Boolean logic page, or the Q bit of the full adder and see that they behave as expected.

Learning More
In theory you now have the fundamental knowledge you need to build any digital device. You can take the basic gates discussed in this article and construct anything. However, it is often more convenient to use larger-scale devices so that you don't have to combine 50 chips to build something common like an ALU. It is also helpful to see examples of different ways to combine gates to create complicated systems.

If you would like to work on a bigger project, you can try building the digital clock described in the How Stuff Works article on digital clocks. If you want to learn more about TTL devices, the following books will be helpful:

TTL Cookbook by Don Lancaster - a great introductory book full of information and ideas.
TTL Logic Data Book from Texas Instruments - complete data on all TTL chips (also available from Jameco).
IC Projects
Find the books at Radio Shack with part numbers ranging from 62-5010 to 62-5026. These books are very inexpensive and have a large number of circuits to try out and play with.

You will be AMAZED at what you can create with just a few ICs and some creativity - Have fun with it!

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