Hands On

Using the various gate chips, experiment with connecting inputs to various combinations of logic levels and observe the outputs. You can either use a logic probe (see the next guide in the series for a DIY logic probe) or connect output to an LED (via a current limiting resistor 220-330 ohm)

7400 - 2 input NAND
7402 - 2 input NOR
7404 - NOT
7408 - 2 input AND
7420 - 4 input NAND
7430 - 8 input NAND
7432 - 2 input OR

An interesting exercise is to get a bunch of 7400 chips and build the other types of gates (AND, OR, NOT, NOR, XOR, and XNOR) just using NANDs.

Next combine gates, connecting the output of some to the inputs of others. Note that you can not connect gate outputs together, and each input can be connected to a single output.* However a single output can be connected to multiple inputs.

One interesting combination is ORing the output of AND gates. This AND-OR combination is so common that there are chips such as 7451, 7454, and 7455 that combine AND, OR, and NOT. They are referred to as AND-OR-INVERT gates.

components_7451.png
7451
components_7454.png
7454
components_7455.png
7455

Practical concerns

A logic circuit has what's called a fan-out. That indicates how many logic inputs an output can be connected to and still work reliably. This is typically high enough with TTL that it won't be an issue for these experiments, but in more complex circuits it can be a concern. You can usually rely on a fan-out of at least 10 for TTL.

Another physical limitation is speed, or rather the lack of speed: latency. Each gate takes a minute amount of time to generate a new output when it's inputs change. This is it's latency. At this point you can completely ignore this, but as circuits get more complex it adds up as each gate takes a bit of time. Eventually a design will need to take latency effects into account.

* We'll look at open-collector outputs in a later guide.

Supplies, parts, and equipment

You can find TTL chips at the major electronics component retailers including Digikey and Jameco (which I have very happily dealt with). Be sure to specify that you want DIP chips, not SMT.

You'll need some solderless breadboard. Adafruit carries various sizes, but I've found that the large has plenty of space to experiment. It may seem like overkill but you'll end up needing it if you continue following along at home.  And having breadboard space available is never a bad thing. I have a habit of throwing 3-packs of full and half sized ones onto Amazon orders.

You'll need an assortment of jumper wires to use with the breadboard. Some alternatives are listed on the right.  The key is they need to be male-male to use the with breadboard. Having an few lengths can be handy.

Next you'll need a good (i.e. with stable, smooth output voltage) 5 volt power supply. I've listed a 2 amp model which will probably do well until toward the end of the series. If you don't mind the price, I've listed a 10 amp model as well that will do quite nicely.

Finally you will need a way to get that power onto the breadboard. If you get/have a 5v power supply with a standard 2.1mm plug, you can get the breadboard friendly barrel-jack I have listed.

There are other alternatives, just be sure it's at least a couple amps and provides a steady output.

Next time

I'll look at some tools that will be handy in the parts to come. Of course, being makers I'll provide circuits and examples of them so you can make your own.

Closing

Have fun and experiment. If you have questions you can find me on Twitter and Adafruit's Discord server.

There will soon be a video to run through techniques and some experiments.

This guide was first published on Mar 22, 2018. It was last updated on Mar 22, 2018. This page (Hands On) was last updated on Sep 11, 2019.