Many people do not realize that it is quite easy to build sample digital electronic logic circuits that demonstrate the concepts that have been presented to you as well as let you try out your own simple experiments. If you have, or are taking, a course in digital electronics, it probably includes a well-equipped laboratory in which you worked through a number of experiments. You do not need to replicate this laboratory at home if you wish to experiment with digital electronics. As I will show in this chapter, you can come up with a very capable digital logic circuit test kit for less than $20 and use parts available in modest electronics stores (like ‘‘Radio Shack’’).
Chances are, you are familiar with a variety of different electrical power sources: the ones that comes to mind first are batteries. There are a confusing number of different batteries that you can choose from, ranging from simple ‘‘AA’’ batteries that cost a few cents to the batteries used in the International Space Station that weigh (on Earth) 1200 pounds and cost over $200,000 each. Along with batteries, electricity can also be produced by generators, solar cells and fuel cells. Within your home you can access electrical power very conveniently through outlets in the walls, although this power is alternating current (‘‘AC’’) and not the direct current (‘‘DC’’) required for digital logic. AC power coming from the sockets in your home will have to be reduced and rectified into DC.
When you are experimenting with simple electronics, I think it’s best to use a power source that is definitely ‘‘low end’’; ‘‘alkaline’’ and rechargeable nickel–metal hydride (‘‘NiMH’’) batteries are widely available to power your experiments. TTL digital electronic chips generally operate between 4.5 and 5.5 volts – you could come up with a combination of batteries that will provide 5 volts to your circuit, or convert a 9 volt radio battery output to 5 volts using a ‘‘regulator’’. Rather than going through this effort and potential expense for TTL, I am going to recommend that you use CMOS digital logic chips that can be powered by 9 volts directly.
A 9 volt battery ‘‘clip’’ (Fig. 3-33) will cost you just a few cents and a bag of them can be bought for a dollar or so. For the purposes of the digital logic circuit test kit, you should look for a 9 volt battery clip that either has wire’s individual strands soldered together (the ends of the wires will look silver, shiny and attached together) or has a single strand. The wires will be covered
in a red and black plastic insulation and the strands will poke out the ends for a 1/4 inch or so.
Make sure the strands of the 9 volt battery clip wires are either soldered together or the wires consist of a single strand, because the wires from the battery clip will be pushed into holes and clamped by copper springs to provide power for the test circuits. Loose, individual strands break easily, can short with other loose wires or become a tangled mess, none of which are good things.
The battery clip is only one part of the wiring that will be used with the digital logic circuit test kit. By itself, the battery clip brings power out of the 9 volt battery conveniently, but is difficult to work with when you are working with chips and even moderately complex circuitry. The ‘‘breadboard’’ and wiring kit (Fig. 3-34) provide a customizable platform in which chips and other electronic components can be inserted into and easily wired together.
‘‘Breadboards’’ allow you to simply and quickly wire up your own prototyping circuits. From the top, a breadboard looks like a sea of holes, but if you were to ‘‘peel back’’ the top (Fig. 3-35), you would see that the holes are actually interconnected, with the central groups of holes connected outwards and the outermost two sets of holes connected along the length of the breadboard.
The central holes are spaced so that DIP chips can be placed in the breadboard and wired into the circuit easily. The outside two rows
of holes, I use as power ‘‘buss bars’’ and connect the power source to them directly.
Along with the breadboard, you can either buy a pre-cut and stripped wiring kit (shown in Fig. 3-34) or a roll of 24-gauge solid core wire and some needle nose pliers, wire clippers and maybe some wire strippers. For convenience, I usually go with the wiring kit as it costs just a few dollars.
Along with buying the battery clip, breadboard and wiring kit, you should also buy:
1. 5 or so LEDs in a 5 mm package 2. 10 or so 1k, 1/4 watt resistors 3. 10 or so 0.01mF ceramic capacitors 4. One 555 oscillator/monostable chip
5. 5 or so SPDT switches, that can be inserted into the breadboard 6. One 74C00 quad two-input NAND gates chips
7. One 74C02 quad two-input NOR gates chips 8. One 74C04 hex inverter chip
9. One 74C08 quad two-input AND gates chip 10. One 74C32 quad two-input OR gates chip 11. One 74C74 dual D-flip flop chip.
All these parts should cost you less than $20 and are available at a fairly wide variety of sources including:
. Radio Shack (http://www.radioshack.com) . Digi-Key (http://www.digikey.com)
. Mouser Electronics (http://www.mouser.com)
. Active Components (http://www.active-electronics.com).
You will not require any test equipment (such as a Digital Multi-Meter) for this kit and the sample circuits that I will present in this book.