Your Laboratory

Hints for laboratory work in electronics

Putting Together An Electronics Laboratory

The fun in electronics is in doing, and it is easy and inexpensive to "do." Laboratory work will point out to you when your ideas and the universe do not agree, but will also support and encourage you when you get it right, and reinforce your ideas. A lot can be done with a few simple things, and your laboratory can continually grow to suit your resources and interests.

The first stage requires the following properties. A digital multimeter should be your first purchase; the least expensive is still useful for most purposes. It will measure at least DC voltage, resistance and current, of which the voltage measurement is the most useful. Spending a lot will add very little except some convenience. You will need a solderless breadboard. One with 47 rows of five holes on each side of center, and busses down both sides for power. With the breadboard, you will want a wire stripper for #22 wire, small pliers and a small screwdriver. The wire should be #22 solid with plastic insulation, in several colors if possible. Stranded wire, and larger wire, should be avoided. A DC power supply is needed, and the most useful will be one supplying 5 volts regulated, at perhaps 500 mA. Such a supply is easily improvised with an unregulated plug-in supply of 6 V and a 78L05 voltage regulator with filter capacitor. For a little more money, a surplus power supply giving ±12 V in addition can be obtained. If this is a switching power supply, remember to draw some current from it with a bleeder resistor so it works properly when you have nothing connected. You are now equipped to make many observations, at a cost of under $100, and it all fits in a shoebox. Buy a second DMM if you can afford it; a second meter is very convenient. Buy some spare fuses for the meter.

Now you will want some components. An assortment of 1/4 W carbon film resistors might be a good buy. They should range from 100 Ω to 1 MΩ, and just the 20% values (10, 15, 22, 33, 47, 68) are really necessary. As you need smaller or larger values, or special values, accumulate them. Potentiometers of 1k, 10k and 100k values will be useful, for picking off variable voltages, and controlling currents. Plastic dielectric capacitors in a few values, say 0.001, 0.01, 0.1 and 1 μF, disc ceramic capacitors from 10 pF to 0.1 μF, and an assortment of aluminum electrolytic capacitors from 1 μF to 1000 μF. A wide range of values is more useful than a large number of values. I have not found inexpensive "grab bags" very useful for a basic supply. As far as active components go, a few LED's of various colors, a few 1N4001 rectifier diodes, some 1N4148 or similar signal diodes, a few 2N4401 npn and 2N4403 pnp transistors, and some 741 op-amps will provide much entertainment. There are many alternatives, and the type numbers mentioned are only suggestions. Acquire special devices as you need them, and add them to your stock.

When you get beyond DC measurements, into the interesting world of pulses and AC, some relatively expensive instrumentation is needed. An oscilloscope is a door to a wide world of investigation. A 20 MHz scope, with 10x probes, is quite adequate for most purposes. However, for advanced work, a 100 MHz scope with triggering and other conveniences is necessary, and this will run $1000 or more. Storage and digital scopes are available, and useful for certain applications, but the analog scope is excellent for general purposes and is easy to use. The oscilloscope is the most expensive instrument you will need, and it is generally worth the price. Much less expensive are a function generator (up to 1 MHz) and a frequency counter, to supplement the oscilloscope. All of these instruments require proper probes and leads--a piece of wire or a coathanger will not do. A variable DC supply is also useful, providing perhaps 2.5 A up to 30 V. Some leads with a banana plug on one end and a grabber on the other are useful. You can make your own leads with a variety of terminations, using stranded wire with rubber insulation, if you have soldering facilities.

Experimenting with integrated circuits requires no special equipment, other than that mentioned above. Construction additionally requires only sockets, with solder lugs or wire-wrapping terminals for mounting other components.

The usual small hand tools are convenient, and a larger variety is necessary if you undertake construction. A hand drill with a set of drills, reamer, and center punch are often needed. The reamer can be used to make a hole larger than your largest drill, which should be at least 1/4". A small power drill, or better a small drill press, are handy. For working with sheet metal (0.625" soft aluminium), good tin snips are necessary, and perhaps a "nibbler" to open out square holes. Flat and round files are needed to smooth edges. A set of nut drivers is more convenient than using adjustable wrenches, but both should be available. Back when electronics was made on aluminum chassis, the electronics shop included a well-equipped sheet metal shop, but this is no longer necessary for the panels and small boxes now used. Small aluminum boxes are a good choice to protect circuits. Parts and components are supported by holes drilled in the boxes, by terminal strips attached by screws, and by printed circuit boards mounted on spacers inside. Power cords exit through grommeted holes, or by special strain relief fittings pressed in the holes.

Construction creates a need for much small hardware, such as screws, nuts and washers, hollow and threaded spacers, soldering lugs, terminal strips, connectors, jacks, plugs, grommets, fuse holders, pilot lights, shrink tubing and sockets. The US sizes of screws commonly used are 2-56, 4-40 and 6-32. The corresponding metric sizes are 2.0, 3.0 and 3.5 mm. The excellent BA machine screws are, alas, no longer with us. It is also nice to have a desk vice. You will also need switches and similar controls for certain projects.

Breadboarding Practice

Breadboarding is assembling and testing circuits put together temporarily from components. Components are now available in two general forms: through-the-hole with wire leads or pins, and surface-mount with small pads. Surface-mount devices have manufacturing advantages, and dissipate heat better, but are not useful for breadboarding. If you must breadboard with them, small boards with inline pins are available to which they can be soldered. Surface-mount devices are also not convenient for breadboarding and manual construction, and are impossible to maintain. They should be left for the volume manufacturers, since they offer no functional advangtages.

The solderless breadboard is a superior way to study simple circuits. Connections are made at groups of five holes, through which leads or pins are passed. The rows of holes are 0.1" apart in both directions. The boards are made for DIP (dual in-line plastic) integrated circuits, which straddle the center groove. To insert a new DIP, press the leads parallel by lying the package on its side on the desktop and rotating it slightly. Then the pins should enter the holes in the breadboard easily, and the DIP should be fully pressed down. Usually, a bus down the right side of the breadboard is the positive supply voltage, and the bus down the left side is ground. Double busses give the benefit of a ± or bipolar supply for op-amps and similar circuits. All you will normally want is +12 and +5 on the right, and -12 and GND on the left. A 1 μF plastic capacitor should be connected between +5 and GND for "despiking" digital circuits.

The breadboard I use is shown at the left. The wooden shelf rests on a surplus switching power supply that provides ±5 V and ±12V. There is a 600 Hz sine and square wave generator at the upper left (the pot varies the sine wave amplitude), 8 buffered LED's at the upper right, with a debounced pushbutton to the left. There is an 8-bit buffered source on the lower right, set by hexadecimal thumbwheels. A 10k potentiometer is at the lower left, its slider buffered with an LF411 op-amp. An IC audio amplifier and loudspeaker could be added easily, but I have this in a separate box.

DIP pins are generally numbered beginning with 1 at the upper left-hand corner of the package, as shown at the right. The "upper" edge of the package is the one with the small semicircular recess, and there is often a depressed circle near pin 1. The numbering proceeds down this side, and up the right-hand side. The DIP is made for manufacturing, not for breadboarding, so the pins are fragile. They are meant for one-time-only mounting, not for repeated mounting and removal. Nevertheless, if treated kindly, they will survive quite nicely. A kind thought is always to remove a DIP from the breadboard by prying with a screwdriver. The channel makes this easy, and it is the professional thing to do. Screwdrivers do not have the reflexes of fingers that invariably cause the pins to be bent when the DIP comes loose.

The +V for many digital integrated circuits is supplied through the upper right-hand pin (highest number), and the ground through the lower left-hand pin. Although this may become a habit, there are enough chips in which the power pins are somewhere else that checking is always a good idea, especially with linear integrated circuits. Op-amps customarily have +V at pin 7 of the 8-pin DIP, not at pin 8. Circuits are known with GND at the upper right-hand corner, and many older digital chips have the power pins in the middle. Making an error usually destroys the chip, so beware or have lots of spares. It is also a bad idea to work with a variable-voltage supply. Sometimes you want this, but rarely, and disasters are frequent when such supplies are used for breadboarding.

#22 AWG wire has a diameter of 0.025" or 0.64 mm, and is ideal for the usual breadboard. Solid wire should be used, never stranded. Stranded wire does not fit well, and bits can come loose. #20 wire, diameter 0.032" or 0.81 mm can be used, but anything larger will damage the spring contacts. Also, #24 wire, diameter 0.020" or 0.51 mm, is satisfactory, but anything smaller will not be held securely. The flat leads on TO-220 transistor packages will fit if they are twisted 90° with pliers. If you have to work with larger leads or stranded wire, solder a short length of #22 to them to make connections with the breadboard. Potentiometers, and certain other components, are available with leads spaced to fit directly in the breadboard.

Circuits on breadboards are not permanent. The connections are not air-tight, and will oxidize with time and become uncertain. A circuit that will see practical use should be soldered or wire-wrapped.

Soldering Practice

The assembly of an individual circuit for testing and practical use is called prototyping. Small circuits can be soldered together on small PC boards arranged like a solderless breadboard, using short lengths of wire for connections. Radio Shack has a variety of these that can hold one or more DIP's, and some have card-edge connectors. Headers are available that make use of DIP sockets for connections, with resistors and capacitors soldered tot he lugs in the header. Phenolic board with a matrix of holes at a spacing of 0.1" can also be used. Terminals are available that can pressed into the holes where necessary, and connections soldered to them.

Serious prototyping is done using a printed-circuit (PC) board prepared for the purpose. This requires facilities for laying out the mask used in making the board, as well as the considerable expense of making the actual board, which may have the features of being double sided and having plated-through holes. Recent, costly, boards may even have multiple layers. Kits are available for making boards at home, exposing the photosensitized board, developing and etching it, but this takes practice and skill to do a good job. The result, of course, has no civilized features, but is a PC board. PC boards will be supplied with construction kits, and it is good practice to make one. This teaches component recognition as well as soldering technique, and is an indispensable experience.

Wire-Wrapping

For complex circuits, and more advanced digital work, wire-wrapping is the best choice for prototyping. For this, you will need a wire-wrapping (and unwrapping) tool that looks like a jeweller's screwdriver, and wire-wrapping posts that are pushed into tinned holes in a circuit board and soldered there. The wire used is #30 with Kevlar insulation, most conveniently supplied in pre-cut and pre-stripped lengths. I do not find motor or squeeze-driven wrapping tools sensitive enough for good work, and stay with the manual tool and a fine touch.

A wire-wrap project begins with a good-quality circuit board with holes at 0.1" spacing and plated through. Power busses are provided on the board, sometimes on opposite sides, registering with each other. These should be connected with despiking capacitors (0.01) at frequent intervals. Components are mounted using DIP sockets and press-in terminals, both with wire-wrapping posts on them. One post can handle two or three wraps, depending on the length. Be sure you have the length you need.

Wire-wrapping is not just winding wire around posts. Using the right wire, the right posts, and the right method, reliable and permanent results will be obtained. Otherwise, you are self-insured. Any connections not made in the legal manner should be soldered. There is usually a considerable amount of soldering involved in making a complete wire-wrapped project, of the socket pins and terminals put through the holes.

Connections can be changed rather easily by unwrapping one wire and wrapping the other. This is one of the great benefits of wire-wrapped construction, that PC boards do not possess. Unsoldering is much more bother than rewrapping.

Notebook And Circuit Diagrams

The experienced reader will need no encouragement to keep a notebook, and to use circuit diagrams. These two things are essential to electronics engineering. The notebook should be a bound or spiral book rather than looseleaf, and entries in it should be made immediately, while working, since that is what it is for. It is impossible to recall all the small but important details even after a couple of hours have passed, not to mention a couple of weeks. Forget school habits: using scraps of notes to "write up" the work later misses the whole point, and is worthless. I keep notes in pencil, on quadrille paper if possible, but if you are looking for patents, keep notes in pen, and date and sign them. I do not usually date my notes. Number each pair of pages, and keep an index on the first page. You will be surprised (if you do not already know it) that you can find things even years later with little trouble, if you only record the date that the notebook was begun, and use meaningful titles. ("Experiment No. 1" is a typical useless title.)

In drawing circuit diagrams, you can either do your own thing and use conventions revealed to you in dreams, or else try to communicate with the rest of us. There are accepted standards, but they may be inconvenient to follow in detail, and keep changing anyway. There is great latitude, but within the bounds of convention. Imitate what you think is best in what you see. Common conventions are shown in the diagram at the right. Generally, DC current flows from top to bottom, and signals flow from left to right, and things are arranged this way when possible. Wires that join are shown with a visible dot. Wires that cross without connection are simply shown crossing, without the dot. There used to be a little semicircular jump, but this is too much trouble and is very old-fashioned. A small open circle or square indicates some kind of connection, such as a terminal or an integrated-circuit pin. Wires run vertically and horizontally in most cases, almost never diagonally. Points where voltages are measured are usually shown by dots, as at a wire junction, even if there is no junction, and the positive direction of currents and voltages is shown where necessary. The arrow meaning connection to the positive power supply voltage always points upward, and the symbol for ground or earth is always drawn as shown, never upside-down.

A resistor is shown here represented by a rectangle, instead of by the zigzag line introduced by radio engineers long ago. This is not only easier to draw, and actually looks like a resistor, but is also the international standard. The polarity of an electrolytic capacitor is shown by a + sign on the positive lead, or by the alternative ISO symbol at the left. Other capacitors have no indication of polarity. Note that the inductance symbol has no loops. A and K refer to the anode (p end) and cathode (n end) of a diode, and the arrowhead points in the direction of easy current flow. Arrows in transistor symbols point from p to n type and always indicate the emitter. On circuit diagrams, each component is identified with its value, or by a symbol consisting of a letter and a number, or both. For example, R₅ or C₂. T indicates a transformer, Q a transistor, U an integrated circuit, S a switch and D a diode. There is only an approximate consistency between the circuit symbols used by different organizations. The components represented by these symbols will be explained in the pages that follow. A surge arrester, whose symbol is shown at the left, is usually a zinc oxide nonlinear resistor, and looks like a ceramic disc capacitor with a shiny coating.

The quality of an engineer can be assessed by a brief examination of his or her lab notebook. If it looks like something that would be handed in by an A student at the Sam Houston Institute of Technology, look out! Circuit diagrams should be sketched with care, so that you could recreate the circuit if necessary. Every circuit diagram should be there for a purpose, and the purpose should be clear, with a written explanation if necessary. There should be measured values, notes when something significant took place, and a logical progression. Tables and graphs are often useful, and the sources of the figures should not be in doubt. Ratings and specifications of the components should be given, as well as pinouts and other necessary information. Sometimes things might not have gone as planned, sometimes notes and figures will be lined out or corrected. There will be nothing erased. You should be able to recreate in your mind the happenings recorded there without difficulty. The typical school procedure of writing up results later is the worst thing that could be found in the notebook. Of course, you can always sit down and analyze the work later, and prepare reports or summaries, but it should always be obvious to the reader that this is what you are doing, and is a very different thing from the actual lab work. The next worst fault is chaos.

Hints From Experience

• Always make the right-hand bus +5, and the left-hand ground (earth). Use red wire for +5, and black wire for ground. Wire power first.
• Use only solid #22 plastic-insulated wire for solderless breadboards.
• Make all connections via the breadboard or a terminal strip, or solder them. Do not twist wires together for this purpose. Screw-on connectors are not recommended for electronics use.
• Set scope probes to 10X to avoid disturbing the circuit under test; almost never use the 1X setting, and then only when you know what you are doing.
• Leave scope probes, properly adjusted, permanently connected to the same place. Do not move them to another position, or to a different scope. Use a separate probe for the trigger, not one of the usual channel probes.

• Use only the wire stripper to remove insulation, carefully avoiding nicking the wire in the slightest. Fixed strippers are best.
• Apply the soldering tip only when it is fully heated, apply it to maximize the heat transfer area, and remove it quickly when everything melts and makes a meniscus.
• Apply flux only to areas where solder is desired; make sure it is absent from areas where solder is not desired.
• Route wires around components, not over them.
• Locate components logically to minimize the lengths of wire runs. Do not make wires too long or too tight, and use similar colors for similar functions.
• Wire-wrap only to proper square posts; in all other cases, solder after wrapping.
• Check that the power dissipation rating of transistors, resistors and potentiometers is not exceeded. It should be possible to touch components without pain, except were they are intended to run hot.
• Use a protective resistor in series with a circuit if there is a chance of a short circuit, especially resulting from mishandling of a control. Potentiometer wipers are specially at risk.
• Segregate 120 V AC wiring from other wiring, and make certain that neither wire contacts any metal parts, using the ground (green) wire to ensure this. Fuse the connection, and follow all steps necessary to reduce the risk of shock or fire. When experimenting with 115 V AC, use an isolating transformer to reduce the risk of accidental contact.

Vanishing Electronics

It is only to be expected that obsolete items, such as vacuum tubes, will be hard to find, and recourse must be had to special sources. However, even new and recent items, often excellent ones, disappear without trace. Companies enter and leave the field continually. For example, Exar, that produced excellent linear IC's, seems to have vanished, and this is only the latest of many examples. Other companies are lost in mergers, such as Burr-Brown in Texas Instruments. The old leaders, RCA and General Electric, appear to be out of the electronics business completely, which is a great loss. General Electric now makes most of its money from financial transactions, as its technical expertise in things electric withers. Motorola, once an excellent company, has also stopped manufacturing components, turning to more profitable lines as it has become cumbersome and noncompetitive. I have also collected interesting items through the years from Radio Shack, and some of them appear in experiments in these pages. I am sure Radio Shack no longer handles them, and do not know if they are available anywhere. One example of this is the TL507 single-slope A/D converter, which is a rather interesting little chip with which you can make a square wave with fully adjustable duty cycle. Radio Shack recently closed their supply stores, so I must rely entirely on mail order, even in a major city. Obtaining components, especially unusual ones, is a continuing difficulty. Even NASA is having trouble getting 8086's.

Sources of Information and Components

The Internet has become the major source of information on electronics components, replacing data books to a great extent, as well as a means of purchasing. The Application Notes provided by manufacturers in .PDF files are a valuable resource. Information on integrated circuits can be found in the websites of the semiconductor manufacturers, for example:

National Semiconductor
Texas Instruments
Intersil
Advanced Micro Devices
Atmel
Maxim Integrated Circuits
Microchip - PIC microcontrollers
Parallax - BASIC Stamp microcontrollers
Zilog - Z8 and Z80 microcontrollers
B&B Electronics Mfg. Co.
Capital Adv. Tech. - Surfboards for SOIC packages
Belden - Wire and cable
Alpha Wire - Wire and cable

A Google source for the name of a manufacturer will reveal the uRL immediately in most cases. A part can be searched for by its number, or there are selection tables. The data sheet can be downloaded in .PDF format. Parts can be obtained from distributors, and ordered from their websites, for example:

Jameco
JDR Electronics - Computer parts
Mouser Electronics
Digi-Key Electronics
All Electronics
Antique Electronic Supply
Sphere
Marlin P. Jones Associates
Newark Electronics
Hosfelt Electronics

Most of these will provide a printed catalog.

Composed by J. B. Calvert
Created 30 June 2001
Last revised 12 March 2010