Last month we talked about tools and basic construction techniques; now it's time to use what we have learned. We're going to build an AC voltage monitor, which uses colored LEDs to display a range of voltages. Electronically it is pretty simple-- a plug-pack transformer supplies DC voltage (which varies with the AC line voltage it's plugged into) to a pair of quad op-amps. The op-amps are configured for voltage comparison, and turn an LED on if the voltage is equal to or greater than a specified voltage. Seven LEDs are set for levels of 100, 105, 111, 118, 123, 128 and 132 Volts AC, with a bar-graph effect-- all LEDs below the measured voltage will be lit, and they are color coded (two red, two yellow, and three green) so you can tell at a glance if the mains voltage is within normal limits. For future reference, there is a similar kit available to monitor DC voltage on a 5, 8 or 12V supply-- you could put both units in one box for a complete station power monitor.

Everything you need to build the VM-110 is in this article-- the schematic, parts list, parts overlay, and circuit board artwork (reprinted with permission), or you can buy a complete kit from Milestone Technologies or Electronic Rainbow.⁽¹⁾

Why this particular kit? It's useful, you probably don't have one already, it's reasonably inexpensive, and it is of moderate difficulty. You'll be working with polarized components and integrated circuits, and along the way you'll have to overcome some "challenges." A Rainbow Kit also conforms to my "Rule Number 1" for kit selection-- never buy a kit from someone who won't fix it if you can't get it to work! You will usually have to pay a repair fee, but at least you know you will not be throwing your money away. And such a service is a good sign that the kit seller has done everything possible to make the kit "buildable."

While it might look like it at first glance, this is not a "project article" as such-- the purpose is to help you learn to build stuff, not just this particular kit. Along the way we'll talk about things that aren't even related to this kit, and that's why it will take so much print space when the actual instructions you get with the kit are printed on a single side of a letter sized page! Besides, the topic is really huge. We're talking mechanical skills, manual skills, a considerable amount of knowledge, and of course experience. Experience is something you can only get for yourself, but you have to start somewhere. The intention here is to keep it as simple as possible, but giving you a base to "build on."

Step One - Read the Instructions

All of them! Even if it is a 30 page booklet, you really need to read through it to get an idea of how to proceed with construction, whether you will need to make any "option" decisions, whether the order of construction is mandatory, etc. And often you will find explanations for little mysteries, like extra parts or unusual parts that you might encounter in Step Two (see below).

There are tremendous variations in the standards of documentation for kits. Sometimes you will be told in excruciating detail exactly what to do, and sometimes the instructions will be very general. Here are a couple of actual examples from kits I've built recently, word for word:

EXCRUTIATING DETAIL "[ ] 52a. Locate resistor R1 (1K, Brown-Black-Red). Bend the leads and insert on the component side of the board in the space marked "R1.." which is near the transformer you installed in step 48. Solder and trim leads."

The "No Instruction" Instruction "Insert and solder all of the board-mounted components."

Sometimes you will have to make choices as you build the kit, so it is a good idea to work your way through the options before you start. Sometimes there are literally options, as for example a transceiver kit where you can select either fixed IF filter bandwidth or variable (or even band of operation), or the VM-1 DC voltage monitor where you choose which base voltage to measure (5, 8, or 12V) and what the steps should be for each LED (.25, .5, or 1V steps). These options often have parts implications, so that's another reason for reading the manual before counting the parts.

There can also be "user modifications," or cases where you might want to do something different from the literal instructions. That's one of the great things about kit building-- it's your kit and you can do anything you want with it! An example of this might be where a board mounted pot is supplied and you would rather wire up an external pot on a control panel. Again, it helps to have these things in mind before you start building.

Step Two - Taking Inventory

If you are gathering the components yourself this is not an issue, but if you have bought a kit it is important to find out right away if you got everything you paid for. Locate the parts list and inventory the supplied components. Check the values carefully, and check them off on the parts list as you go. In my experience wrong or missing parts are something of a rarity-- but it does happen, and reputable kit suppliers will fix the problem fast. In fact, if you start by checking the parts you will often be able to obtain a replacement before you actually need the part, especially with larger projects.

Usually there are only two problem areas in checking component values-- resistors and caps. Resistors because there are often a lot of them and the value is indicated by colored bands, caps because there are often a lot of them, too, with multiple standards for labeling, and often you will need a magnifying glass to read the lettering. A complete rundown on component identification would take more space than the editor will let me have, so I'll refer you to the ARRL Handbook.

You should learn the resistor color code, so I've provided it for you in table 1.

Don't hesitate to use the process of elimination when you must. If you can't absolutely identify the nature or value of a component, go on and do the rest of them and see what's left. Often it helps to look at the quantity of a component too-- for example, you might be working on a kit (not the VM-110) that has .01 and .001uF capacitors which you can't tell apart, but since there are supposed to be two of the former and fifteen of the latter you should be able to figure it out.

Sometimes you will find extra parts, and parts which are not listed on the parts list. Case in point-- the VM-110 I built while developing this article had an extra set (7) of 1K resistors.

I've done kitting myself, and with inexpensive components like resistors it is occasionally easier to throw in extras "to make sure" than to re-count everything. Also in my VM-110 kit the plug-pack transformer was not listed on the parts list, nor were sockets for the integrated circuit chips.

You can test, or measure the value, of many components as you go. Until you have a lot of experience working with resistors, use your multimeter to confirm the value that you have deduced from the color code. Your multimeter will also tell you whether a diode is good (and confirm that it is in fact a diode), and some of the more elaborate multimeters will measure capacitance and inductance and even test transistors. Actual testing of components is generally a waste of time with commercial kits, but don't overlook it as a means of identifying parts.

As a more or less last resort (it's tedious work) you can cross check the parts against the circuit diagram. And if all else fails, don't hesitate to get in touch with the kit supplier-- most of them are happy to clarify things for you and even happier to find out where there are problems in the documentation.

About Printed Circuit Boards...

Printed circuit boards (PCBs or just "boards") come in a wide variety of types, colors, and materials. Every board has two sides, and usually the components go on one side (the "component" side) and the solder connections are made on the other (the "solder," "foil", or "track" side). Sometimes, though, you might need to attach a component to the solder side of the board, or make solder connections on the component side..

And sometimes connections are needed on both sides of the board; this is called a double sided board (actually there can be layered boards, too, with circuitry on one or more layers within the board). With double sided boards, there is usually a requirement for connections between the two sides. Sometimes you will need to install "vias" or "feed-through" wires, which are soldered on both sides of the board. In other cases, the board comes with "plated through" holes, meaning a metal grommet has been inserted in the component holes, which makes the connection between the two sides.

The "tracks" or circuit connections on the solder side of the board can be of pure copper or an increasingly wide variety of alloys. Pure copper boards occasionally carry enough oxidation that you should clean them before you start working with them. You can clean them with hot water and detergent, and scrub lightly with a scouring pad, but in my experience it's seldom necessary with commercial kits.

There are two "premium treatments" that can make boards easier to work with. A board can be "solder masked," which means the the tracks and areas between the solder pads have been covered up with a material that solder won't stick to (making solder bridges a lot easier to avoid.) And the component side of the board can be silk-screen printed with an outline of the parts and identifiers for each part, making it hard to get a component into the wrong holes. Unfortunately not all boards have these features (the VM-110 being an example of a single sided board without solder-masking or silk screening).

Moving right along...

But first, as Norm Abrahams would say, "a word about shop safety." We're not working with power tools here, but you do need to be careful with a couple of things. When you clip a lead from a circuit board, the clipped end is likely to go flying and could easily hit you in the eye. Wear safety glasses, if you have them. Ordinary glasses are probably of some value, and you can also put a fingertip on the lead before you clip it to keep it from flying. I've had a lot of things hit my glasses, so I know there really is some risk here. Soldering? Well, you're working with molten metal-- what can I say? Be careful! And don't expose yourself to the fumes any more than you have to. Don't panic, normal hobbyist exposure to solder fumes should be harmless, but on the other hand don't take any unnecessary chances.

At this point, we should have inventoried all the parts, have our tools in place, and generally be ready to start working on the circuit board.

Populating the Board

I've always loved that expression, "populating" a circuit board. It conjures up images of real creativity and procreation, or something. When my children were small they thought I was building little "cities," and they loved to build their own cities with junk box components and a piece of styrofoam. In any case, that's the term we use when we're talking about soldering components onto a circuit board.

Order of construction can be important, so you can treat these following guidelines as "rules of thumb" which are applicable to just about any kit. A good case to watch for is where there are "progress checks" as different parts of the circuit are completed, in which case you will have to do everything in the order described in the instructions (except maybe the IC sockets!).
 

• SOCKETS FIRST. Start with sockets for the integrated circuits, regardless of what the instructions say. I have yet to encounter a situation where this is a bad move, whereas I have often run into trouble trying to hold the darned things flat amid a forest of previously installed components. The solder pads for ICs are also very close together and it's a lot easier to solder them if the board is otherwise empty! Double check the orientation of the socket (there should be a notch at one end corresponding to the notch on the chip itself). Insert it into the component side of the board, then flip the board over so that it lies flat on top of the socket and hold it while you solder pins on diagonally opposite corners. Then solder the rest of the pins, going back and forth from side to side-- soldering adjacent pins puts an awful lot of heat into a small area of the socket (or chip) and alternating sides allows for a bit of cooling. Remember to check the soldering as described last month before you go on to anything else. Most kits which use integrated circuits will include sockets for them. If not, run down to RS and buy sockets! They make working with ICs a whole lot easier and safer. Very rarely the circuit may be so sensitive that the additional "lead length" for a socket will cause problems, but usually the instructions will tell you that. Do NOT install the chips into the sockets (a) until you are instructed to do so or (b) as the very last step before applying power to the completed project.

 
• LITTLE FLAT THINGS. Next, install the small, flat components, like resistors and diodes. The reason for doing these next is that after you have installed taller components, it can be difficult to get the lower ones into position. You can do all of the resistors at this point, even vertically mounted ones, or you can leave the vertical ones for later. For the flat-mounted components, hold the component in one hand and use your long-nose pliers to bend the leads at a ninety degree angle, close to the body of the component. Don't hold the head of the pliers right up against the body, though, because you can damage the component that way. You will be bending the component into a "U" shape which should slip right into the holes on the circuit board. If it doesn't fit, straighten the leads and do it again-- don't try to adjust it by forcing it into position. Once the body of the component is flush with the surface of the board, bend the leads outward at about 45 degrees on the solder side. Install three or four components, then solder them.

• EVERYTHING ELSE. Do the rest of the components in no particular order or in the order listed in the instructions or in whatever order you prefer. Three common approaches are (1) to work your way across the board from corner to corner, or (2) to work your way upwards in component size, or (3) do all the components of a given type. Each approach has it's fans, and you'll eventually decide which works best for you.

• IT DOESN'T FIT! Sometimes you will find that a lead or wire is too thick to go through the hole in the board, or the spacing of the holes is slightly off. This happens because the kit suppliers work with batches of kits, and exactly the same component might not be available. You can narrow the lead diameter slightly by scraping it with your hobby knife. Or you can widen the hole with a small drill or even by pushing a small screwdriver through it. If you widen the hole, be careful that you don't damage the solder pad, though. If the holes are improperly spaced, you may be able to fix the problem by soldering the component onto the tracks on the other side of the board (be extra careful about orientation, though).

Take a Break

Your eyes are tired, your hands are getting shaky, you're getting a headache... It's a huge temptation to do "just one more resistor" or "finish up the caps," or go ahead and "fire it up," but when you are tired you are far more likely to make mistakes Don't try to finish the project in one session if more time is needed. You'll probably complete the VM-110 in under an hour, but for larger projects I find a ten or fifteen minute break every hour is very helpful. Traditionally, I finish building kits late at night, and I have learned over the years that it is best to ignore the temptation to test them immediately. They almost always work better the next day!

The VM-110 Board in Particular

You can probably build the VM-110 in accordance with the generic construction steps described above, but let's look at some specifics.

You've probably noticed that the instructions are pretty rudimentary. For example, step 4 says "INSERT AND SOLDER THE LED'S AND JUMP (WATCH POLARITY OF LEDS)." How high are you supposed to jump? Well, they mean you should install the "jumper" between two holes on the right side of U2. It's any little scrap of bare wire, such as the trimmed lead from a resistor. Do it when you do the resistors, or you will find it pretty awkward.

Several of the components (resistors and diodes) were supplied on "ammo strips," or held together with paper tape on the ends. Generally, you cut them free of the paper strip with your flush cutting pliers, and generally you never pull them lose (or you could damage the component.) But there are always exceptions, and in my VM-110 kit the 1K resistors had unusally short leads. If you cut them from the tape, you will find that the there isn't enough lead left on R6 to mount it vertically. For just this one resistor, scrape the tape off of the ends of the leads rather than cutting it free. This is the kind of problem that is almost impossible to anticipate, so treat this warning as a freebie. Since I cut the resistor before discovering the problem, I had to find a solution, which turned out to be a spare 1K resistor from the junk box. It's also possible to solder a new piece of wire onto the original one if you're careful.

Installing the Light Emitting Diodes (LEDs)

We need to think a bit about the LEDs before installing them. First, we need to select an order, since we have three green ones, two reds, and two yellows. The order suggested in the parts list is fine for me (reds on each end, then yellows, and three greens in the middle (D3-5), but feel free to do it differently if you want.

You should be able to detect the flat side of the LED (just a tiny flattening on one side of the plastic ridge at the base of the body), but you can always check with a battery-- touch the leads to the terminals of the battery (1.5 or 9V) briefly, and if the LED lights, the positive lead is the one in contact with the positive terminal. If it doesn't light, turn it around.

The other thing we need to think about is what we are going to do with the board when it is finished. Ordinarily you might install the LEDs flush against the component side of the circuit board, but if you do it that way how will you mount it in a box? The IC's, vertically mounted components, and trim pot (R16) all stand higher than a flush-mounted LED, so it will be difficult to mount the board in an enclosure in such a way that we can actually see the LEDs. Mount the LEDs above the board, with the base of each LED about half an inch above the surface of the board. Visually check that the base of the LED is higher off the board than the other components.

The leads could be bent 90 degrees, but we are going to mount the board by inserting the LEDs into holes and gluing. Install the two end ones first (D1 and D7), measuring carefully, then you can just line the interior ones (D2-6) up visually.

Installing the Integrated Circuits

Integrated circuits are sometimes tricky to install, and as a rule the more pins they have the trickier they are. Some (particularly CMOS devices) are extremely sensitive to static electricity, so you should make sure you have provided a static discharge path to ground before handling them. You can use a clip lead from your watch band to a convenient ground, but if you do that you are really grounded and if you should happen to come into contact with a live wire, the results can be pretty drastic. A much better bet is a commercial electrostatic discharge strap (disposable ones are inexpensive and can in fact be used many times). A commercial ESD strap has a resistance built into it, so while static will drain away to ground through it, a large current will not. It's also a good idea to ground the circuit board before inserting the chips. Connect a clip lead between the ground track (usually around the outer perimeter of the board) and a convenient electrical ground.

Make sure the IC's pins are straight-- sometimes one or more will be bent out of line, and they are likely to fold under or outward as you try to insert the chip if you don't straighten them. Usually you will see that all of the pins are bent out at a slight angle from vertical. In a commercial environment they are inserted with a special tool or by machine. To do them by hand you must bring the pins to vertical first. Grasp the chip firmly with your thumb on one end and middle finger on the other end, and press the pins at an angle against a flat surface such as the top of your workbench. Turn the chip around and do the same on the other side.

When you insert the chip watch closely to see that all pins are going into slots and not bent under or outward. If a pin bends under it can be extremely difficult to find later when the device doesn't work! Press the chip firmly into the socket.

Cleaning Up

You're nearly through building, now, so it's a good time to tidy up. Clean away any trimmed leads lying around your desk top (they can easily get into places they shouldn't and short things out!). Some builders will suggest that you clean the excess solder flux off of the board, but I usually don't bother. The flux removal process is mechanically rough on the board, and seems to cause more problems than it solves. Sometimes, though, I will go back and clean a board where I suspect, but cannot find, a solder problem. I don't like doing it because it is messy and dangerous (unless I use a commercial flux remover which is messy, dangerous, and expensive). I use acetone (readily available in grocery stores as fingernail polish remover) in a well ventilated area and a paint brush for most ordinary fluxes. There are water soluble fluxes available, but if you use those please be sure the board is thoroughly dry before it comes into contact with any electricity!

Probably the best alternative is to use solder containing a "low-residue" flux. It's a little more expensive, but worth it.

The Smoke Test

The smoke test is a time honored tradition in ham radio and electronic construction-- it's the point at which you apply power to a device for the first time, and see if anything catches fire or emits smoke. And we're ready for the Smoke Test on our VM-110 now, right? Wrong! There are still two things to do. First, take one more close look at the board, the component orientations, and the soldering. Second, use your multimeter across the power connections on the circuit board to make sure there is no short. On the power input of most circuits you should find either an infinite or at least a very high resistance. If you measure no resistance (or if your multimeter has an audible continuity tester and you hear the tone), look for a short. If there is a low resistance, look at the circuit diagram and trace the current path from the power supply. Sometimes a low resistance will still allow the tone to sound on an audible continuity tester, so if you hear the tone you might just want to measure the resistance and see if it is zero (a short) or a few ohms (possibly ok).

When you are ready to connect power for the first time, you should be ready to disconnect it very quickly, or apply it only for a moment. With the VM-110 I'd suggest that you connect 12V from your station power supply using clip leads. But if you don't have a 12V supply, you can use the plug pack transformer and plug it into the wall outlet as a way of switching it on and off. Note that the instructions say that the "SOLID WIRE IS THE '- INPUT.'" By this they mean the solid colored wire (without the white stripe); they don't mean solid wire as opposed to stranded wire!

Connect the ground (negative) lead first and then just momentarily touch the positive lead to its connection on the board, or if your power supply has a switch, connect both leads and turn the switch on momentarily. If you blow a fuse, you know you have a short circuit somewhere on the board. If you have a "protected" power supply, it would shut itself down (sometimes you can see this on the supply's voltmeter) without blowing a fuse. If a short circuit is evident, review the soldering and component orientations (especially diodes near the positive power input connection).

If you haven't blown a fuse, you can connect the positive lead and "check for smoke." Look and listen! Sometimes you will hear a crackling sound before your see or otherwise detect a problem. Look for actual smoke, for a resistor turning brown. If everything looks ok, you can make the power connection permanent and continue to set up and operate your device.

I've said "look and listen," but in fact you should use all of your senses with the possible exception of taste. Sometimes you might see or hear a problem, and sometimes you might touch a resistor or transistor to see if it is hot. But don't overlook your nose, in a manner of speaking. Even if everything else seems ok, you might notice what we sometimes call a "brown smell." New electronic equipment does have a characteristic smell, but a brown smell is unmistakable and not a good thing.

A light brown smell indicates that disaster is imminent. A dark brown smell indicates that the disaster has already happened.

Finally, with power applied to the VM-110 circuit, adjust the variable resistor (R16) back and forth and you should see all of the LEDs progressively light up and go out as you adjust R16.

Congratulations! Your VM-110 is built!

Calibration

At this point, you have a working VM-110 and all that is left is to calibrate it. The instructions say that to "properly calibrate your kit you must be able to monitor and adjust the incoming AC."

The ability to adjust the incoming AC is probably beyond most of us, but fortunately it is enough to be able to monitor it. You do have a multimeter, right? Set it on the AC voltage range and carefully measure (and note) the voltage at the outlet you will plug the transformer into.

SAFETY FIRST! Unlike most of the electricity that you will be working with when building kits, AC mains current is DANGEROUS. Make sure you are holding the test leads well back from the metal tips!

Now connect the VM-110 (plug in the transformer) and adjust R16 so that the appropriate LEDs are lit, according to the table on the instruction sheet. For example, if you measured 120 volts, adjust R16 until LEDs one through three are lit. Continue until LED 4 is lit and then tweak it just the tiniest fraction farther (but not enough to light LED 5). That's it-- we're done!

Next month we'll look at putting the finished VM-110 into an enclosure. We'll be using a plastic box, Radio Shack's 270-271⁽²⁾, or any other little box you might happen to have lying around.

But now it's time to pat yourself on the back, grab a vessel of your favorite beverage, kick back and admire the pretty lights.
 
 

Table 1: Basic Resistor Color Code
Mnemonic(3)	Color	Value	Multiplier
Big	Black	0	1
Boys	Brown	1	10
Race	Red	2	100
Our	Orange	3	1,000
Young	Yellow	4	10,000
Girls	Green	5	100,000
But	Blue	6	1,000,000
Violet	Violet	7	10,000,000
Generally	Grey	8	100,000,000
Wins	White	9	1,000,000,000
	Silver		10% tolerance
	Gold		5% tolerance
Notes
1. Most resistors you encounter in kits will have four color bands-- two significant digits, a multiplier, and a tolerance band. No tolerance band indicates 20% tolerance, but these are rare now. An easy way to calculate these values is to write down the first two digits, then add the number of zeros represented by the third (multiplier) band.  2. Resistors also carry power ratings, and if your kit contains resistors of more than one rating you will have to guess at those based on the physical size. Most that you encounter will be 1/4 watt, and size is relative to power rating, so for example a 1/2W resistor will be larger than a 1/4W one. 3. Silver and gold can be multiplier bands if they appear as the third band. In that case multiply the first two digits by .01 for silver and .1 for gold. For example, red/red/gold/gold would be a 2.2 ohm resistor, and red/red/silver/gold would a 0.22 ohm resistor, both having 5% tolerance..  4. Precision resistors can have five bands, with three significant color digits, and a wider space between the multiplier and tolerance bands.

 

1. Rainbow Kit VM-110, $10.95 (+$5 s/h) available from Milestone Technologies Inc., 2460 S. Moline Way, Aurora, CO 80014, or call 800-238-8205 for credit card orders. Also available from Electronic Rainbow Inc., 6227 Coffman Rd., Indianapolis, IN 46268 or call 317-291-7262.

2. Radio Shack-- call 800-842-7422 for store location.

3. An entire generation of military service personnel learned a different mnemonic phrase, but it is no longer politically correct. I'll reproduce it here for purposes of historical accuracy only; you must not learn it. "Bad boys rape our young girls but Violet gives willingly."