Understanding Schematics

Understanding Schematics

15-10-2012

From Schematic to Reality Understanding Schematics Schematics are the lingua franca of electronics. They provide a concise and comprehensive diagrammatic description of a circuit. Plus, they are mostly standardized so once you learn the general idioms of a schematic, you can decipher almost any schematic. Schematics are especially important to stompbox building, because so many schematics are available. Of course, the most popular designs are represented well with PCB layouts, perfboard layouts, vero-board, etc. But if you want to enjoy the true wealth and diversity of designs, you’ll want to understand how to read schematics. This article describes schematics, their symbols, layout and tips and tricks for reading them. From there, we’ll work on how to translate schematics into the real world in the form of things you build on a breadboard, point-to-point, or some type of perfboard media.

Behold, The Schematic As a starting point, let’s look at a schematic of a very simple boost pedal based on the ElectroHarmonix LPB-1.

Figure 1.1: A Schematic You can see that there are various bits represented by symbols, all connected in various ways. Let’s look at some of the big picture concepts: Left to Right: The first thing to notice is that you read the schematic left-to-right: the input on the left feeds the signal through parts and pathways in the middle to an output on the right. This left-to-right convention is not universal, but it is probably the most common layout for a schematic. Power and Ground: The top area of the schematic shows some type of power (in our case, 9 volts Direct Current, the same thing that comes out of a 9 volt battery). The bottom of the schematic shows grounds. This directly maps to the physical arrangement of our power source, again, a 9 volt battery. The top of the schematic is showing the positive (+) voltage, and ground represents the negative (-) side. Symbols: Components are denoted by a standardized set of symbols, each representing a specific type of component. For example: a resistor:

Each symbol shows a part number and a part value or type. R1 denotes two things. First, the “R” signifies a resistor. Even though the schematic symbol itself is unique to a resistor, it is helpful to denote the part type. This is also a somewhat standardized format: R for resistor, C for capacitor, Q for transistor, VR for variable resistor, etc. The number part is just a sequential counter that makes it easy to cross reference against a parts list. The number also makes it easier to talk about schematics. (It’s a lot easier to say “change the R1 value to 500K for more bass” than to say “change the first resistor that is connected from the input to the ground, before the first capacitor, for more bass.) Connections: The connections between components are shown by lines. That is easy enough—anywhere there is a line, you are reading that there is a conductor (a wire or the http://www.beavisaudio.com/techpages/SchematicToReality/

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copper trace on a PCB). Where the connectors cross over can be kind of tricky because there is no real standardized way of showing it. Is it just crossing over with no connection, or is it connected? The following diagram shows the three most commonly used connection representations:

Figure 1.2: Various Ways of Depicting Connected Lines In the first example on the left, a dot shows interconnecting lines. So A, B, C and F are all connected together. Lines that pass over another line are not connected, so D is only connected to E. In the second example, dots are not used. Instead, a line that intersects without the little “hump” pass over, is connected. So the first and second diagrams are the same. The third example shows another where the dot signifies a connection, and non-connected crossing lines do not use the hump pass over convention.

Inputs and Outputs For stompbox designs, you almost always have an input and an output. Unfortunately, how these inputs and outputs are represented on schematics is all over the place. In the most standard form, some of the details about input and outputs are left off schematics because these details remain standard across stompboxes. So when you look at a schematic like this, you are dealing with a sort of shorthand that the schematic author used.

Figure 2.1: Shorthand Depiction of Inputs and Outputs If you look at the input side of the schematic, it is one wire. But the plug on the end of your guitar cable has two connectors. WTF? This is an example of shorthand, and here’s how the schematic maps to the real world.

Figure 2.2: Mapping Shorthand to the Real World The tip of the plug always carries the signal, and the sleeve of the plug is always connected to ground. So when you see the simplified form, it is assuming you will connect to tips of your plugs and jacks to input and output, and both sleeves will be connected to ground. There are other ways of representing inputs and outputs on schematics. For example:

Figure 2.3: Another Way to Show Inputs and Outputs In this example, a more literal form of schematic symbol is used for the input and outputs. It shows the jack part connected to ground. So Figure 2.3 is electrically identical to Figure 2.1.

Power Your stompbox circuits will mostly use a very simple power scheme: a battery or AC/DC adaptor that provides a positive voltage and a negative voltage. The positive side of your power supply goes to the part of the schematic that shows power input, and the negative side goes to ground. In the case of bi-polar supplies, that is not the case, but such a supply is not that common so http://www.beavisaudio.com/techpages/SchematicToReality/

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we cover that separately. Referring to our simplified schematic form again:

Figure 3.1: Power Representation You can see that the positive side of the battery is represented by a symbol denoting + voltage. The negative side of the battery is ground. There are other forms you will see in schematics, such as when batteries are actually shown as a schematic symbol.

Figure 3.2: Battery on the Schematic So as with other forms of shorthand, Figures 3.1 and 3.2 are electrically identical. One of the drawbacks on Figure 3.2 is that it is showing a battery, whereas you may want to connect your circuit to a battery and an AC/DC adaptor. A small point to be sure, but it illustrates another example where “schematic shorthand” can be useful. To round out this discussion of power and input/output shorthand, here’s the bo0ster schematic re-drawn to show grounds in the non-shorthand way:

Figure 3.3: The Revised Booster Schematic

Switching Another confusing aspect can be the switching arrangement. For example, when you look at the schematic in Figure 1.1, there is no on/off switch for the power, nor is there any switching for bypassing the effects. As with input and outputs, the design of power switching and bypass switching is usually assumed. In other words, we assume that when we build an actual pedal from the schematic, we will use the standard 9 volt battery clip wired to the standard 2.1mm DC jack, all in a standard way. Because this power scheme hardly ever changes, there is no real reason to repeat it on each and every schematic. Similarly with bypass switching: the ubiquity of 3PDT true-bypass switching is such that it doesn’t make sense to draw it out in every schematic. So how do you translate the shorthand of schematics to the real world of switching and power? We’ll cover that a little later when we talk about the Stompbox Harness.

Schematic Symbols So now that we have the general lay of the land for schematics, let’s delve into the mysteries of the symbols themselves. By and large, symbols are fairly standardized. However there are exceptions that are introduced to cover the huge array of component types. In this section, we’ll cover the most commonly used symbols and point out any variations you might see.

Resistors, Potentiometers, and Trimmers Resistors are not polarized devices, they work either way. Resistors are shown as a wavy line, like the R3 value below. http://www.beavisaudio.com/techpages/SchematicToReality/

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Figure 4.1: Resistor, Potentiometer, and Trimmer Schematic Symbols Potentiometers have three connections, so you need to know how to match up the three connections on a schematic with the actual pot, like this:

Figure 4.2: Matching Potentiometer Lugs to the Schematic Symbol Trimmers, as shown in TR1 above are potentiometers also, but they are usually small plastic devices soldered to the board as a “set and forget” type of affair. The identification of resistors is simple: The letter R followed by a sequential number. Potentiometers are often denoted as VR for “variable resistor” but may also show up as R. It’s easy to spot the difference just by looking at the schematic symbol. Additionally, potentiometer values are shown using standard code. Potentiometers have very simple codes: a Letter and a Value. The code is: A single letter, A for audio/log, B for linear A Numeric value, i.e. 10K So a 100kΩ linear taper would be B100K. A 1k audio taper would be A1K. Finally, potentiometers and sometimes trimmers) will have an additional label that denotes their function. So in Figure 4.1 we can see that the VR1 potentiometer controls the volume.

Capacitors Capacitors appear on schematics using one of two basic symbols: parallel lines or a straight line and a curved line. In the case of parallel lines, the type is unpolarized, so for our purposes that will mean ceramic or film capacitor. When the symbol is a straight line and a curved line, the capacitor is polarized and the straight line side represents the positive side. Polarity may also be indicated by a + symbol.

Figure 4.3: Capacitors on Schematics

Diodes Diodes are polarity sensitive, and the cathode side is indicated by a colored band.

Figure 4.4: Diodes on Schematics The following graphics illustrates mapping between the schematic symbol and the actual device:

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Figure 4.5: Diode Polarity Mapping For stompbox use, you are typically going to use small signal diodes. These can handle about 100mA of power. Since a LED is just a special type of diode, it follows the same convention in terms of having an anode and a cathode. In terms of packaging, the longer leg is always the positive side. There is also a flat side, which denotes the negative side.

Figure 4.6: LED Polarity

Transistors Transistors almost always have three legs, and the pin outs (i.e. which leg is the Base, which is the Collector, and which is the Emitter) can be confusing. One of the most common reasons a transistor-based circuit won’t work for you is that you inserted the transistor wrong. So it is important to look at the pinout for the specific device.

Figure 4.7: Transistors on Schematics

Integrated Circuits Integrated Circuits (known as ‘chips’ in the vernacular) are even more amazing the transistors, because inside, they contain hundreds or thousands, or even millions of transistors. ICs are roughly divided into linear and logic types. Linear types include operational amplifiers, and logic types include counters, logic gates, etc. Because integrated circuits come in some many configurations, you’ll find there are several representations for them. The most common IC used in stompbox circuits is the operational amplifier or opamp. This has a pretty standard pin out and configuration across types so it has its own schematic symbol.

Figure 4.8: Opamp Schematic Symbol We can see that the opamp symbol is a triangle with two inputs and one output. Opamps have negative and positive inputs, so those are shown. Also shown are the pin numbers for the specific opamp. There are many types of ICs that are specialized enough that they don’t have their own specific schematic symbol, so they are drawn as a rectangle or square with pins shown in whatever order makes sense in the schematic layout:

Figure 4.9: Generalized IC Schematic Symbol There are also logic and other types of integrated circuits that have their own schematic symbols, like these: http://www.beavisaudio.com/techpages/SchematicToReality/

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symbols, like these:

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Figure 4.10: Other IC Symbols Most ICs you will use in stompbox projects are plastic dual inline package (DIP) devices with a variety of pin counts and pin outs. Note that the chip orientation is always denoted by a notch, or printed dot, on one end.

Figure 4.11: Identifying Pin 1

Schematic Cheat Sheet

The Big Picture: What does Each Part Do? So now we have a good feeling for how to read schematics. But what do each of these parts do? Learning about the function of each component and its complex interactions both within a circuit, and with the things that it connects to is the purview of electrical engineering, and beyond the http://www.beavisaudio.com/techpages/SchematicToReality/

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scope of this article. However, it is useful to look at a simple example to try and weave all the things we’ve learned so far back into a coherent example. So let’s look at the booster schematic again.

Figure 5.1: A Schematic We can easily identify the input and output. The signal you want to modify is presented to the input, the goo in the middle does the work, and presents is modified signal to the output. Let’s look at each component, generally left to right. After the input jack, there is R1, a large value resistor that connects to ground. This is something you will see very often in stompbox schematics—it helps set the input impedance of the circuit to a level where it doesn’t drag the guitar’s pickups down to much. C1 is the input capacitor which filters and DC out of the signal. It also controls the frequency response of the input signal as it is presented to the transistor. R2 and R4 form a voltage divider. This simple snippet is in charge of providing half of the 9 volt source voltage as a reference point to the base of the transistor. This reference point helps tell the transistor how much to amplify the signal. R3 and R5 set the gain factor of the transistor, which simply means that it tells the transistors how many times to amplify the signal. The signal then goes to C2 which removes the DC component of the signal. Finally, we are off to the potentiometer for volume. The pot is wired as another voltage divider. Depending on where you turn the knob, you are balancing how much of the output signal goes tor ground (i.e. thrown away or attenuated) and how much goes to the output. That’s it—a single transistor and a handful of components give you a nice linear boost circuit.

From Abstract to Reality: Let’s Put it on a Board One of the key reasons to learn how to read schematics is to be able to speak the language of electronics, the ability to look at a picture and get a general idea of what it does and how. But the other more concrete reason is that you want to actually build something. Which leads to the central point of this article: how do you turn a schematic from abstract symbols to an actual working thing? The good news is that schematics are not all that abstract. In fact, in most cases you could lay out your physical components in an arrangement pretty much the same as the schematic and then connect wires just like in the schematic. While that makes sense, it is not really practical. There are much easier ways to do it.

On the Breadboard Probably the easiest way to transfer the conceptual schematic to a physical dimension is to use a breadboard. Breadboards also have the advantage of non-permanence—unlike solder you can undo mistakes easily and experiment with different values. Most breadboards are conveniently organized in a way very conducive to stompbox hacking. Take a look at the following diagram:

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Figure 6.1: A Typical Breadboard You can see that we have positive and negative strips running down the left and right edges of the board—very convenient for connecting our various bits to power and ground. There are also a bunch of “strips of 5”. These are the places where we can insert components and wires to form a physical arrangement that maps to the schematic. (Note that the above breadboard is representative of one of the most common types, but others have different arrangements.) So, to build our LPB-1 Booster on the breadboard, we simply work through the schematic and arrange components and wire jumpers. Like this:

Figure 6.2: The LPB-1 Booster on the breadboard As you trace through the schematic, compare it to the breadboard. Usually there is an “aha!” http://www.beavisaudio.com/techpages/SchematicToReality/

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moment when you realize how simple it actually is.

Non-Breadboard Reality Once you have traced a schematic, tried it out and want a more permanent solution, there are various options. This section outlines some of the more common board techniques.

Perfboard There are various types of perfboard and the term itself loosely covers a lot of different designs. The most common type is pad-per-hole. It looks like this:

Figure 7.1: Pad Per Hole Perfboard The board itself is made of a rigid insulating material, and there are rows and columns of holes. On the pad-per-hole layout, each hole is surrounded by a copper pad. None of the copper pad/hole combinations are connected to any others. So you stick your component through the top side of the board, flip it over, and solder it to the pad on the other side of the board. You then solder bare wires to the underside to form the connections. For example, the following diagram shows the connection between a resistor and capacitor on a per-per-hole layout:

Figure 7.2: Pad Per Hole with components Pad per hole has the advantage that you can layout your components and wires much like a schematic. The grid of holes that you work on presents a great way to match up components on a schematic to an x/y grid on a board. The disadvantage of pad per hole is that it can be somewhat tricky to get all the soldering clean and not have it run and create unwanted solder bridges. Also, unless carefully planned, pad per hold can lead to larger board sizes as compared to other mediums. Other than that, pad per hole is a great way to turn schematics into reality. There are other types of boards that fit into the perfboard category. These usually have bus connectors—copper traces that connect a group of holes in interesting ways. For example, Radio Shack sells a number of perfboards that make it much easier to build on than pad-per-hole designs. For example, their IC prototype board makes it easy to supply power (+ and ground) and use ICs and other devices:

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Figure 7.3: Radio Shack Prototyping Board Prototyping boards like the Radio Shack version shown above have a big advantage over pad per hole designs: they have pads pre-connected in ways that make build a lot easier. For example, look at the middle of the board. There are two strips of connected pads that run the length of the board. These are very useful for power and ground. Similarly, there are groups of 3-connect pads and groups of 2-connect pads. These make it easy to connect multiple component terminals which means a lot less wiring. Here’s an example of using the Radio Shack board with an integrated circuit to create a tone generator:

Figure 7.4: Radio Shack Board in Use

Veroboard Veroboard (also known as stripboard) is a specialized form of perfboard. It is a name-brand product that arranges holes along a connected bus. To form circuits, you make small cuts in the bus trace to match the schematic you are working on.

Figure 7.5: Veroboard Veroboard diagrams show where to make trace cuts (usually with a small Xacto type knife) and where to place and solder the components. For example, here’s a layout that shows red dots that signify where to cut the traces, and a few components shown.

Figure 7.6: Veroboard Explained

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Printed Circuit Boards (PCBs) are probably the best way to build things if you are doing more than one, or want a more professional result. But they require skills that are sometimes impractical for beginners. In other words, you can do a lot more learning, testing and experimenting with the other types of “reality” devices discussed here. If you want to make your own PCBs, there are many resources on the interwebz to help you. Additionally, lots of DIY sites, like General Guitar Gadgets and TonePad have PCB layout artwork you can download and use. Here’s a layout for my Noisy Cricket PCB. Generally, a layout file will contain both the PCB layout artwork itself, and a graphic showing the location and orientation of components for the board.

Figure 7.7: PCB Transfer Artwork

Figure 7.8: Parts Layout Diagram

The Stompbox Harness Earlier, we talked about all those interesting shorthand notations found in schematics. Like the fact that true-bypass switching is usually not shown. Same for power on/off switching, the battery connector and the power jack for an AC adaptor. The following diagram shows a “Stompbox Harness”, a generalized component and wiring diagram that forms a generic shell to place your circuit board in. It features true-bypass switching, and dual power: either a 9 volt battery or an AC/DC adaptor.

Figure 8.1: Stompbox Harness Note that there are several ways to accomplish true bypass wiring. Check out the following link from General Guitar Gadgets for loads of information on true-bypass wiring options. http://www.generalguitargadgets.com/index.php? option=com_content&task=view&id=33&Itemid=27

DIY Layout Creator No discussion of creating circuit boards would be complete would be complete with a nod to a fine http://www.beavisaudio.com/techpages/SchematicToReality/

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fellow named Bancika. He created a free piece of software called DIY Layout Creator that is a work of genius. DIY Layout Creator allows you to graphically draw layouts for projects, using pad per hole, veroboard, or printed circuit boards as your medium.

Figure 9.1: The Incredibly Cool DIY Layout Creator Software As you can see from the above screenshot, you have a list of drag-and-drop components on the left, a design area in the middle, and an explorer on the right. DIY Layout Creator would be cool if it was the product of a team of software engineers from a big company. But from a single guy toiling away to develop a free program, it is simply incredible. If you are going to be doing anything of consequence in stompboxes, I highly recommend you download this program, give it a try, and then send a quick donation to Bancika—he’s a cool guy. http://www.storm-software.co.yu/diy/index.php?project=software Finally, there is also a huge library of layouts for you to freely download, including pad per hole, veroboard and PCB layouts at the same site. http://www.storm-software.co.yu/diy/index.php?project=layouts

Resources Thanks to google, the world really is at your doorstep. Here are some useful places to go as you work with schematics, layouts, and boards. What Great gallery of layouts including pad per hole, veroboard, and PCB designs Bancika’s DIY Layout Creator Software Layout Library for DIY Layout Creator General Guitar Gadgets: Lots of projects and layouts Runoffgroove: Lots of projects and layouts Tonepad: Lots of projects and layouts DIY Stompboxes: THE forum for diy stompbox stuff PCB Layout for Musical Effects: R.G. Keen's comprehensive book on doing layouts right.

Where www.aronnelson.com/gallery/main.php www.storm-software.co.yu/diy/index.php?project=software www.storm-software.co.yu/diy/index.php?project=layouts www.generalguitargadgets.com www.runoffgroove.com www.tonepad.com www.diystompboxes.com/smfforum www.smallbearelec.com/Detail.bok?no=679

Conclusion I hope that this short article has cleared up some of the mysteries of schematics for you. Of course there are a thousand more details, variations and confusions as you start learning to read http://www.beavisaudio.com/techpages/SchematicToReality/

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schematics and transfer them to the real world. But hopefully you have a basic understanding of how they work, and how they map to the real world. As always, I love to hear feedback, corrections, and even the occasional flame. Pop me an email at dano/ at / beavisaudio.com (c) 2005-2012. Some Rights Reserved - This work is licensed under a Creative Commons License

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