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.
As a starting point, let’s look at a schematic of a very simple boost pedal based on the Electro-Harmonix LPB-1.
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.)
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.
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.
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.
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:
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.
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 we cover that separately.
Referring to our simplified schematic form again:
Battery on the Schematic
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.
Battery on the Schematic