Resistors are the most common component in DIY synth design. As the name implies, they provide resistance in a circuit by acting in opposition to the flow of electric current. This resistance is measured in Ohms, though most circuits use resistors that are rated in the thousands of Ohms (kilo-Ohms) or millions of Ohms (milli-Ohm). Resistors are typically cylindrical, slightly compressed around the center axis. It has two leads, one out of each end of the cylinder. Resistors are not polarized, so there’s no distinction between the leads when installing it in a circuit board.

“Fixed” resistors have a single resistance value, which can be deciphered using the colored bands on the resistor. I find that the colors are not always easy to read, and there’s no absolute standard for the tint of the colors, so “orange” bands from one manufacturer might look exactly like “brown” bands for another. I find it much easier to store my resistors in clearly marked bags. If I end up with strays that I can’t readily identify, I use a multimeter to measure the resistance. One thing to note is that all resistors have a tolerance rating, generally between 0.1% and 5%, which is akin to a +/- rating for the resistance value. In other words, a 100k resistor with a 5% tolerance might actually fall anywhere between 95k and 105k in real resistance. When using a multimeter, you might find that the resistor is off a few ohms as a result. This almost never has meaningful impact on the circuit and 1% tolerance is completely acceptable rating.

In addition to the resistance and the tolerance, resistors also have a power rating in Watts. Most anything you would be building at home would not consume enough power to warrant anything greater than 1/8W, though I find that 1/4W resistors are easier to physically handle with my clumsy hands.

Potentiometers

Potentiometers, or pots, are a form of “variable” resistor, where the resistance can be changed on the fly. Common applications include anything that you turn and colloquially call a “knob” (not to be confused with what the British colloquially call a knob). Pots have three terminals that connect to the circuit, either by soldering wire to the terminal (solder lug/panel mount) or by inserting the terminals into holes on a circuit board (PCB mount).

Pots consist of a resistive element and a contact that moves along the resistive element, like a track. Internally, each end of the resistive element is connected to one of the outside terminals (lugs) on the potentiometer, while the inner terminal (wiper) is connected to the contact that moves along the track. Controlling the resistance is done by either turning a shaft that rotates the wiper (standard, radial pot) or by sliding the wiper along a straight track (slider or sliding pot). In most common applications, a pot will have a sweeping range of a single turn (i.e., the shaft turns almost 360 degrees to complete a full sweep of the available resistance). Some special applications might call for multi-turn pots or pots that have less than a single-turn range.

Resistance Tapers

The taper of a pot refers to how it responds to changes in resistance along the full sweep of the shaft. There are two main types: logarithmic and linear. A linear taper pot has a flat taper, where the resistance changes are proportional to the distance the wiper moves along the track (when the knob is turned halfway, the output is one half the total value of the pot’s resistance). This is good for non-audio applications where the affected portion of the circuitry needs consistent changes. Logarithmic pots have a taper where the response is more curved and not proportional to the distance between lugs (when the knob is turned halfway, the output does not equal half the resistance).

There are also “anti-log” or inverse log or reverse log pots, where the logarithmic curve is inverted.

Potentiometers can also come with multiple “gangs”, or sets of terminals. A two-gang pot allows you to control two different gangs of terminals with a single knob.

Most potentiometers will have their resistance value printed somewhere on the pot, either on the top, above the shaft, the back of housing, or stamped on the bottom. The resistance value should be followed by a letter to indicate its taper: A = linear taper; B= logarithmic taper; C = anti/inverse/reverse logarithmic

Trim Pots

Trim pots are small potentiometers that are generally used in applications where the variability of other components in a circuit make it difficult to determine the exact resistance necessary to achieve a desired outcome. For example, a trim pot might be used in a circuit to allow for precise calibration. Trim pots typically do not have shafts and knobs that are adjusted by hand, but instead the shaft is a small cylinder that is adjusted with a screwdriver.

Trim pots can be single-turn–the range of adjustment occurs in a single rotation of the shaft–or multi-turn–the range of adjustment occurs over several rotations of the shaft. Multi-turn trim pots can range from as low as 5 turns to as many as 25. The more turns available, the more precise the control over the output.

Thermistors

Thermistors are resistors whose resistance value changes when the ambient temperature around the thermistor changes. These are useful in circuits where other components are sensitive to temperature changes and the resistance needs to be variable to account for that sensitivity. You’ll commonly find thermistors in older analog oscillator designs that don’t use temperature compensated ICs (like the AS3340/CEM3340).