Gear Potentiometers – A Quick Introduction

Gear Potentiometers are also called Thumbwheel Potentiometers. Let’s investigate them.

There are many varieties of compact gear potentiometers available. Most of them are single and dual – this time we’re going to opt for a dual (stereo) gear potentiometer.

The gear potentiometers are popularly used in audio systems. They can be easily connected to the analog input of a microcontroller like an Arduino or a Raspberry Pi. Below are the quick technical specifications of the gear potentiometer demoed here.

  • 16mm Round Dial Potentiometer
  • Dual potentiometer with common ground
  • 5 Pin interface
  • Resistance: 50KB (B503)

Note, the “B” denotes a “linear” taper potentiometer (https://www.mvorganizing.org/what-is-the-difference-between-a-and-b-potentiometers/).

This appears to be a standard dual potentiometer, widely used as a volume control in portable stereo audio systems. Since it contains two individual potentiometers, connected to a common shaft, it has two input terminals, two wiper terminals, and one common/ground terminal. Identifying the terminals of this type of dual-potentiometer without looking at the datasheet is not easy, but a multimeter can help you determine the pinout. Below you will find a sample datasheet snip.

This is the pinout of my unbranded and cheap 50KB gear potentiometer that came without a datasheet!

Although commonly used in audio applications, gear potentiometers can also be used “limitedly” to test some other hobby-level concepts such as simple elevation encoders. Note that potentiometer and encoders are both electromechanical devices that can measure position. The key difference is that a potentiometer outputs analog signals whereas an encoder provides digital signals. When it comes to the merits of the potentiometer, there are times when it’s better to employ a cheap potentiometer rather than an expensive encoder.

Below is the image of GE’s (https://services.gehealthcare.in/gehcstorefront/) potentiometer gear assembly for Computed Tomography (CT).

An encoder is a sensing device that provides feedback as it converts motion to an electrical signal that can be read by some type of control circuitry in a motion control system (encoders may produce either incremental or absolute signals). The feedback signal can be used to determine position, count, speed, or direction (https://www.encoder.com/).

This is another nifty example. It is a little pedal for speed control with a common potentiometer (https://www.thingiverse.com/thing:4300675).

Today, inexpensive rack and pinion mechanism kits (see below) are available everywhere, so with a little skill and patience, it’s easy to build linear encoders from standard gear potentiometers.

Simply put, the mechanism involves a regular gear wheel rotating and impinging on what could be called a rack (in this application the rack and pinion mechanism translates linear motion into rotation). The knob/wheel of the gear potentiometer in most cases acts as a pinion, otherwise use it after attaching it to another pinion. A rough pointer is shown below.

Since the 50KB gear potentiometer under inspection is a “stereo” potentiometer, it consists of a pair of circular tracks of a resistive material, terminated at each end and a pair of moving contacts known as a wipers that move along the track to set the resistance presented to the external circuit relative to the fixed connections. For a quick test, one part of the dual-potentiometer was used with the help of a simple code to operate an Arduino Uno as an analog threshold detector.

Using a10mm green LED as the threshold indicator, the 50KB gear potentiometer was operated while observing the voltage at the wiper during its excursions from 0V to 5V. The results were mostly smooth and promising!

See the Arduino Sketch:

const int POT_PIN = A0;

const int LED_PIN = 3;

const int SET_THRESHOLD = 512;

void setup() {

pinMode(LED_PIN, OUTPUT);

}

void loop() {

int analogValue = analogRead(POT_PIN);

if(analogValue > SET_THRESHOLD)

digitalWrite(LED_PIN, HIGH);

else

digitalWrite(LED_PIN, LOW);

}

This is the “WOKWi” simulation link https://wokwi.com/arduino/projects/314128806677840448

A few hiccups along the way?

Potentiometers pose significant challenges in long-term use because they’re simple electro-mechanical devices.

The common fault introduced by aged (or dusty) gear potentiometers is the jitter in the signal, both when being operated and when static. This nettling noise can manifest as crunching in audio circuits, or instability in settings for key parameters.

Now to the result of a quick examination using an oscilloscope (for the test, a bit aged potentiometer was operated while monitoring its wiper voltage). The legion vertical elongations to the trace below signals that wiper of the potentiometer is disrupting its touch with the resistive track for substantive periods, which will badly the operation of the circuitry which it controls (always an undesirable behaviour).

This can be tricky. If they’re cleaned sensitively and lubricated with the correct greases, their performance can be sustained over a long time!

I hope this little post inspires you to tackle your own gear potentiometer projects – whatever your skill level. As always, be positive and constructive 👍

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