Introduction

3.5 mm cables are used very frequently in assistive technology (AT) design. Over years of designing and working with AT, Makers Making Change has created a resource on the basics of these cables to help makers, clinicians, and users. This resource will primarily focus on the core basics of 3.5 mm cables, such as usage in cables in AT, types of cables, and the basic circuitry involved. This will give the reader a great starting point to understand why 3.5 mm cables are an important part of assistive technology. The main topics described in this resource are listed below:

  1. Usage of 3.5 mm Connectors in Assistive Technology
  2. 3.5 mm (1/8 inch) Connector Components
  3. Cable Types
  4. Basic Circuitry

Usage of 3.5 mm Connectors in Assistive Technology

Audio cables with 3.5 mm phone connectors have been used to connect assistive technology devices for a long time and have become an unofficial standard or convention. Since the cables and connectors are used in many consumer devices, they are easy to source and inexpensive.  Having a standard connector type allows devices from one manufacturer to be connected with devices from other manufacturers. For example, a switch interface can be used with a variety of different switches. This modularity provides flexibility to the user and helps reduce costs. Below are open source devices that exemplify how 3.5 mm cables allow for the connection between assistive switches to devices with 3.5 mm jacks.

Graphic showing three columns. The left column showing three adaptive switches. The center column showing a 3.5 mm cable. The right column showing three devices that have a 3.5 mm jack and can be controlled via switch access.

[1] – See Attribution

To be compatible with existing commercial devices, DIY and open-source Assistive Technology typically use the same connector convention. This is exemplified through the use of an Interact Switch and Dice Spinner device (image seen below). The Interact Switch uses a cable with a 3.5 mm plug so it can be connected to any device that allows for switch access through a 3.5 mm jack. The assistive switch is able to connect to the Dice Spinner with a 3.5 mm cable as the dice spinner has a 3.5 mm jack available for switch access. Since the 3.5 mm jack is incorporated into the Dice Spinner design, it allows this device to be used with commercially available switches or open-source DIY switches that use a 3.5 mm cable like the Interact switch. This setup does not require an external power source as the dice spinner itself has batteries to run.

A top down view of an adaptive switch (interact switch) plugged into a dice spinner device.

 

3.5 mm (1/8 inch) Connector Components

The 3.5 mm (1/8 inch) connector is a type of phone connector and gets its name from the outside diameter of the cylindrical sleeve of the plug. This type of connection may also be known as a phone jack, audio jack, headphone jack, or jack plug. These cables connect to a jack for the input. There are three main components that are used to create connections when using 3.5 mm cables, the plug (described as male), cable, and jack (described as female). The plug and cable are often attached (but not always). The cable carries the signal and can have multiple wires inside. The plug is used to make the connection of the wires. The jacks are a separate component that is used to make the connection of the plug and cable to the device.

Three seperate photos labeled cable, plug, and jack.

 

The origin of this connector type can be traced back to the late 1800s for use with telephone switch boards. The 3.5 mm size is considered the “mini” connector. The original 6.35 mm (1/4 inch) version is still used for audio connections like electric guitars, and there is also a “sub-mini” connector with a diameter of 2.5 mm.

Cable Types

There are several types of audio cables that are used to create DIY assistive technology. The cables are specified by the number of conductors/wires and the type of the connectors on the end. The connectors may have 2, 3, 4, or even more contacts.  The plug on the end of the cable will have a tip, a sleeve and may also have 1 or 2 rings. Connectors may be referred to using letters corresponding to the tip, ring, and sleeve combination. e.g., T – tip, R – Ring, S-Sleeve.

Designation Conductors Name Common AT Uses
TS 2 “Mono” Assistive Switch
TRS 3 “Stereo” Dual Assistive Switches
TRRS 4 “TRRS” Analog Joysticks

 

 

These three types of 3.5 mm connectors look very similar at first glance. Although, a maker must be careful when building or using one of these cables as a design may specify a specific type of cable to use. TS cables (mono) are the most common in assistive technology design. This is due to these connectors being able to easily be incorporated into an adaptive switch to trigger devices as it only has two conductors. However, TRS and TRRS cables can be used as a replacement for TS cables, but this will not be covered in this resource. For now, let’s see how to tell the difference between these three types of 3.5 mm connectors.

How to tell the difference:

  • TRS cable (stereo) will have three distinct sections on the plug. A TRRS cable will have four. TS (mono cables) will have two.
  • When stripping the wire, there will be three wires inside a TRS cable (stereo) and four in a TRRS cable. A TS cable (mono) will have two.

TRS, TS and TRRS cables are shown in a graphic. Pointing out the components of the plug and showing the corresponding cables

[1] – See Attribution

If a 3.5 mm cable is used to build an assistive switch, it may require direct soldering to a microswitch, PCB board, or any other component. For this they must first strip the wire. A side-by-side comparison of a stripped stereo cable and mono cable can be seen below. They look different as there is an inner wire for tip, ring, and sleeve on a stereo cable, but a mono cable only has sleeve and tip. There is often extra casing on the inside of stereo cables that can be removed.

In the photos below, a multimeter was used to identify which cables relate to the different conductors for example purposes. Unfortunately, there is no industry standard around colors for identification of which cable relates to tip, ring, or sleeve.

Two images of a TS and TRS cable that have been stripped. There are labels identifying the cables as sleeve, tip, and ring.

Basic Circuitry

The basic circuit that is often created when using 3.5 mm audio cables creates a connection between a device (output) and an input. In AT, the common input is an adaptive switch or adaptive device that allows the user to trigger a switch. This switch then will then send a signal along the cable to the output when active. A basic diagram of this type of connection can be seen below. This image also reflects the setup that was described above with the interact switch and dice spinner device.

Simple circuit diagram showing the input connected to the output via a 3.5 mm cable.

[1] – See Attribution

The above diagram shows the input device is being connected to the output device via a 3.5 mm connector and jack. In this case, the output has a jack connection that the adaptive switch can be plugged into to trigger. The output will be activated when the input is engaged until the trigger is unengaged. This can be seen in the animation below; the toy car has been connected to the adaptive switch via a 3.5 mm connection. When the switch is engaged, the toy car lights up.

[1] – See attribution

Attribution

This resource was created by Makers Making Change while using materials from Loreto Dumitrescu of Zero Day Camp and icons from The Noun Project.

[1] These graphics were created using the help of icons from Noun Project.