University of Edinburgh

Vision for Doing

Assessing Functional Vision of Learners who are Multiply Disabled

Part 3: Topics Revisited

Introduction to part three

In the final part of the book we present a few topics in a little more depth. It comprises four short Appendices dealing with:

Appendix I 'Switches and Interfaces'


At several points in the main text we mentioned the use of switches. Here we delve into a little more detail. Nowadays, any site of the body which is under voluntary control may be enabled, through choice of the right switch, to operate a variety of devices. Switches may be effected through blowing and sucking, eye movement, head pointing to a target, ear wiggling, not to mention foot kicking and other more conventional means of operation. Unfortunately, the use of this technology is not always carried out in a principled fashion. Consider a learner lifting an arm, which then turns on a light source. Seems simple enough. However, you might remember the questions which were first raised in Section 9, in the discussion on Improving Awareness. The questions were:-

  • Should the event ( light switch on) occur immediately the arm is moved?
  • In which direction is the movement to occur? On the same side as the arm movement or the middle or the opposite side - above or below?
  • Should the light be bright or dim, coloured or white?
  • For how long should the light be activated?
  • What if the learner lowers his arm, should the light stay on or go off with the lowering?
  • Should every arm raise result in the light being activated, if not how often should they occur?
  • Should the light always be presented in the same position relative to the learner?
  • Is it best to have a switch which is latched, or momentary or timed; and should it be normally on or normally off?

The answer to each of these questions will present different levels of difficulty to a learner, and the answer will depend on what his or her previous experience of similar and different activities have been. In what follows we outline some information which may be helpful when you come to consider these questions. Finding your way and making sense of catalogues, reference materials and reports dealing with technological devices operated by 'switches' requires some knowledge of the range of different terms that are used. For a more detailed description of switches and interfaces the reader is referred to the ACE Centre documents 'Switches and Interfaces' and 'A Common Terminology for Switch Controlled Software'.

What are switches?

The term 'switch' is usually used to refer to a mechanically-operated device, which, when pressed, touched, pulled, squeezed, approached, yelled at, or jumped on by a user, will send an electronic signal to a toy, computer or other piece of equipment.

'Switch' should really only be applied to devices which actually switch signals from one state to another, eg from off to on or vice versa. Switches can range from simple 'lever' types to devices that detect the angle of a user's limb, to sensors which detect muscle activity itself. There are single, double, 3 or 4 way and multiple switches: keyboards and concept keyboards are simply multiple switch arrays for example.

However, 'switch' is sometimes also used to refer to devices whose signal changes gradually between 'on' and 'off' as the user controls it (like the volume control on your hi-fi which can vary anywhere between zero and maximum). This set of devices are more properly classed as 'analogue', 'proportional', 'pointing' or 'direct selection' input devices. Examples are mice, analogue joysticks, light pointers and touch screens.

There are advantages and disadvantages of both: analogue devices obviously offer more control and speed over the computer/wheelchair/toy/appliance, but usually demand quite fine motor control.

Types of switches

The ACE 'Switches and Interfaces' document categorises switches as follows:

Limb & head operated


- simple mechanical switches operated by pressing, pushing or pulling. Important considerations are the size of the switch, the travel (how far you have to move it until it operates), and the operating force required.


- switches which respond to being squeezed, eg between fist, or thumb and finger.

pressure pad

- rather than being activated by clicking a mechanical lever or plunger, these switches operate by detecting a change in air pressure. They have a different 'feel' to mechanical switches.

light sensitive

- switches which detect the presence or absence of light - the user therefore points a light source at them, or uses a body part to block a beam of light.


- switches which react when a user comes close, or just touches them.

foot switches

- foot operated switches are basically heavy-duty lever switches.


- wobble switches or sticks are robust wands which can detect movement or a blow in any direction.


- other switches exist which detect very small muscle movement ('P-switch, eye movements ('Twinkle, eye-brow wrinkling, etc.


The switches above translate one user action into one signal. Joystick switches are different in that their operating lever can move in different directions, each of which can give a different signal. Joysticks are usually more efficient than the equivalent number of separate switches, but require better motor control.

Mouth and tongue

For users with no other motor control, there are a large number of 'suck-puff' switches available, together with other switches that can be operated by the tongue.


Sound switches can be set to detect sounds of certain volume (and sometimes pitch) produced by the user.


Contact and non-contact

Switches can either require the user to make physical contact with a target, but can also detect the user's activity from a distance. Such 'non-contact' switches demand different cognitive, perceptual and physical skills from the user, because the only feedback available to the user is from the device that is being controlled - there is not necessarily any tactile, visual or auditory response from the switch itself. This can make the switch hard to use because neither the user nor the teacher/therapist can tell when the switch is being activated: an example is a mercury tilt switch, which tends to produce a jittery 'on-off-on-off-on-off' sort of signal. It's therefore a very good idea to either purchase a switch which does have some form of added feedback, or to obtain a separate unit to provide such feedback.

Bounce and other imperfections

The jitter from the mercury switch is an example of 'switch bounce', which is something that almost all switches suffer from. For most of them, the signal settles down within a few milliseconds, so the effect on the toy or whatever is unnoticeable, but for some devices or computer programs, the effect of switch bounce may cause problems. Various electronic boxes and some programs have settings which can remove or damp out switch bounce.

Some switches produce a 'pulse' rather than a change of state, so that the toy activates momentarily, then goes off again and again, there is hardware and software to make the switch more useful.

There are several other more obscure switch characteristics which are described more fully in the ACE 'Terminology' document.

Normally on/normally off

Most switches are 'normally off' ie. they are off and when pressed or activated, turn on. It is also possible to obtain switches which are 'normally on', and which turn off when they are activated.


Many switches and toys are described as 'momentary', 'timed' or 'latched'. Momentary switches operate like bell-pushes -they stay on for as long as the user activates them. Latched switches are like on/off power switches - a first press and release switches on, and a second switches off. Latched switches are sometimes said to 'toggle' between the two states. Timed operation means that the user activates the switch, and the device comes on for a set time period, then goes off by itself.

Each type requires different skills and can be used for different purposes and devices.

Momentary switches are most common and possibly most useful to the user full control is offered over the switching on and off of the device. Momentary control is the same as 'user scan' a form of 'scanning'. However, momentary control requires that the user be able to activate, hold, and deactivate the switch when desired. These switches may be beyond the abilities of many users who are physically disabled, and they are cognitively more difficult.

Latched switches also enable the user to have full control over the device, but rather than holding the switch on then releasing to switch off, the user presses the switch a second time to switch it off. The technique can therefore be used by learners who have difficulty maintaining the switch activation - however it does require more accurate activation. A variation on latched control using a single switch is latched control using two switches: one switch controls 'on', and the other 'off'. Latched control is the same as 'auto-scan'.

Timed control only requires that the user activate the switch, and leaves the deactivation of the switch up to the electronics. In addition, the device normally ignores any further inputs from the user until the timed period has elapsed, and so it can be useful for jittery switches (or jittery users). Not all 'timed' devices are created equal however - for some, if the user holds down the switch after the timer has finished, the device is still kept on. Others will switch the device off regardless of the condition of the switch, and so the user can only switch it on again by releasing the switch then activating it again. The former method gives more control to the user but may be more confusing both for user and for teacher/therapist. Timed control may provide the most useful introduction to switch and toy control for many learners because the 'reward' is maximised for minimum input. On the other hand (depending on the switch and the device being controlled), momentary control may give a better and more consistent relationship between cause and effect.

1Further Reading: Switches and Interfaces ACE Centre; Ormerod School; Waynflete Road; Headington; Oxford OX3 8DD.

2'A Common Terminology for Switch Controlled Software' ACE Centre.


Most switches have a cable with a 'jack' plug on the end and most toys and other hardware have corresponding jack sockets into which you plug the switch. However, some equipment is supplied with different connectors and using it may require an adaptor which can be obtained from audio or electronic shops such as Tandy.