Biomedical Engineering Reference
In-Depth Information
FIGURE 2-3
Push-button switch
configurations.
(a) Normally open.
(b) Normally closed.
(c) double throw.
FIGURE 2-4
Examples of limit
switches.
(a) Micro-switch.
(b) Optical. (c) Reed.
(d) Hall device.
In the interests of reliability, mechanical limit switches have mostly been superseded by
optical or hall switches (discussed later), which have no moving parts and are generally
easier to interface. Figure 2-4 shows photos of some examples of microswitches and their
electronic equivalents.
2.2.4 Rotary Switches
Rotary switches consist of a number of circular wafers containing many poles operated
by a spindle that passes through the center of each wafer. There are many types, including
both shorting (make-before-break) and nonshorting (break-before-make). However, as
with most other mechanical switches, these are not particularly reliable and are being
replaced by their electronic counterparts.
2.2.5 Optical Switches
Optical switches consist of an infrared light-emitting diode (IR LED) and a phototransistor
sensitive to a similar optical wavelength, mounted into the same plastic holder. These are
mostly configured so that the IR LED is directed toward the phototransistor across a slot
in a transmission configuration, but some are mounted at an angle to operate in a reflective
configuration. In the transmission configuration, the phototransistor operates as a switch
that is conductive (on) if it is illuminated by the IR LED. In the transmission configuration
if an opaque object is introduced into the slot, then the light path is blocked and the
phototransistor turns off, whereas in the reflective case the switch is off until a reflective
material is introduced to reflect the light from the IR LED back into the phototransistor.
2.2.6 Other Switches
Reed switches generally consist of a normally open contact pair mounted on ferromagnetic
reeds in a hermetically sealed glass tube so that, in the presence of a strong magnetic field,
the contacts are drawn together to close the switch.
The Hall effect generates a potential difference orthogonal to the direction of current
flow in the presence of a magnetic field. Hall switches are integrated circuits (ICs) that
exploit this effect to produce the electronic equivalent of reed switches.
