Re: How do proximity sensors and optical transceivers work?

There are a lot of different kinds of sensors around that detect the 
presence of people or objects.  While some of them actually sense that 
something has moved to a position near the sensor, most do not -- they 
usually just detect that something has changed near the sensor.

Probably the most common kind of proximity sensor -- the one in home 
intrusion alarms -- uses infrared sensors.  Basically, several infrared 
detectors stare continuously at an area under surveillance.  Because these 
things have to be inexpensive, there are not enough detectors to form a 
good image -- usually there are something like 10 or 20 detectors that 
cover different adjacent areas in a room.  Electronics monitor the output 
of the detectors and notice when their output changes relatively quickly --
 either getting lots larger or brighter.  When an appropriate change 
occurs, a switch is closed which actuates an alarm or opens a door or 
whatever.  The basic idea is that a person passing through the area that a 
detector is looking will either block some of the infrared radiation that 
the sensor is seeing or will emit more infrared radiation that the 
background that the sensor is looking at, so that the change will be 
noticeable.  Most sensors are pretty smart, in that if something moves too 
slowly (the light from the sun changing position during the day) or too 
quickly (something falling through the field of view), they will ignore 
it.  Some also even notice whether the changes occur sequentially moving 
from one detector to another in the way that a person might move.  In any 
case, these sensors work pretty well.

Other sensors send out a beam of light and look for reflections from 
objects that they might hit.  In the simplest form of this sensor -- used 
to automatically flush toilets after use -- a weak infrared diode beam is 
projected and if a receiver detector located next to the transmitting 
diode sees a return, it closes a switch and flushes the toilet.  The diode 
output is weak enough so that reflections off the walls of the restrooms 
are too puny to trigger the receiver, but just strong enough so that the 
reflection off a person will be adequate to actuate the device.  Other 
forms of this device use an infrared laser beam projected across a room, 
where it is reflected off a mirror or bike-reflector-kind-of-array and 
back to the receiver located near the transmitter (usually in the same 
box).  When someone moves between the transceiver (trans-mitter and re-
ceiver... get it?) and the mirror, the return reflected from the person is 
too weak for the receiver to see it.  The receiver notices that 
something's wrong and triggers the desired effect -- in the case of a 
garage door opener, these devices stop the door from closing to protect 
any animal or kid that may have walked under the door as it was being shut.

There are also tons of other variants on these two themes, but the basic 
two ideas --(1) a passive receiver, or (2) a transceiver -- are involved 
in almost all motion or proximity sensors.  Particular units may use 
visible detectors or millimeter wave detectors or microwave devices, but 
the principles are the same.

Hope this helps!