IR Sensors & Interrupters
Reflective IR sensor
This type of sensor is commonly found in cassette decks & VCRs.
It works by reflecting the IR beam off a patterned surface and
detecting the change in returned IR level caused by the alternating
silver & black areas on the rotating disc. When the pulses stop,
the circuitry knows that the tape is at the end & stops the player.
Some VCRs also use the output from the sensor to derive speed
information that can be used to calculate how much tape is on each
cassette reel or to optimise other functions.
One possible use for these sensors in robotics would be to mount
one or more underneath (facing down) to detect the table top and
take action to prevent your robot falling off the edge of the table !
You may be able to strip these sensors out of dead cassette decks.
The sensor is usualy comprised of an infrared led and a matching
IR sensitive photo-transistor. When the IR light shines on the photo-
transistor, it turns on just like a normal transistor. One way to hook
it up is to connect the LED cathode & transistor emitter to 0V. Then
add a resistor from the LED anode to V+ (1K for 5V) and another
(4K7) to act as a pullup from the transistor collector to V+. Take
your signal from the transistor collector. Try different value resistors
for the LED & transistor to get the best sensitivity for your application.
Photo Interrupter
This type of sensor is most often found in carousel CD players.
It works by transmitting the IR beam across a small (<5mm) gap.
A slotted disc or ring passing through this gap interrupts the IR
beam in a pattern that indicates position information to the micro.
By detecting the relative widths of the gaps in the plastic ring, the
micro can determine the absolute position of the rotating CD tray.
Think of it as morse code, only done with flashes of IR light that
can be short or long. The relatively constant speed of the rotating
CD tray make it easy for the micro to decode this into 0's & 1's.
Sometimes photo interrupters are used in place of conventional
switches. A sliding or swinging lever simply moves into the gap
to activate the switch. Floppy drives often use them to sense when
the read/write head is back at the 'home' position. Having a sensor
with zero backlash (often a problem with mechanical switches)
means that the activation point is very precise and repeatable.
One possible use for these sensors in robotics would be to have
'feelers' on the outer edges that push into the gap when pressed.
This could trigger the robot to change direction if it encounters an
obstacle or perhaps seek out objects in a room.
Main Robotics Page