A close-up of the proximity detector on my robot. There are two IREDs in
the white plastic tubes on the very left and very right. The black tape over the
plastic tubes prevents stray light from triggering the detector in the middle.
In this line follower, there is no
insulation around either of the legs of
the phototransistor or IRED. That’s
because I don’t think the robot will ever
crash hard enough to bend the leads
until they touch. If that is a risk with
your robot, then you ought to slide a
length of thin tubing over one of the
leads before you solder. Notice that
there is an aluminized plastic tube
covering the IREDs and phototransistors.
40 kHz. In the schematic; a red box
surrounds the oscillator circuit.
The values of the resistors and
capacitors set the circuit’s oscillation
frequency. To account for variations in the
resistor and capacitor values, a trimmer
resistor is added to the circuit to tweak
the final oscillation frequency. When the
robot controller sets either the L (left) or
the R (right) signal to high, it creates a
ground that allows the IRED to blink at
40 kHz. The detector — positioned between the IREDs —
reports whether it sees a reflection from either of the IREDs
(i.e., if there’s an obstacle in front of them). The two widely
spaced IREDs allow the robot to detect and determine if
obstacles are on the left, the right, or in front of the robot.
The schematic for a 40 kHz obstacle detector. By monitoring when the detector
sees a reflection from the IREDs, a robot can determine if an obstacle is located
on either side or across the front of the robot. That gives the robot three ways
to react to obstacles.
I recommend this book even though it was published 10
years ago. I learned enough reading it to develop my first
robot controller (it was for LEGO-based robots).
The proximity detector is essentially an infrared bumper.
Since it uses infrared, your robot doesn’t need to make
physical contact with an obstacle to detect it. A close
approach to the obstacle is enough to trigger a reaction
from the robot. The proximity detector consists of three
parts: two (a left and a right) IREDs and a centrally located
40 kHz IR detector. Driving the IREDs is an oscillator
consisting of a combinational inverter circuit operating at
Assembly Directions
There are three jumper wires in the proximity detector.
A cut resistor lead can be used for the shortest one, but for
the other two jumpers you’ll have to cut and strip thin
Parts placement
for the
CheapBot
proximity
detector.
54 SERVO 08.2009
