This exercise is based on the
NXT Mindstorm robot’s infrared proximity sensor. By
isolating this component from the robot’s environment we
can conduct much analysis of the component and gain
sufficient knowledge of the operation of each individual
electronic component and how it is integrated into the
The foundation of this exercise is based
on both math and physics. All design principles are
derived from Kirchhoff’s voltage law (KVL) which relies
on basic mathematic operations such as fractions and
relations. The infrared light (IR) is based on the
principles of optics.
Principle of Operation
An IR proximity sensor works by applying
a voltage to a pair of IR light emitting diodes (LED’s)
which in turn, emit infrared light. This light
propagates through the air and once it hits an object it
is reflected back towards the sensor. If the object is
close, the reflected light will be stronger than if the
object is further away. The sensing unit (for this
experiment a Sharp IS471FE will be used), in the form of
an integrated circuit (IC), detects the reflected
infrared light, and if its intensity is strong enough,
the circuit becomes active. When the sensing unit
becomes active, it sends a corresponding signal to the
output terminal which can then be used to activate any
number of devices. For the purpose of this exercise, a
small green LED will turn on when the sensor becomes
Sharp IS471FE ´
1 kΩ, 10 kΩ ´ 1 each
Capacitor = 0.33 µF
´ 1; Green (or Red) LED
QED234 Infrared LED x 2;
5V Power Supply
Bread Board; Wire Cutters; Red and Black Wire
Figure 1: Schematic of IR Proximity
With the flat side of the Sharp IS471FE
(the IC) facing you, gently spread each leg so they are
aligned and place at the edge of the bread board.
Place the IR LED’s (D1 and D2) along the
same edge of the bread board such that they face the
same direction as the IC. It helps here to place a small
“cone” of dark paper at the base of the LED to prevent
light from shinning backwards.
Connect the cathode (shorter leg) of D1
should be connected to the Anode (longer leg) of D2. It
is recommended to use a wire to make this connection.
* Note, “Cathode”
refers to a negative terminal, while the term “Anode”
refers to a positive terminal.
Connect the cathode of D2 (the shorter
one) to pin 4 of the IC.
Connect C1 between pins 1 and 3.
Connect the cathode of D3 (the green or
red LED) to pin 2.
Connect resistance R1 to the Anode of D3,
and the other end to a common point to be later connect
Connect the Cathode of D1 and pin 1 of
the IC to same common point to be connected to +5V.
Connect resistance R2 from pin 2 of the
IC to a common point to be later connected to ground.
Connect pin 3 to the same grounded
CHECK ALL CONNECTIONS!
Before connecting the power supply make sure it is set
If all connections
are complete and correct, turn on the power supply and
notice that when an object is brought within
approximately 15cm from the IC, the green/red LED turns
Remember to always
double check every connection and make sure your circuit
is properly grounded. Never work with high voltages as
it might damage the components and/or cause bodily
harm. If you are unsure of anything about your circuit
or about working with electricity, contact the lab
This type of circuit is commonly used as
a switch where physical contact is not an option. For
example, we commonly see infrared proximity sensors on
public drinking fountains and in public washrooms.
Proximity sensors have found homes in a variety of other
applications as well from collision detection and
avoidance systems in robotics and cars, as well as
position and distance sensors for a variety of
applications. One specific example of where proximity
sensors have been integrated into an interactive system
is the Lego NXT Mindstorm robot. Lego has integrated
proximity sensors to prevent the robot from colliding
with any objects when it is active. Short of a few
extra cables and a slight change in components, the IR
Proximity sensor as described and built in this
experiment is very similar to that used on the Lego