| Infra-Red
Proximity Sensor
(I)
Using
an IR LED as a sensors
By
Ibrahim Kamal
Last update:
4/4/08
 Overview
 |
Based
on a simple basic Idea, this proximity sensor, is easy to
build, easy to calibrate and still, it provides a detection
range of 35 cm (range can change depending on the ambient
light intensity).
This sensor can be used for most indoor applications where
no important ambient light is present. For simplicity, this
sensor doesn't provide ambient light immunity, but a more
complicated, ambient light ignoring sensor should be discussed
in a coming article. However, this sensor can be used to
measure the speed of object moving at a very high speed,
like in industry or in tachometers. In such applications,
ambient light ignoring sensor, which rely on sending 40
Khz pulsed signals cannot be used because there are time
gaps between the pulses where the sensor is 'blind'...
|
|
The solution proposed doesn't contain any special components,
like photo-diodes, photo-transistors, or IR receiver ICs, only
a couple if IR leds, an Op amp, a transistor and a couple of resistors.
In need, as the title says, a standard IR led is used for the
purpose of detection. Due to that fact, the circuit is extremely
simple, and any novice electronics hobbyist can easily understand
and build it.
Object
Detection using IR light
|
It is the same principle in ALL Infra-Red proximity
sensors. The basic idea is to send infra red light through IR-LEDs,
which is then reflected by any object in front of the sensor.
Then
all you have to do is to pick-up the reflected IR light.
For detecting the reflected IR light, we are going
to use a very original technique: we are going to use another
IR-LED, to detect the IR light that was emitted
from another led of the exact same type!
This is an electrical property of Light Emitting Diodes
(LEDs) which is the fact that a led Produce a voltage difference
across its leads when it is subjected to light. As if it
was a photo-cell, but with much lower output current. In
other words, the voltage generated by the leds can't be
- in any way - used to generate electrical power from light,
It can barely be detected. that's why as you will notice
in the |
 |
schematic, we are going to
use a Op-Amp (operational Amplifier) to accurately detect very
small voltage changes.
The
electronic Circuit
|
Two different designs are proposed, each
one of them is more suitable for different applications. The main
difference between the 2 designs is the way infra-red (IR) light
is sent on the object. The receiver part of the circuit is exactly
the same in both designs.
Note: Both the sender and the receiver
are constructed on the same board. They are separated in the schematics
for simplification.
Design
1: Low range, Always ON |
As the name implies, the sensor is always ON,
meaning that the IR led is constantly emitting light. this design
of the circuit is suitable for counting objects,
or counting revolutions of a rotating object,
that may be of the order of 15,000 rpm or much more. However this
design is more power consuming and is not optimized for high ranges.
in this design, range can be from 1 to 10 cm, depending on the
ambient light conditions.
|
As
you can see the schematic is divided into 2 parts the sender
and the receiver.
The sender is composed of an IR LED (D2) in series
with a 470 Ohm resistor, yielding a forward current of 7.5
mA.
The receiver part is more complicated, the 2 resistors
R5 and R6 form a voltage divider which provides 2.5V at
the anode of the IR LED (here, this led will be used as
a sensor). When IR light falls on the LED (D1), the voltage
drop increases, the cathode's voltage of D1 may go as low
as 1.4V or more, depending on the light intensity. This
voltage drop can be |
detected using an Op-Amp (operational
Amplifier LM358). You will have to adjust the
variable resistor (POT.) R8 so the the voltage at the positive
input of the Op-Amp (pin No. 5) would be somewhere near 1.6 Volt.
if you understand the functioning of Op-Amps, you will notice
that the output will go High when the volt at the cathode of D1
drops under 1.6. So the output will be High when IR light is detected,
which is the purpose of the receiver.
In case you're not familiar with op-amps, here
is shortly and in a very simplified manner, what you need to know
to understand how this sensor functions: The op-amp has 2 input,
the +ve input, and
the -ve input. If
the +ve input's voltage is higher
than the -ve input's voltage, the
output goes High (5v, given the supply voltage in the schematic),
otherwise, if the +ve input's voltage
is lower than the -ve input's voltage,
then the output of the Op-Amp goes to Low (0V). It doesn't
matter how big is the difference between the +ve
and -ve inputs,
even a 0.0001 volts difference will be detected, and the the output
will swing to 0v or 5v according to which input has a higher voltage.
Some applications of the 'low range Always ON' Design:
Notice how in both devices, the IR leds are encapsulated
to protect them from ambient light. this kind of encapsulation
was totally sufficient to overcome all noise due to ambient light
for indoor applications.
Wheel
Encoder
This is a simple wheel encoder based on the idea that white
stripes will reflect IR light, while black ones will absorb
it. this will result in a series of electrical pulses as
the wheel is rotating, providing the microcontroller with
precious information that can be used to calculate displacement,
velocity or even acceleration. It is now clear that this
kind of sensor has to be Always ON, to detect every single
white stripe passing in front of it, to achieve accurate
results. |
Contact-Less
tachometer
This is a tachometer, that counts the revolutions per minute
of a rotating object, given that the object has a reflective
stripe glued on it, that will pass in front of the IR sensor
for each and every revolution, giving a pulse per revolution.
Again a microcontroller will have to be used to 'understand'
the data provided by the sensor and display it. Many commercial
contact-less tachometers, that are sold for more
than $200 rely on this simple idea!
[Build your
own one for less than $20 in this article...]
|
|
|
Design
2: High range, Pulsed IR
|
In this design, which is oriented to obstacle detection
in robots, our primary target is to reach high ranges, from 25
to 35 cm, depending on ambient light conditions. The range of
the sensor is extended by increasing the current flowing in the
led. This is a delicate task, as we need to send pulses of IR
instead of constant IR emission.The duty cycle of the pulses turning
the LED ON and OFF have to be calculated with precision, so that
the average current flowing into the LED never exceeds the LED's
maximum DC current (or 10mA as a standard safe value).
The duty cycle is the ratio between the ON duration of the
pulse and the total period. A low duty cycle will enable
us to inject in the LED high instantaneous currents while
shutting it OFF for enough time to cool down from the previous
cycle.
Those 2 graphs shows the meaning of the duty cycle, and
the mathematical relations between the ON time, the Total
period, and the average current.
In the second graph, the average current in blue is exaggerated
to be visible, but real calculations would yield a much
smaller average current. |
|
Pulsed IR, Duty cycle, Average
and Instantaneous current. |
Now, hands on the circuit that will put all this theory into practice.
The CTRL input in the figure, stands for Control,
and this pin should be connected to the source of the low duty
cycle pulses discussed above, whether it is a microcontroller
or an LM555 timer that generates the pulses.
The calculations yielded that a 10 ohm resistor is series with
the LED D2, would cause a current of approximately 250 mA to flow
through the LED. A current this high, would destroy the LED if
applied for a long period of time (some dozens of seconds), this
is why we have to send low duty cycle pulses.
The first Op-amp will provide voltage buffer, to enable
any kind of device to control the
|
sensor, also, it will provide the 30mA base current required
to drive the base of the transistor. The calculation of
the the base resistor R3 depends on the type of transistor
you use, thus on how much current you need on the base to
drive the required collector current.
The receiver part of this schematic functions in the exact
same way as in the first design, refer to the first, 'ALLWAYS
ON' design for a detailed description. |
Software
based ambient light detection.
|
When the sensor is controlled by a microcontroller
to generate the low duty cycle pulses, you can benefit from the
High and Low pulses to be able to detect any false readings
due to ambient light. This is done by recording 2 different outputs
of the sensor, one of them during the ON pulse (the sensor is
emitting infra red light) and the other during the OFF time. and
compare the results.
The
Idea is enlightened by this graph, where in the first
period, there is low ambient noise, so the microcontroller
records a "1" during the on cycle, meaning that
an object reflected the emitted IR Light, and then the
microcontroller records a "0" meaning that
during the OFF time, it didn't receive anything, which
is logic because the emitter LED was |
|
OFF. But study the second period of the graph,
where the sensor is put in a high ambient light environment. As
you can see, the the microcontroller records "1" in
both conditions (OFF or ON). This means that we can't be sure
whether the sensor reception was caused by an object that reflected
the sent IR light, or it is simply receiving too much ambient
light, and is giving "1" whether there is an obstacle
or not.
The following table show the possible outcomes of this method.
Output
recorded during: |
Software based
deduction |
On
pluse |
Off
time |
1 |
0 |
There is definitely
an Obstacle in front of the sensor |
1 |
1 |
The sensor is saturated
by ambient light, thus we can't know if there is an obstacle |
0 |
0 |
There is definitely
Nothing in front of the sensor, the way is clear |
0 |
1 |
This reading is un
logical, there is something wrong with the sensor. |
Example C Code for 8051 microcontrollers
#include <REGX51.h>
#include <math.h>
unsigned char ir; // to store the final result
bit ir1,ir2; // the 2 recording point required for our algorithm delay(y){ // simple delay function
unsigned int i;
for(i=0;i<y;i++){;}
} void main(){
//P2.0 IR control pin going to the sensor
//P2.1 IR output pin coming from the sensor
while(1){
P2_0 = 1; //send IR
delay(20);
ir1 = P2_1;
P2_0 = 0; //stop IR
delay(98);
ir2 = P2_1;
if ((ir1 == 1)&(ir2 == 0)){
ir = 1; // Obstacle detected
P2_3 = 1; // Pin 3 of PORT 2 will go HIGH turning ON a LED.
if ((ir1 == 1)&(ir2 == 1)){
ir = 2; // Sensor is saturated by ambient light
}else{
ir = 0; // The way is clear in front of the sensor.
}
}
}
|
Components
positioning:
|
The correct positioning of the sender LED, the receiver LED with
regard to each other and to the Op-Amp can also increase the performance
of the sensor. First, we need to adjust the position of the sender
LED with respect to the receiver LED, in such a way they are as
near as possible to each others , while preventing any IR light
to be picked up by the receiver LED before it hit and
object and returns back. The easiest way to do that is
to put the sender(s) LED(s) from one side of the PCB, and the
receiver LED from the other side, as shown in the 3D model below.
This 3D model shows the position of the LEDs. The
green plate is the PCB holding the electronic components
of the sensor. you can notice that the receiver LED is positioned
under the PCB, this way, there wont be ambient light falling
directly on it, as ambient light usually comes from the
top.
It is also clear that this way of positioning the LEDs prevent
the emitted IR light to be detected before hitting an eventual
obstacle. |
 |
Another important issue about components positioning, is the distance
between the receiver LED and the Op-Amp. which should be as small
as possible. Generally speaking, the length of wires or PCB tracks
before an amplifier should be reduced, otherwise, the amplifier
will amplify - along with the original signal - a lot of noise
picked up form the electromagnetic waves traveling the surrounding.
Here
is an example PCB where the distance between the LED and
the Op-Amp is shown. Sure this distance is not as critical
as you may think, it can be up to 35mm without causing serious
problems, but trying to reduce this distance will Always
give you better results.
Actually, when I design the PCB, I start by placing the
receiver LED and the Op-Amp, as near to each others as possible,
then continue the rest of the design. |
 |
An
example PCB construction
|
Here is an example construction of the PCB for the
High Range, Pulsed IR proximity sensor.
You can download here the project
folder containing the schematic, the PCB
design, and an example code for 8051 microcontroller
to send the low duty cycle pulses.
In this design, the LM358 Op-Amp is mounded on the copper
side, to save some space. The POT is the potentiometer
used to adjust sensitivity.
As explained before, the sender and receiver LEDs are on
both sides of the PCB.
|
|
Testing
the High range Pulsed IR sensor
|
The
last step, is to test the performance of the pulsed IR proximity
sensor. To do this, I connected the sensor to a 89S52 microcontroller,
loaded with a program to generate pulses with a duty cycle of
approximately 1.6. at a frequency of 3Khz. LEDs are deigned to
operate at very high frequencies, so you don't have to worry about
the response time. To make sure your duty cycle calculations are
correct, let the sensor running for a minute, and check with your
fingers the temperature of the IR sender LED. If its not hot,
then everything is alright. On the other hand, if the LED is getting
hot, to an extent that you can feel it, there is probably something
wrong, you should then try to decrease the duty cycle, or increase
the series resistor, in order to decrease the average current
flowing into the LED.
Then, you can start testing the range of the sensor, and experiment
it in different ambient light conditions, but the potentiometer
may have to be adjusted carefully, to cope with ambient light.
In the example C code
above, the final output of the sensor appears on the pin P2_3
of the microcontroller, as explained before.
Related
Tutorials
|
Download
the zip file for the High range sensor.
containing the PCB, Schematic and Example
8051 C51 code.
[note: i use ExpressPCB(FREEWARE)
to design the schematics and the PCB]
|
Join the Mailing List |
| Let us get in
touch with you when we upload new interesting content.
|
Preview of the last 15
messages discussing this page. Messages are sorted from the newest to
the oldest. |
Posted
by:
rob
on:
23 Jul 2008 |
|
|
|
Posted
by:
ikalogic
on:
18 May 2008 |
|
Re: My receiver LED rises voltage to OPAMP when IR received |
|
|
Quoting mrkiss: My receiver LED rises voltage to OPAMP when IR received, in the case of the multi tester prove placed in OPAMP pin 6.
It means It works opposite direction only with the tester prove.
Without the tester prove, receiver LED doesn't changes voltage to OPAMP.
So I changed circuit and it works
My working circuit is like below
VCC
|
|
1K R
|
|
pin 6__ OPAMP__pin 5 --- variable resistor
|
|
Receiver LED
|
|
Ground
Does your IR receiver LED really drops voltage to OPAMP when IR received?
And what the tester do to the circuit? |
Thanks for sharing the info..
most probably, the tester probe drops the voltage because it sinks a small amount of current.
|
|
|
|
|
Posted
by:
mrkiss
on:
18 May 2008 |
|
My receiver LED rises voltage to OPAMP when IR received |
|
|
My receiver LED rises voltage to OPAMP when IR received, in the case of the multi tester prove placed in OPAMP pin 6.
It means It works opposite direction only with the tester prove.
Without the tester prove, receiver LED doesn't changes voltage to OPAMP.
So I changed circuit and it works
My working circuit is like below
VCC
|
|
1K R
|
|
pin 6__ OPAMP__pin 5 --- variable resistor
|
|
Receiver LED
|
|
Ground
Does your IR receiver LED really drops voltage to OPAMP when IR received?
And what the tester do to the circuit?
|
|
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
|
Posted
by:
ikalogic
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting hmd:
Quoting ikalogic:
Quoting hmd: man.. idetect the problem
i unpower the irled sender then set the pot. untill the red led off
the re powerd the ir sender.....then....its work.....3.6V on the output...
thanks for your replays... |
wait wait wait! I didn't understand what was the problem.. please say again in detail!  |
ok
1.unpower the sender IRLED
2.reset the potentiometer untill the REDLED off
3.reset the IRLED sender.
use the circuit in dark ambient light......
the result......good output with 3.8V |
I am happy you finally made it! It was beginning to get seriously strange!
|
|
|
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting ikalogic:
Quoting hmd: man.. idetect the problem
i unpower the irled sender then set the pot. untill the red led off
the re powerd the ir sender.....then....its work.....3.6V on the output...
thanks for your replays... |
wait wait wait! I didn't understand what was the problem.. please say again in detail! |
ok
1.unpower the sender IRLED
2.reset the potentiometer untill the REDLED off
3.reset the IRLED sender.
use the circuit in dark ambient light......
the result......good output with 3.8V
allright man???...
|
|
|
|
Posted
by:
ikalogic
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting hmd: man.. idetect the problem
i unpower the irled sender then set the pot. untill the red led off
the re powerd the ir sender.....then....its work.....3.6V on the output...
thanks for your replays... |
wait wait wait! I didn't understand what was the problem.. please say again in detail!
|
|
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
man.. idetect the problem
i unpower the irled sender then set the pot. untill the red led off
the re powerd the ir sender.....then....its work.....3.6V on the output...
thanks for your replays...
|
|
|
|
Posted
by:
ikalogic
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting hmd: .....no man......i see it brighted by the camera....not naked eyes..
thats not stuiped questions man.....thats troubleshooting |
Okay, so that tell us you'r IR LEDs are working.
So you're telling me that the Op-Amps output is still 2V even if your swing your potentiometer from one limit to the other...
hmm.. Beleive it or not, it happened to me before, it can be the potentiometer that is dead (internally disconnected).. can you measure the voltage output from your POT (or the voltage at the PIN 3 of your LM358) and tell me if it varies with potentiometer variation?
|
|
|
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting ikalogic:
Quoting hmd:
Quoting ikalogic: Tell me, is that left LED glowing? or is it because of the flash of your camera??
yes its white LED
It seems very bright, it seems like it is glowing... IS IT A WHITE LED?
sorry the questions seems stupid.. but i just can't find a clue to your problem! |
the LED is brighted,yes...not from the flash of the camera...its the sender IRled
and the led near of it is the recievr IRLED.which is not brighted |
Okay i think now i understand the problem... just one litle question, even more stupid than the question before it!
Can you see the sender IR LED glowing with your naked eye? or it just appear glowing when pictured by your digital camera?
My friend, if you see it glowing with your bare eyes, then it not an IR LED! |
.....no man......i see it brighted by the camera....not naked eyes..
thats not stuiped questions man.....thats troubleshooting
|
|
|
|
|
Posted
by:
ikalogic
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting hmd:
Quoting ikalogic: Tell me, is that left LED glowing? or is it because of the flash of your camera??
yes its white LED
It seems very bright, it seems like it is glowing... IS IT A WHITE LED?
sorry the questions seems stupid.. but i just can't find a clue to your problem! |
the LED is brighted,yes...not from the flash of the camera...its the sender IRled
and the led near of it is the recievr IRLED.which is not brighted |
Okay i think now i understand the problem... just one litle question, even more stupid than the question before it!
Can you see the sender IR LED glowing with your naked eye? or it just appear glowing when pictured by your digital camera?
My friend, if you see it glowing with your bare eyes, then it not an IR LED!
|
|
|
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Quoting ikalogic: Tell me, is that left LED glowing? or is it because of the flash of your camera??
yes its white LED
It seems very bright, it seems like it is glowing... IS IT A WHITE LED?
sorry the questions seems stupid.. but i just can't find a clue to your problem! |
the LED is brighted,yes...not from the flash of the camera...its the sender IRled
and the led near of it is the recievr IRLED.which is not brighted
|
|
|
|
Posted
by:
ikalogic
on:
03 May 2008 |
|
Re: Infra-Red proximity sensors PART 1 |
|
|
Tell me, is that left LED glowing? or is it because of the flash of your camera??
It seems very bright, it seems like it is glowing... IS IT A WHITE LED?
sorry the questions seems stupid.. but i just can't find a clue to your problem!
|
|
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
|
Posted
by:
hmd
on:
03 May 2008 |
|
|
You have
to be a member to post replies. |
|
|
|
