Cognoscis

After the LDRs, I now come to the next level of sensors, the Infra red sensors. I am going to describe the use of IR LED and IR photodiodes today. Now, if you have my post about LRD sensors(http://cognoscis.wordpress.com/2008/05/01/sensor-ldr/), you will notice that the working of the IR sensors in exactly the same. Then whats the advantage of using these sensors one might wonder. The main advantage of IR sensors in that it will not be affected by noise as much as the LRDs would be. Since LRD use white light, there can be many sources of noise for it.

IR LED(left) and IR detector(right)

The picture shows an IR LED and a photodiode that can detect IR radiations. We use the same principle as that of the LDR when we use these IR sensors to follow a white line. Once you have seen its working, you can apply it for different purposes. Similar to LDR, the photodiode will conduct only when IR light falls on it. That is, its resistance is very high when IR light DOES NOT fall on it; and its resistance is very low when IR light falls on it. Here again the output will be analog in nature as in the LDR circuit. So, if you are using this output to drive any IC, you will need to convert this to a digital signal first. That can be done in two ways, using comparators or using ADCs(Analog-to-Digital Converters). I will be using the comparator(similar to the LDR circuit) as its very easy to handle and also cheap. After the comparator, you will get a signal that can be fed to any IC, be it a micro-controller or logic gates.

IR Sensor circuit (Click to view)

The above circuit shows a simple design of sensor circuit. Towards the left is the emitter part. The 120? resistance is to limit current through the diode. You can make the LED grow brighter of dimmer by changing the value of resistance. But the problem with IR is that we cannot see it. But, the cameras can. If you have digital camera, mobile camera or a web camera, just see the image being formed on the screen, you will find that the LED glows with a green or blue colour. See, how easy it is

Now, lets see what happens in the detector part. This is the same as the LDR circuit. The comparator used here is the ever popular LM339D. its a quad comparator IC package. It has four comparators and here only one of them is shown. The potentiometer R2 acts as the reference voltage for the comparator. You can use a potentiometer of 10k?/10 turns so that you can set a suitable reference value. The IR detector is placed between VCC and ground. Now this acts as a switch. When OFF, the input to the comparator is VCC. When ON, input to the comparator is zero. Now, after you set up the circuit( preferably on your bread board) check the various voltage levels once.

See the voltage at the center pin of the pot(potentiometer) that is given to the pin 7 of LM339D. Now, by turning the pot you will be able to vary this voltage. Set it to 2.5V initially. Now, you check the voltage at pin 6 that is due to the detector. First, when the IR detector is covered, you will see that the voltage is equal to VCC. Now, allow the light to fall on the IR detector and see the fall in the voltage. It will around 0.5 to 1 V. Now, that you have tested your inputs, its time to see if you are getting the proper output.

The resistance R3 at the output is called the pull up resistor. The chip provides an output of only ground. hence, you pull up the output line to high using this resistor unless the IC pulls it down. Now, when the light falls on the IR detector, the input at in 6 will be less than the reference voltage which we have set at 2.5V. Now, the output will be low. If you take the light off the IR detector, the voltage at pin 6 rises above the reference. Hence, the output will be high. Thus, we get a digital output from the circuit. Now, one more good thing about this circuit is that you can set your reference voltage level by varying the pot. This will help to operate the sensor in any environment. All you need to do is check the minimum and maximum voltage from the LDR as described before and set the reference voltage value somewhere in between the two values obtained. Thats how you calibrate your sensors to a given environment.

Any problems or additions, please post as comments