How to make IR Sensor Modules using comparator

Knowledge of sensors are very essential part of robotics.And IR sensor  is very useful if you are tying to make a obstacle avoider robot or a line follower. In this post we are going to make a simple IR sensor which can detect a object around 6-7 cm. or you can change range of sensor to 1-2 cm if you are using for line follower.

Why IR sensor

• Because it have good range which is fulfill our requirements for robots
• It is very low cost and can be constructed on general purpose PCB
• It is of very small size. which is best for robots
• You can increase numbers of transmitter as you want for good result
• Good immunity to ambient light and waves are invisible to eyes.

 How it is work

Working of IR sensor is very simple and working principle is totally based on change in resistance of IR receiver which is similar to LDR ( light dependent register ) and reflection of light.

Here in this sensor we connect IR receiver in reverse bias so it give very high resistance if it is not exposed to IR light. the resistance in this case is in range of Mega ohms.

but when IR light reflected back and fall  on  IR receiver. The resistance of Rx it comes in range between  Kilo ohms to hundred of ohms. 

We convert this change in resistance to change in voltage . Then this voltage is applied to a comparator IC which compare it with a threshold level (defined by us according to required range and sensitivity ).

if voltage of sensor is more than threshold then output is high else it is low (depends on your connection to comparator ) which can be used directly for  microcontroller.

IR transmitter and receiver

IR LED is used as a source of infrared rays. It comes in two packages 3mm or 5mm. 3mm is better as it is requires less space.

IR sensor is nothing but a diode, which is sensitive for infrared radiation. This infrared transmitter and receiver is called as IR TX-RX pair.

It can be obtained from any decent electronics component shop and costs less than 10Rs. Following snap shows 3mm and 5mm IR pairs. Colour of IR transmitter and receiver is different.

However you may come across pairs which appear exactly same or even has opposite colours than shown in above pic and it is not possible to distinguish between TX and RX visually. In case you will have to take help of multimeter to distinguish between them.

How to identify IR Tx and Rx

Here is how you can distinguish between IR TX-RX using DMM :

• Connect cathode of one LED to +ve terminal of DMM 
•  Connect anode of the same LED to common terminal of DMM (means connect LED such that It gets reverse biased by DMM )
•  Set DMM to measure resistance upto 2M Ohm. Check the reading. 
•  Repeat above procedure with second LED. 
•  In above process, when you get the reading of the few hundred Kilo Ohms on DMM, then it indicated that LED that you are testing is IR sensor. 
• In case of IR transmitter DMM will not show any reading.

But there is a simple way to identify Tx and Rx by connect to supply. Connect  +ve to 5volt and -ve to GND with a series resistor around 330Ohms or higher.  Then look  both through us a  camera of mobile. One which glowing is transmitter.

Sensor Circuit

This is the circuit of sensor which have IR Tx and Rx. you can test different  value of Resistor and Capacitor according to your requirement. it will effect range and sensitivity of sensor only.

click to zoom

Solder Tx and Rx on a zero PCB and connect these three wire. which you can connect to comparator and mount this sensor any where on robot. 

You can solder Tx and Rx like this 

Now it time to connect output of sensor to comparator to complete our sensor. I  use LM324 comparator while making this you can which you like. but LM324 have 4 comparator in one IC so you can connect 4 sensor using a small size PCB with one LM324.

This is connection of sensor output to comparator. -ve terminal have a potentiometer which use to set threshold level (range and sensitivity)

sorry for image error: it is DMM not DMC

Comparator LM324

this is specially for you if you don't know how a comparator work else you can skip it. comparator have a internal circuit which compare two input. one is called +ve and second is -ve. when you give +ve terminal voltage higher than -ve terminal  voltage then the output of comparator is Vcc (supply voltage to IC). and if -ve voltage higher than +ve voltage then output is -VEE ( zero volt  in our case ).

This is pin diagram of LM324

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