Intro to IR Circuits




Introduction: Intro to IR Circuits

About: I am a electronics engineer as well as a hobbyist is many other areas. My Instructables are based on engineering and electronic circuits. I also like to do DIY projects involving some electronics.

IR is a complex piece of technology yet very simple to work with. Unlike LEDs or LASERs, Infrared cannot be seen with the human eye. In this Instructable, I will demonstrate the use of Infrared through 3 different circuits.

The circuits will not be using IR receivers or microcontrollers, instead, they will use a photodiode to detect the IR signal because it is more simple.

Step 1: IR LED & Photodiode Basics

The three projects all depend on the IR LED and Photodiode. The IR LED emits infrared radiation in all directions, the photodiode is placed next to it so if an object gets too close to it, it will reflect the infrared radiation into the photodiode, the photodiode turns the infrared it absorbed into a signal, the signal then can activate other things. Note the diagram above has a black IR LED and a transparent photodiode, this is not very common as it is usually the other way around, but the following 3 projects use normal type of IR pairs (IR LED: transparent, Photodiode: Black/dark purple). The colours of the diodes do not matter but just make sure you remember which one is which.

Important things to note (Please read the following):

IR LED: The infrared LED emits IR radiation, we cannot see the radiation because it contains lower frequency than visible light, humans can only detect infrared as heat (so the IR LED can get a bit hot, that's normal), and the radiation is not harmful because it's just heat.

Photodiode: The photodiode is like an LED but it does not give out light, instead, it is a light sensor (like an LDR but not quite). The photodiode can come in many forms: it usually looks like a black LED but it can also be transparent (in which don't get it mixed up with other LEDs). The photodiode is connected differently from normal LEDs, instead of Vcc to the anode of the LED, it's Vcc to the cathode of the photodiode (like how you connect batteries).

When purchasing IR LEDs and photodiodes, try to buy them in pairs because sometimes the IR LED does not work with the photodiode.

Step 2: IR Circuit 1

The first IR circuit will just show how the pair (IR LED & Photodiode) works. By using a transistor, we can turn dirty analogue from the photodiode into clean analogue which the output LED likes better. The circuit is very simple, all it needs is:

Resistor: 2x 220ohm (or similar), 1x 10k

Diode: 1x IR LED, 1x Generic LED, 1x Photodiode

Transistor: 1x BC547 (or any equivalent NPN transistor e.g. 2n2222A)

A 5v power source (USB is fine), jumper wires and a breadboard.

Step 3: IR Circuit 1 Test

Before you finish the circuit, make sure the IR LED and Photodiode are placed next to each other.

Once the circuit is complete, test the sensor by hovering an object or your finger about 5cm above the two diodes, then slowly move the object/finger towards the diodes till you touch them both. The generic LED should light up more the closer you get, this is because the object is reflecting more infrared into the photodiode.

If this does not happen, check you have put the photodiode in correctly, check your wire connections, check your power source, if none of this help, the problem might have occurred between the IR LED and the photodiode (you should buy new ones or try a different pair).

Make sure you do not run the circuit under the sun or very bright light because that will confuse the photodiode.

Step 4: IR Circuit 2

Now you understand how the IR LED and Photodiode works together as a sensor, we are going to transform the previous circuit into an alarm circuit. This circuit will use an OP Amp to Amplify the photodiodes signal, a buzzer is connected to the output of the OP Amp but that can be modified and replaced with another component/circuit.

This circuit will need:

Resistor: 1x 220 (or similar), 1x 10k

Potentiometer: 1x 10k

Diode: 1x IR LED, 1x Photodiode

IC Chip: 1x LM358

Others: 1x Buzzer or replace it with your own circuit.

5v power supply (USB is fine), Breadboard, jumper wires.

Step 5: IR Circuit 2 Test

Remember the two diodes have to be next to each other as the last circuit. To test the circuit, move an object or your hand above the two diodes, this should trigger the alarm. You can also adjust the sensitivity of the photodiode by turning the potentiometer, there will be a point when the alarm will always be on, this is because the photodiode is so sensitive to IR it detects it from the atmosphere around it. It is not possible for me to show the circuit functioning in the picture above but just imagine you can hear the sound of the buzzer.

Do not operate the circuit under the sun or very bright light because that can confuse the photodiode.

To troubleshoot, repeat step 3.

Step 6: IR Circuit 3

In this circuit, we will activate an LED (or any output) without pressing a button. This time two pairs of IR LEDs and Photodiodes will be used. Instead of using an OP Amplifier, we will use a 555 timer for simplicity. We will also bring back the transistors for smoothing the analogue signal.

This circuit will require:

Resistor: 3x 220ohm, 2x 10k, 2x 1M, 2x 3M

Capacitor: 1x 10nf

Diode: 2x IR LED, 2x Photodiode, 1x generic LED

Transistor: BC547 (or equivalent)

IC Chip: 1x 555 timer

5v power supply (USB is fine), Breadboard, jumper wires

Make sure the two pairs of diodes have distance between them so they won't interfere with each other. Also, make sure you pair the right diodes up.

Step 7: IR Circuit 3 Test

The circuit consists of two pairs of diodes, one turns the output on, the other turns it off. You must first figure out which pair of diodes controls what. Once you do, you can turn the output on by hovering an object over one pair of diodes. The output will stay on even after you have taken your object away from the sensor, the output will only shut off if you hover an object over the other sensor, it will then stay off until you repeat this process.

Again, do not operate under sunlight.

Step 8: More IR Stuff

There is a much bigger world to IR circuits, its not very complicated but is quite fascinating. Instead of IR LEDs and Photodiodes, better circuits would consist of IR remotes and IR receivers, these devices can cover much more range and can transfer more information as well.

Should there be any questions, please feel free to ask.

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    2 months ago on Step 1

    Step 1 would be less confusing if you redo the third and fourth photos so that the red lines indicate that the clear device is the transmitter and the "black" device is the receiver. Then you can remove the sentence about the components in the photos not conforming to the usual standard.


    2 years ago

    Perhaps correct the schematic is step 4
    Further, I might be wrong, but I think that what you are using are phototransistors, not photodiodes.

    Have a nice day :)


    Reply 2 months ago

    The hobbyist kits commonly available use IR LEDs (the devices in the clear package) and photodiodes (the devices in the "black" package). You might have phototransistors instead but the article is unaffected by that.


    Question 2 months ago on Step 6

    Can you explain how Q1 works, please? The circuit shows its collector at a lower potential than its emitter, which doesn't seem correct for an NPN device.


    1 year ago

    The connections for +V and -V are incorrect. +V is on pin 8 and -V is on pin 4. I wondered why the LM358 was getting hot.