How infrared sensors work

Here is a brief explanation of how the infrared sensor works.

First, the infrared LED emits infrared light forward. This infrared light hits the wall and reflects, and the phototransistor detects the reflected light. The distance to the object is estimated from the intensity and angle of this reflected light.

How infrared sensors work

Generally speaking, the stronger the reflected light, the closer the wall is, and the weaker it is, the farther away it is. It's a pretty simple and easy-to-understand system.

Anode/Cathode

Sorry for the wait. Now we can finally talk about the main topic, the anode/cathode problem.

LEDs have a polarity known as anode and cathode. Since current flows from the anode to the cathode, the anode must be connected to the positive side and the cathode to the negative side. If the connection is incorrect, the current will not flow, which can even damage the LED depending on the type and voltage, so it is important to be careful.

So how do you distinguish between an anode and a cathode? Generally, the longer leg of a diode is the anode, and the shorter leg is the cathode.

Which is the anode?

Because I had learned this information in training prior to the production workshop, when I was building a test circuit on a breadboard to check the operation of the infrared sensor, I got to work enthusiastically, saying, "The longer one is the anode!!" This is the test circuit I actually created.

Infrared sensor test circuit

When I put this circuit together, I thought that the current would flow from hole j to hole f, and from hole e to hole a, so I wired the wires in exactly the right order, which resulted in a pretty messy layout. This is another typical rookie mistake.

Misunderstanding diagram (only LED circuit for simplification)

That aside, even though the circuit was correct, we encountered a problem where infrared light was not being emitted from the LED. Some of you may be wondering, "Infrared light is invisible to the naked eye, so how did you know it wasn't being emitted?" In fact, you can easily check this by using your smartphone camera! And it's super simple! Just look at the infrared LED through your smartphone camera! Smartphone cameras and digital cameras can visualize infrared light, so if you see a white or purplish light on the screen, you can confirm that infrared light is being emitted.

*Some cameras may not be able to check this.

Infrared rays emitted from an LED seen through a camera

Test Circuit LED When I actually looked at it through the camera, I could not see any light, even though I should have seen a pinkish light as in the image above, so I concluded that no infrared light was being emitted. I wondered if I had installed the anode and cathode in the wrong place, so I removed it and checked, but it was not working properly. Connect the longer end to the positive side No matter how many times I checked the circuit, I couldn't find anything strange. LED When I checked the data sheet, I found that LED teeth The shorter one is the anode I never thought that the longer one was the anode. 1 I never thought it would be overturned in less than a month. LED I changed the direction and reconnected it, and it worked just fine. LED Fortunately, I was able to notice it before it broke. LED Really? 1 Current can only flow in one direction.