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Topic: Infrared LED x Normal LED with visible light (Read 185 times) previous topic - next topic

gilperon

Hi,

I have an infrared sensor that reads 38kHz. I connected it to my arduino in order to get HIGH/LOW from the infrared sensor when it detects 38kHz.

To generate the 38kHz I used a normal LED (with visible light) that blinks 38000 times per second using PWD. Why it doesnt activate the infrared sensor if it has the right frequency? I used an osciloscope to check it and the frequency from my LED was indeed 38kHz.

On the other hand, when I generate the same frequency with an infrared LED the infrared sensor detects the 38kHz frequency and it all works fine.

So why the normal LED (with visible light) even producing 38kHz does not work?

Runaway Pancake

It's simply because the "normal LED" isn't emitting in the infrared range in the first place.
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bad_crc


The 38KHz is the frequency of the LED switching on an off, NOT the frequency of the light emitted by the LED.  The sensor is designed to pick up light in the IR wavelength which is not the same as the visible wavelengths coming from normal LED, which is why using an IR LED to transmit @ 38KHz works but the normal LED does not.

gilperon

Thank you so much! I thought that the infrared light had a frequency os 38kHz. Now it's clear to me that infrared differs from others lights only in the wavelenght.

Is there any sensor that can read 38kHz of green light wavelenght? Do you know anything like that?

Grumpy_Mike

#4
Nov 21, 2014, 08:41 pm Last Edit: Nov 21, 2014, 08:42 pm by Grumpy_Mike
Quote
Now it's clear to me that infrared differs from others lights only in the wavelenght.
Well if that is clear you are not understanding electromagnetic radiation. Read what bad_crc said again.
Different sorts of electromagnetic radiation have different wavelengths and frequencies.

Wavelengths and frequency are related by
 wavelength = frequency / C
Where C is the speed of light.

 
Quote
Is there any sensor that can read 38kHz of green light wavelenght?
Not like the TV remote sensors there isn't, not a lot of commercial call for that.

If you want one you have to make one. A photo transistor or diode will detect green light. Then you put a green filter in front of it. Then behind it you build a tuned amplifier that will amplify just 38KHz.

gilperon

Grumpy_Mike now I am confused. An infrared LED will emit light with a specific wavelenght but the frequency is not necessarily related to the wavelenght.

If the wavelenght is 1 meter, the frequency cannot be calculated just from the frequency. We would need to know the period of that wavelenght to get the frequency. A wavelength of 1 meter can have different frequencies depending on the period of each wavelenght, right? Am I so wrong?

I say this cause I have here in my arduino kit an infrared sensor and it reports HIGH only when infrared light is emited with frequency of 38kHz. But in my kit I also have a reflexive optical infrared sensor that reports HIGH whenever there is infrared light (it does not matter the frequency).

I would like to be right cause I had a tought time trying to understand this, but if I am wrong I would appreciate all your help!

Grumpy_Mike

Quote
An infrared LED will emit light with a specific wavelenght but the frequency is not necessarily related to the wavelenght.
Yes it is. It is physics.

Quote
If the wavelenght is 1 meter, the frequency cannot be calculated just from the frequency. We would need to know the period of that wavelenght to get the frequency.
Yes we do know the period. The period is simply the reciprocal of the frequency. ( the reciprocal is one over ).

Quote
A wavelength of 1 meter can have different frequencies depending on the period of each wavelenght, right? Am I so wrong?
Yes you are so wrong. Because the velocity of all electromagnetic radiation is the same the period and wavelength are locked together.
The wave length times how many wavelengths you get in a second ( frequency ) is the velocity of the wave, which is the speed of light.

Quote
I have here in my arduino kit an infrared sensor and it reports HIGH only when infrared light is emited with frequency of 38kHz.
No you do not.

You have a sensor that reports HIGH only when infrared light is MODULATED at 38KHz. In this case modulated means being turned on and off 38,000 times a second. This is known as amplitude modulation or sometimes on / off keying or in fact CW (carrier wave). It is nothing to do with the frequency of the emitted light. You were told that in reply #3 by bad_crc and again in reply #5 by me.


gilperon

Grumpy maybe I am so wrong... but I am getting really upset. Look: if infrared LED already emmits 38kHz (as you say) would do I have to send 26 period wavelenghts manually? Look, in my code I send 13 microsecnnds of high and 13 microseconds of LOW in order to my infrared sensor (and my TV infrared sensor) reads it. If you were right, turning on the infrared led would be enough to activate the infrared sensor BUT NOT. I need to turn on/off my infrared light precisely at every 26 microseconds in order to my infrared sensor reports it. What is wrong?

I understand what you said that the speed of the wave should be the same, I never thought that way. If speed is the same so wavelenght is indeed related to the frequency directly. But please read the above paragraph and tell me what is wrong.

Grumpy_Mike

#8
Nov 23, 2014, 04:27 pm Last Edit: Nov 23, 2014, 04:27 pm by Grumpy_Mike
Quote
if infrared LED already emmits 38kHz (as you say)
I never said that! Please pay attention to what is told to you.

Quote
If you were right, turning on the infrared led would be enough to activate the infrared sensor
No I never said that and that is wrong.

You do not seem to have grasped the concept of modulation.

Here the act of turning the LED on and then off is the modulation.

It has nothing to do with the wavelength / frequency of what you are modulating. IR light has a wavelength of 840nm and so has a frequency of about 3.5 x 1014 Hz. It emits a frequency of 3.5 x 1014 Hz.


The off part is just as important as the on part. It is the on and off (the modulation) that the amplifier in your sensor is filtering for. It is the lights wavelength and frequency that the sensor is detecting.

Runaway Pancake

An infrared emitter having a wavelength of 940nm has a frequency of approximately 319 THz.
A green LED with a nominal wavelength of 525nm has a frequency of approximately 571 THz.

How many cycles of 940nm wavelength could (be) fit into one repetition of 38 kHz?
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

bad_crc

Grumpy maybe I am so wrong... but I am getting really upset. Look: if infrared LED already emmits 38kHz (as you say) would do I have to send 26 period wavelenghts manually? Look, in my code I send 13 microsecnnds of high and 13 microseconds of LOW in order to my infrared sensor (and my TV infrared sensor) reads it. If you were right, turning on the infrared led would be enough to activate the infrared sensor BUT NOT. I need to turn on/off my infrared light precisely at every 26 microseconds in order to my infrared sensor reports it. What is wrong?

I understand what you said that the speed of the wave should be the same, I never thought that way. If speed is the same so wavelenght is indeed related to the frequency directly. But please read the above paragraph and tell me what is wrong.
Again, the frequency that you are modulating the LED ( switching it on and off ) is 38kHz.   But the light coming from the LED has a frequency that is MUCH much higher.  When we say frequency of the light we are NOT talking how fast the LED ( IR or otherwise ) is being turned on and off, rather an intrinsic property of the light coming from the LED when it is on.

Think of an analogy of your eye.  Your eye is tuned to pick up visible light, but you can not see x-rays or radio waves etc...  It's exactly the same for your IR sensor.  It is tuned to pick up infrared, it can not see the visible light emitted from a normal LED.

For the visible light LED, it does not matter how fast you turn it on and off, by design it will not emit light at the frequency that the IR sensor is designed to pick up.   The sensor is designed to respond to infrared light, not visible light.  That in a nutshell is why the IR led works and the normal, visible light led does not.



See image above.  The visible light is just a small band of possible frequencies along the whole electromagnetic spectrum.  The left of visible light ( lower frequency, higher wavelength ) is infrared radiation.  This the the area that your sensor is designed to pick up.

gilperon

Finally!!!!!! :) You are amzing. So there are 2 frequencies! The frequency of the wavelength and the frequency at which the led is turned on/off! They are completely different things! :) I got it, you helped me A LOT!

JimboZA

How did you think radio works? The sound coming out of the speakers is at human ear frequencies of 20Hz to 20kHz, but you don't hear that sound whizzing through the air from the transmitter to your radio. When you tune your radio, you pick a frequency somewhere in the 100MHz region for FM or 1000kHz for AM, yet somehow the human ear spectrum is hidden in there.

That's what modulation is, and is the "M" in FM or AM: it's how the message (the part the human wants to hear or see) is carried in the radio frequency signal.

"Could you do the egg bacon spam and sausage without the spam then? "

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