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Remote control air conditioner (AC)

Picture source.

I was told by my AC's technician who several times came to my house to fix my air conditioner, especially if it got problem with the remote control or the board on the AC equipment. Once he explained how to see the infrared light by using a smart phone's camera. He demonstrated how to see if the remote control is working or not by pressing any button on the remote control and pointing it to the smart phone's camera, and we will see a something like violet color blinking fast.

I tried to fix it my self after the same problem previously occurred, the remote control is not working, then I searched over the net, especially on YouTube, and I found that nothing to do with infrared frequency with the remote control. The remote control is using an LED which is called IR LED, while in the AC's board installed TSOP4838, which it works on 38KHz, not in the range of 300GHz-430THz (above microwave but below visible light), the infrared spectrum in the electromagnetic spectrum.

Does that remote control really work in IR spectrum? If not, then why is the LED installed on the remote control itself and the TSOP4838 IR receiver on the AC equipment are called IR LED and IR receiver respectively?

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    \$\begingroup\$ It's two frequencies. There is the light waves which are in the infrared spectrum. And that light blinks 38000 times a second. Like when you have a laser pointer and you press it on and off once a second that's 1Hz but the light is still 430THz or whatever \$\endgroup\$
    – user253751
    Apr 27, 2023 at 23:01
  • \$\begingroup\$ @user253751, Have you watched the video I mentioned on my post? \$\endgroup\$ Apr 27, 2023 at 23:06
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    \$\begingroup\$ No, but I know how IR remote controls work. IR LEDs make IR light just like green LEDs make green light. If you turn the green LED on and off 38000 times per second, it still makes green light. \$\endgroup\$
    – user253751
    Apr 27, 2023 at 23:11
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    \$\begingroup\$ probably because they assume you know it. If it was a green LED would you mention green spectrum? No, why would you mention green spectrum? Everyone knows what green is - no need to talk about it \$\endgroup\$
    – user253751
    Apr 27, 2023 at 23:17
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    \$\begingroup\$ I didn't think this deserved a downvote, although its tone may have seemed a bit belligerent. Fortunately, the OP has accepted a correct answer. \$\endgroup\$
    – PStechPaul
    Apr 28, 2023 at 0:40

1 Answer 1

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The IR light emitted by the IR LED in the remote is a bunch of (incoherent) photons, each with frequency in the hundreds of THz (i.e. wavelength around 950 nm). The LED is turned on and off rapidly at 38 kHz, meaning that 38000 times per second, the stream of hundreds-of-THz photons starts and stops. Finally, that 38 kHz pattern is itself turned on and off with various rhythms to encode different messages from the remote (i.e. power, temperature up, fan on, ...).

The 38 kHz frequency has a practical purpose - it makes it easier to detect the remote's signal against the background of IR radiation from sunlight, reflection, and other background sources.

There is no contradiction to the content in your linked video, which is focused squarely on the practical aspects of IR remote controls that an electronics hobbyist would care about. The photodiode/phototransistor in the receiver detects the photons, and produces a current proportionalto how many photons are arriving (not a hundreds-of-THz current!)1. As a result, it produces a 38 kHz signal observed by the oscilloscope/microcontroller.

Here's another analogy: just like how we modulate 300 THz (IR) with 38 kHz, you can shine a green flashlight (600 THz) at me, while turning the switch on and off at 1 Hz.

Even though your nerves, muscles and fingers cannot operate at 600 THz, you are still modulating a 600 THz signal. On the other end, the retinas in my eyes contain a light-sensitive molecule called rhodopsin, which produces a nerve signal when it gets hit by light around 600 THz. My nerves also cannot carry 600 THz signals, but they detect the signal from my retinas getting stronger and weaker at 1 Hz.

This is just like the photodiode/phototransistor in the electronics example - it gives off a 38 kHz signal when 300-THz photons start and stop arriving 38000 times per second.

In regard to your confusion about the TSOP4838:

in the AC's board installed TSOP4838, which it is works on 38KHz, not in the range of 300GHz-430THz

Actually, it does work at around 300 THz according to this figure from the datasheet (it's just given as wavelength2 around 950 nm, which is equivalent): enter image description here

This device has two frequencies in its datasheet specifications: The light frequencies that its light-sensitive component is physically sensitive to (in the THz range), and the modulation frequency that its electronic filter is most sensitive to (around 38 kHz).

1 In a photodiode, each photon produces an electron-hole pair (thus causing an electron to flow through the circuit), not a 300 THz wave. A 300 THz electronic signal cannot be practically measured or sustained on a circuit board or electronic chip, let alone a hobbyist grade breadboard. It cannot be directly measured by an oscilloscope either. The modulation is optoelectronic - a photon-producing device is turned on and off at a rate that a circuit can achieve, and the demodulation is likewise optoelectronic - a photon-detecting device produces current as a result of electromagnetic radiation, even though that electromagnetic radiation cannot directly be passed to electronic circuits.

2 Wavelength and frequency are related via the speed of light using the equation \$c = f\lambda\$. 950 nm light has a frequency of approximately 315 THz.

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  • \$\begingroup\$ How could a lower frequency modulated the higher frequency? In the video I mentioned in my post, there is no IR Frequency, just the 38kHz. To be able to capture IR range required a high quality measurement tools/osciloscope. \$\endgroup\$ Apr 27, 2023 at 23:11
  • \$\begingroup\$ @AirCraftLover There IS an IR frequency. It's in the literal air between the LED and photodiode. It just doesn't excite a 300 THz frequency in the receiving circuit. Read my last paragraph very carefully. \$\endgroup\$
    – nanofarad
    Apr 27, 2023 at 23:13
  • \$\begingroup\$ Do you have video on YouTube explains/demonstrates it to support your explanation? Because I have searched all I could, I could not find any one. \$\endgroup\$ Apr 27, 2023 at 23:16
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    \$\begingroup\$ @AirCraftLover: IR (Infra red) is just light - it is a little outside the range of light visible to humans, but it is still light. \$\endgroup\$ Apr 27, 2023 at 23:16
  • \$\begingroup\$ @PeterBennett, you no need to explain it, I put it already on my post. \$\endgroup\$ Apr 27, 2023 at 23:17

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