IR Remote Frequency?

[jwhitmore]

Hello all,

I'm trying to re-purpose a handheld IR remote control for another function and wondered about the IR Receiver selection. I stuck a scope onto the IR LED's of the remote and they seem to be transmitting at 31.25KHz

I have an IR Receiver in my parts bin, but it's a 38KHz I looked at its datasheet and that family of IR Receivers has a 30KHz and 33KHz part. I've looked at a number of parts but that seems to be the standard 30 and 33KHz. So what part do I need I guess, a 30KHz or a 33KHz? They've probably got a band pass in there so perhaps either would do but I assume my 38KHz is no good for this remote.

Any help would be very gratefully received

[jwhitmore]

I just had a thought that I'm looking at the wrong number. I looked at the voltage wave form being sent from the uC to the IR Transmitter LED and that's being driven at 31.25 KHz. I just had a thought that perhaps the Frequency listed for an IR Transmitter or Receiver it's the frequency of the light that's being talked about? I might have this totally wrong, so then it gets even more difficult to talk to communicate with the remote.

[oPossum]

CIR is usually 940 or 950 nm.  31.25 kHz is a rather unusual carrier frequency. 36, 38, and 40 kHz are the common ones. The only CIR receiver I know of that will explicitly work at 31.25 kHz is the Vishay TSMP1138.

https://www.vishay.com/docs/82484/tsmp1138.pdf

[Brumby]

The frequency of infrared light is much, much higher.  For a 940nm wavelength, this works out at roughly 318928146808.5 kHz - or close to 319 THz.

FYI - While you will not be able to see IR light directly, every phone camera I have tried IS.  So, if you ever want to check the IR signal is being transmitted from your remote, just fire up your phone camera, point the IR emitter at the camera and press a button or two.

[soldar]

I checked one of my remotes. The IR has a base carrier frequency of about 37 KHz and is modulated with a series of 27 pulses which take about 240 ms. Different keys send different combinations of pulses.

The three pin IR receiver already detects the signal and outputs the modulating pulses, not the carrier.  Neat.

[jwhitmore]

Just getting back to this project and still a bit confused by IR Specs.

I managed to identify the detector IR sensor as a TSOP1730 which according to the datasheet [1] is a 30KHz part. I'm afraid I'm still confused by the numbers. I'd have thought that the frequency of the pulses sent to the LED, (and therefore received by the receiver) is dictated by the uC that's driving the LED Transmitter, with pulses. So if I wrote the code to transmit a square wave of 38KHz and the TSOP1730 is a 30KHz part it's not going to detect it?

I thought that the LED IR Transmitter and the IR Detector would have to match the light that's being transmitted and received, but perhaps the bandwidth of IR is so narrow that they all just Tx and Rx. There is a figure in the datasheet [1] which shows the wavelength of light as being 950nm (figure 12)

Actually looking at the datasheet again there is a bandpass filter built into the sensor! I think that the penny has dropped. So my look at the scope on the LED Tx side showed pulses of 31.25KHz and the Receiving TSOP1730 has a 30KHz bandpass filter, so maybe there's a bit of a mismatch there but what should be happening is the remote Tx 30KHz pulses and they are band pass filtered in the receiver.  I think this might be starting to make sense at last.


[1] https://media.digikey.com/pdf/Data%20Sheets/Vishay%20IR%20PDFs/TSOP%2017...pdf

[soldar]

The TSOP1730 receiver includes the detector so in the output you will see the detected signal, not the 30 KHz carrier.

[rstofer]

There IR Remote projects are all over Google.
There are receiver projects for the Arduino and there is at least one library available.

Figure 1 in the TSOP1730 datasheet shows the effect of not having exactly the right frequency.  For 31 kHz vs 30 kHz nominal, the difference is essentially nothing.  They also show f = f_o +- 5%  In other words, if you are withing 5%, everything is fine.

http://pdf.datasheetcatalog.com/datasheets/208/301092_DS.pdf

ETA:  Since you are between 30 kHz and 33 kHz, buy one of each and see which works best.