IR Wavelength
- Thread starter eagre
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I am sorry to repost the same question. I was blocked from the internet for a time because of dial-up isp problems. When I returned a search of this forum did not seem to reveal Stan's response, or any other. I would appreciate a lead to finding it.
It is hard to believe that IR wavelength does not matter, since IR LEDs and receptors are frequency specific, usually at 880 or 940 nm.
Ed Grens
It is hard to believe that IR wavelength does not matter, since IR LEDs and receptors are frequency specific, usually at 880 or 940 nm.
Ed Grens
As posted 27th April -
<BLOCKQUOTE><font size=1 face=arial>quote:<hr height=1 noshade>Just like say swapping a green for a red LED, the wavelength emitted is a function of the IR LED & has nothing to do with the energising circuit! Most IR data senders & the traditional 3 legged receivers are pretty broad responding anyway. Hence I doubt it'd make a difference which wavelength was used.
Thought: I assume you ARE aware that the Picaxe-08M has enhanced Sony style 38kHz IR sending & receiving already inbuilt? IR data comms has been pretty much revolutionised by this-it's almost too easy! <hr height=1 noshade></BLOCKQUOTE></font><font face='Verdana, Arial, Helvetica' size=2> Stan
<BLOCKQUOTE><font size=1 face=arial>quote:<hr height=1 noshade>Just like say swapping a green for a red LED, the wavelength emitted is a function of the IR LED & has nothing to do with the energising circuit! Most IR data senders & the traditional 3 legged receivers are pretty broad responding anyway. Hence I doubt it'd make a difference which wavelength was used.
Thought: I assume you ARE aware that the Picaxe-08M has enhanced Sony style 38kHz IR sending & receiving already inbuilt? IR data comms has been pretty much revolutionised by this-it's almost too easy! <hr height=1 noshade></BLOCKQUOTE></font><font face='Verdana, Arial, Helvetica' size=2> Stan
I think you are a bit confused.
What does matter is that the IR LED transmits at the same wavelength as that which your receiver receives at.
That is, if your receiver states that its sensor picks up 940nm waves, then make sure that your LED is a 940nm LED.
Think of it as a language:
There are two people, Person A and Person B. They are trying to send a message to each other. So long as Person A speaks a language Person B understands, then all is good - ie. the goal was to send a message, successful.
Say, Person A speaks in German. Therefore, Person B must understand German. Person A speaks in English, Person B must understand English. LED transmitts at 940nm, IR receiver must be able to receieve at 940nm.
Alternatively, if Person A speaks German but Person B doesn't understand German, then we have aproblem. If the LED transmits as 940nm but the IR receiver is sensitive to only 890nm, then we have aproblem.
Hope that clears it up.
<b><i>ylp88 </b> </i>
What does matter is that the IR LED transmits at the same wavelength as that which your receiver receives at.
That is, if your receiver states that its sensor picks up 940nm waves, then make sure that your LED is a 940nm LED.
Think of it as a language:
There are two people, Person A and Person B. They are trying to send a message to each other. So long as Person A speaks a language Person B understands, then all is good - ie. the goal was to send a message, successful.
Say, Person A speaks in German. Therefore, Person B must understand German. Person A speaks in English, Person B must understand English. LED transmitts at 940nm, IR receiver must be able to receieve at 940nm.
Alternatively, if Person A speaks German but Person B doesn't understand German, then we have aproblem. If the LED transmits as 940nm but the IR receiver is sensitive to only 890nm, then we have aproblem.
Hope that clears it up.
<b><i>ylp88 </b> </i>
Another good analogy is Radio Frequency carrier.
Consider LED wavelengths as the equivalent to the radio frequency of a particlular radio station. (In principle, not numerically.)
On your FM radio you can tune into certain frequencies - the 'carrier' part.
The music/chat you listen to is the modulation part.
Similarly the LED/Detector you choose operate at a certain frequency/wavelength - your 'carrier'. And when you put a signal on it (like a TV Remote Control does) then this is your modulation.
At the receiving end (where your IR sensor is) the photodiode senses the carrier part and the electronics demodulates the modulation part.
I know I bang on about reading/rereading data sheets but they will show you graphically their wavelength outputs and sensitivities. Nice BIG graphs.
You don't have to be absolutely exact with matching IR LEDs and Photodiodes.. but it helps especially where you're trying to get the best range. Look at the graphs and you will see what I mean (hopefully).
Why doesn't have to be exact? Because an LED will be sending out a distribution of frequencies/wavelengths of light. The figures quoted are the peak value. Ditto with photodiodes etc. When you look at the graphs you will see that LED output wavenlength distribution graphs will 'overlap' the photodiode sensitivity graphs. The closer the graphs 'fit' then the better the overall system efficiency will be.
Consider LED wavelengths as the equivalent to the radio frequency of a particlular radio station. (In principle, not numerically.)
On your FM radio you can tune into certain frequencies - the 'carrier' part.
The music/chat you listen to is the modulation part.
Similarly the LED/Detector you choose operate at a certain frequency/wavelength - your 'carrier'. And when you put a signal on it (like a TV Remote Control does) then this is your modulation.
At the receiving end (where your IR sensor is) the photodiode senses the carrier part and the electronics demodulates the modulation part.
I know I bang on about reading/rereading data sheets but they will show you graphically their wavelength outputs and sensitivities. Nice BIG graphs.
You don't have to be absolutely exact with matching IR LEDs and Photodiodes.. but it helps especially where you're trying to get the best range. Look at the graphs and you will see what I mean (hopefully).
Why doesn't have to be exact? Because an LED will be sending out a distribution of frequencies/wavelengths of light. The figures quoted are the peak value. Ditto with photodiodes etc. When you look at the graphs you will see that LED output wavenlength distribution graphs will 'overlap' the photodiode sensitivity graphs. The closer the graphs 'fit' then the better the overall system efficiency will be.
Well put! To expand the analogy further the listener could be so multilingual that they could handle almost anything thrown at them. Hence this relates to the IR receivers being very broadly responsive - a check on Vishay data specs. shows them tolerant between 850 - 1050nm.
hippy
Ex-Staff (retired)
<i>That is, if your receiver states that its sensor picks up 940nm waves, then make sure that your LED is a 940nm LED. </i>
Reading between the lines, I think the wrong question is being answered ( albeit correctly answered ), and the question wasn't explicit so I may be wrong.
I think the question was intended to be; if using the TOPS18 IR Receiver chip, what wavelength does the IR Transmitting LED have to use ?
I don't know that answer.
Reading between the lines, I think the wrong question is being answered ( albeit correctly answered ), and the question wasn't explicit so I may be wrong.
I think the question was intended to be; if using the TOPS18 IR Receiver chip, what wavelength does the IR Transmitting LED have to use ?
I don't know that answer.
The TSOP18 (according to the datasheet) has maximum sensitivity at 940nm, dropping in a roughly symmetrical bell curve to zero at 800nm and 1125nm. So it's best to use a 940nm LED. Relative sensitivity at 880nm is only ~65%, so if you use that LED, you can expect to pick up more interference from ~940nm sources.
Long story short: your system WILL work with unmatched Tx/Rx, but you're far better off with having them matched.
Long story short: your system WILL work with unmatched Tx/Rx, but you're far better off with having them matched.
Michael 2727
Senior Member
What does the IR Detector data sheet say ?
They probably have a center frequency on the filter, but they can pick up a broad
range light, even some visible light.
They probably have a center frequency on the filter, but they can pick up a broad
range light, even some visible light.
Thanks to everyone for their responses. Fowkc answered my question completely. I did not find the identification of the Picaxe IR receiver/demodulator as a TSOP18 in axe040.pdf. Now I will use a 940 nm IR LED driven by a Picaxe 08M through an IRF530 and will look for a datasheet.
Ed Grens
Ed Grens