# help needed in IR demodulation circuit

Discussion in 'Electronics Homework Help' started by student, Mar 23, 2011.

1. ### student

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Mar 23, 2011
hi guys

my project consist of two IR sources (A & B) and one recevire, each of IR sources has a different frequency. when one of the beams interrupted a digital signal should be produced. i need help with the demodulation circuit that can tell which signal is which and ignore the other sources such as sun light.

could any one tell me what i need to construct this demodulator and how?

(new member and beginner)

any help is appreciated

2. ### student

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Mar 23, 2011
too many had a look. anyone with a comment or an idea

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
OK, I'll bite.

When you say the two IR sources have a different frequency, do you mean they are modulated with a different frequency, or that their light output is a different frequency (usually given as wavelength)?

Will both be transmitting simultaneously? If they are, then you may see a far more complex input signal.

4. ### student

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Mar 23, 2011

i mean that they are modulated with different frequency.

yes the will trnsmit simultaneously

5. ### student

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Mar 23, 2011
hi steve

i want to do this using a microcontroller. but i now little about the modulation and demodulation. so if u could help me with that part or guide me where to find the right information, please.

thanx alot

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The problem is relatively simple to solve if only one device is transmitting.

You simply look for a series of pulses with a specfic period.

so for a signal like this:

____-____-____-____-____ (20% duty cycle)

you would look at the period (___-) and see if this repeats a number of times (you want to ignore noise) and then you can say that the signal is present. Essentially you track the time between positive (or negative) edges.

Note that this allows your duty cycle to differ without affecting the detection. So this signal would also be detected:

__---__---__---__--- (60% duty cycle)

These may appear to be different lengths because your browser bat show _ and - as a different length. Cut and paste them into something with a fixed pitch font (notepad?) and it will become clearer.

The duty cycle can sometimes seem to change as you move the transmitter and receiver closer or further away, so eliminating this as a part of how you detect the signal is a good thing.

Imagine we have a second signal. It has a slightly shorter period

___-___-___-___-

If we can detect that duty cycle as another signal, then we can trigger some other output when it is received.

There are some things to resolve before going any further. The transmitter frequency (or the receiver clock) may drift over time, temperature, and/or battery voltage. Thus the duty cycle may seem to drift. Your code should look for a sensible range of duty cycles, not a specific single one.

Now the next problem.

imagine they are transmitting together

____-____-____-____-____ (signal A)
___-___-___-___- ___-___ (signal B)

___--__-_-_-__--___-___--_

This is far more difficult to decode.

Before we discuss that, do you understand all of the above?

7. ### student

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Mar 23, 2011
Hi Steve

thanx alot for your reply. i have one question in the following part other than that I'm all right.

have you meant here the duty cycle or the period? because you said up that i will be looking for a specific period and any duty cycle will be detected if it have the same period.
so, the driftid duty cycle wont be a proplem as lang as it has the right period.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Sorry, in that paragraph, I should have said FREQUENCY, but period is a suitable alternative. Well spotted

Here is where I lie and say I put in that deliberate mistake to see if you understood

9. ### student

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Mar 23, 2011
lol, thats a good one.

10. ### student

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Mar 23, 2011
now that i uderstood that part, shall we proceed on the next part, please?

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The next part involves you explaining more stuff.

1) are you making the transmitter(s) as well as the receiver?

2) is range an issue (if so, what range do you require?)?

3) what research into detector circuits have you done?

4) Is there a purpose for this? Just for fun, or something more serious?

12. ### student

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Mar 23, 2011

1) are you making the transmitter(s) as well as the receiver?
yes, i have to do them both.

2) is range an issue (if so, what range do you require?)?
the required range is about 8 - 10 m

3) what research into detector circuits have you done?
i have been reseaching for almost 10 days but all IR detectors i found is for remote controlls which only include one IR source, one detector and couple of timers. i felt kind of lost so i seeked help here.

4) Is there a purpose for this? Just for fun, or something more serious?
i do need it for my uni project. i need to detect when the first beam is cut and when the second is cut and caculate the speed of the passing opject using a microcontroller.

13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
OK, here is where you need to explain things clearly because I can't see why you need a single detector. 2 detectors and 2 sources seems like the usual way. And then you don't need anything really fancy to decode the signals.

If you only have 1 detector then it would seem logical that the beams are not parallel, and therefore the speed will be quite hard to calculate.

14. ### student

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Mar 23, 2011
yes you are right. but i havent explain my whole project, this is why it seems un logical. for my project i need to use three beams and two detectors. two beams will be parelle and one will be daiodnal. so one of the detector needs to see only one beam and the other needs to see two beams. i know how to do the easy part, so i just asked about the hard one.
i thought that i dont need to explain the whole project, as i only need help in this part. so why to bother you ? this what i was thinking

is it clear now.

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No it's not clear at all.

The issue I see is that any decoding of signals will slow the response time. If you have to deal with multiple interacting signals then that delay could be variable. Both of these point to sources of potentially significant errors in your timing.

I would only be using a single sensor if there were absolutely no other choice. It complicates things horribly.

Why don't you start by explaining the actual problem you're trying to solve, then how you intend to solve it, and then how this dual receiver fits in.

These facts are important or you may end up with a perfectly working but totally useless device.

A simple statement of what you were doing at the beginning could have been
You have avoided answering the questions I have asked about WHY you're doing this, so I'm going to presume it's a school project.

16. ### student

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Mar 23, 2011
I haven't avoided any question. i have answerd that it is for university project.

i know that using two detectors is much easier, but i don't have the choice it is required to be this way. the facts you asked me to explain won't help as you can't change or add to the design.

i hope you can help with this complicated situation

25,449
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Jan 21, 2010