IR Modulation

morris4019

I have a question about a modulated IR Detector. I built the circuit below, which uses a 555 timer to modulate 2 IR LEDs to 40kHz. Now at the same time, I need to be able to check that it truly is modulated, right? I though, since there was a fairly easy way of modulating the IR light, there probabl is a an easy way of making an IR Detector only look for a modulated signal. I have not been able to find anything. All the circuits i find use a sharp, or panasonic modulated (38 - 40kHz) IR·detector. Does anyone know of where to find a circuit to use a standard IR detector, such as an SFH506, to filter out everything but the modulated signal.

The reason I ask is I can of course recieve the modulated signal with this detector, but, light effects it, and if anyone tries to turn on the tv within 10 feet I detect that too. I have searched through the stores around me for one of the Sharp or Panasonic modulated detectors, but no luck. So, i figure I would try and see if it can be done using something else, before I spend $10 shipping for the one part.

-Mike



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JReed

Can you use a diode detector? I had a similar setup with a project I did in school where I used 5kHz to modulate an IR led that you would place on your earlobe and then used the diode detector to received it on the other side of the earlobe. Basically and AM signal to detect the blood for figuring out your pulse.

kind of like this: http://faculty.kc.devry.edu/twheeler/eet368/lab5.pdf

morris4019

You know I'm not sure, how bout i explain my situation. I am going to build a beacon, emmitting a frequency that I can modulate and detect from a mobile robot. I figured I should be able to make a very simple mobile robot detect and head for a stationary beacon without the use of programming. Just like a an ir emitter reverse biased to detect and head for light, I thought I could modulate the signal comming from the beacon so that the robot would only detect the beacon.


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JReed

yeah, that's basically what the diode detector would do, I'll see if I cant dig up what I had used (it was about 15 years ago). I think you would need to bandpass filter for the 40KHZ and then use the detector to figure out if there is any signal there.

morris4019

So let me see if understand this correctly about the circuit below. Your saying that the RF INPUT would be the output of my IR Detector? Then the end result (AUDIO OUTPUT) would be the modulated signal if any? But wouldn't this just pick up any frequency?

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Manetheren

That is·a standard envelope detector circuit.· It has a bandpass of a standard AM signal.· You would need to figure out your bandpass and create a RLC circuit that will pass your 40KHz signal (not the AM signal).· But the principle is the same.

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Manetheren

Leon

Try a PLL, like a 74HC4046, as a demodulator. If you have the transmitter and receiver close together, you won't need an amplifier. You can add one when you get the basic system working.

Leon

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