infrared RX-TX circuit.

I am using an infrared tx-rx circuit as a speed sensor for my project. the equipment used are infra red led, an infrared receiver module and resistors. the fact is that when i use the the circuit, i only get a high output from the infrared rx irrespective of whether when the IR led is on or off. I don't know which one is faulty and moreover do I need to know more about IR Tx-Rx circuits since i have already change the led and Rx module twice?

Can you provide the part number for the module you are using?

As I recall most of the IR receivers I have used output a logic high when no IR input is being received. Most of these receivers respond only to IR that is modulated at the carrier frequency (38-40 kHz for the parts I have used). This is done to filter IR noise or ambient conditions.

I would assume that if the TX is in the same module as the RX that there would be an oscillator to drive the TX at the correct frequency so that the user simply has to modulate the signal with a logic input. The data sheet would help tremendously here.

I don't think I read your post correctly the first time. After re-reading it sounds as if you have a receiver module and you are transmitting using a IR LED. Knowing this, I would have to say your problem is with the way you are driving your LED. You need to drive the LED at the carrier frequency which will be specified in the datasheet of your receivier module. I would bet money that it's in the range of 38kHz-40kHz.

You could perform a quick test by simply powering the receiver module and hooking the output of the receiver to the scope while mashing some buttons on a remote you have laying around. You should see the module pulse low while the remote is transmitting. I think the majority of consumer remotes transmit in this 38kHz-40kHz range (based on my experience).

neiji said:

I am using an infrared tx-rx circuit as a speed sensor for my project. the equipment used are infra red led, an infrared receiver module and resistors. the fact is that when i use the the circuit, i only get a high output from the infrared rx irrespective of whether when the IR led is on or off. I don't know which one is faulty and moreover do I need to know more about IR Tx-Rx circuits since i have already change the led and Rx module twice?

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You mention that your receiver only outputs a HI level. How are you measuring the output? Are you using an oscillsoscope or are you using an DVM? If you are using a DVM then you are probably reading a high because the DVM cannot respond to the very short pulses that are present on the output.

hgmjr

This guy is using an IR receiver IC but is not modulating the IR at 38KHz because he doesn't know what he is doing. So the output of the receiver is high all the time. It will be high all the time until he learns about electronic circuits.

Audioguru said:

This guy is using an IR receiver IC but is not modulating the IR at 38KHz because he doesn't know what he is doing. So the output of the receiver is high all the time. It will be high all the time until he learns about electronic circuits.

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What Audioguru meant to say was "try modulating the IR at 38KHz." He also meant to provide a link for further reading. I am confident that he will correct this communications error. Like the rest of us, he does want to be helpful to those new to electronics. That's part of what this forum is about, after all.

thingmaker3 said:

What Audioguru meant to say was "try modulating the IR at 38KHz." He also meant to provide a link for further reading. I am confident that he will correct this communications error. Like the rest of us, he does want to be helpful to those new to electronics. That's part of what this forum is about, after all.

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It is difficult to help a noobie who didn't attach a schematic and didn't say which parts he is using. A communications error.

If he did then I would have highlighted the section in the IR receiver's datasheet where it talks about 38kHz modulation.