about infrared red

[blibala]

i have doubts about infrared transmitter and receiver. hop someone can help me.
i did a lot of research on the net and found that most of the infrared trasmitter using 555 timer to generate a square pulse with high frequency. what is the purpose of the 'frequncy'?is it like the RF trasmission?or use to avoid the noise?if i direct connect the infrred LED in series with resistance, is it ok?
and at the receiver side, i saw that some people using PLL method to detect the signal.what is the purpose of PLL at the receicer side?used to eliminate the noise?
and anyone has better infrared tranceiver circuit that willing to share with me?i build a lot but none of them work fine

 

[StupidDum]

hm.. i've seen similiar things, but it is not infrared, it uses ultrasonic
ehh.. not very sure about that. I think PLL is used to detect certain frequency, so that your receiver not confuse with signal from other source

 

[Nigel Goodwin]

hm.. i've seen similiar things, but it is not infrared, it uses ultrasonic
ehh.. not very sure about that. I think PLL is used to detect certain frequency, so that your receiver not confuse with signal from other source

Yes, that's why it's done, so it only detects the correct signal - I've always suspected the frequencies used were chosen simply to use existing ultrasonic remote control chips, there appears to be no particular advantage gained by using those particular frequencies.

 

[Russlk]

The frequency of the 555 oscillator is modulated by the signal, the PLL at the receiver is the frequency demodulator. Look up National Semiconductor's application note: AN-46 for info on PLL as demodulator.

 

[Nigel Goodwin]

The frequency of the 555 oscillator is modulated by the signal, the PLL at the receiver is the frequency demodulator. Look up National Semiconductor's application note: AN-46 for info on PLL as demodulator.

In normal IR RC operation the frequency isn't modulated, it's simply keyed on and off - frequency modulation wouldn't work with the usual IR receiver chips.

 

[blibala]

then what is the exact answer to my question???!!!!!
i have seem a circuit on the internet using a 555 tmer at the transmitter side and a PLL circuti on the receiver side. then what is actually the fucntion of 555 timer?why they use PLL or other filter at the receiver side???i heard that it is used to eliminate the noise.

is it the receiver side will only receive the signal at frequency generated by 555 timer (transmitter side) and filter out the others????

 

[Nigel Goodwin]

then what is the exact answer to my question???!!!!!
i have seem a circuit on the internet using a 555 tmer at the transmitter side and a PLL circuti on the receiver side. then what is actually the fucntion of 555 timer?why they use PLL or other filter at the receiver side???i heard that it is used to eliminate the noise.

is it the receiver side will only receive the signal at frequency generated by 555 timer (transmitter side) and filter out the others????

I already answered the question in my first reply!.

YES!.

 

[Russlk]

Can you post the URL of the circuit you saw?

 

[blibala]

http://www.geocities.com/lemagicien_2000/elecpage/irvis/irvis.html

 

[Russlk]

The 555 oscillates at 18kHz to provide a light source that is different than any other around. The 567 PLL selects only 18 kHz reflections and ignores 60 (or 50) Hz lights. The circuit should work OK, what is your problem?

 

[blibala]

if i seperate the transmitter part, is it possible to modify the circuit so that it has the range at about 3 meter? and the output voltage can vary with the distance between the transmitter and receiver???
i need it urgently.can anyone help me?

 

[Russlk]

You could raise the sensitivity with more gain in the receiver (add another BC548 stage). What do you mean "vary the output with distance"? Do you not want a relay output?

 

[blibala]

what i mean is that when the transmitter is at different distances, the signal received by the reciver side will vary. if the receiver is near the receiver, the output strength will be highest.when the transmitter is far away, the signal received is poorest.
what i want to ask is that how to make the receiver side knows the distance (how far) of transmitter?

 

[crust]

In a typical IR remote demodulator, an automatic gain control (AGC) circuit is used to handle this problem.

 

[blibala]

i dont understand. can u explain again?
do u have any link to the circuit u decribed?

 

[Nigel Goodwin]

i dont understand. can u explain again?
do u have any link to the circuit u decribed?

If I understand you correctly, you want to be able to measure distance in this way - IR isn't a good choice, it's far too fast to measure the time taken for the reflection to return. Ultrasonics would be a better choice.

 

[blibala]

i knew that the transmitter infrared LED needed to be encased in a tube of some sort for better transmission. but for the receiver side, do i need to do the same thing with the photodiode sensor????

 

[Russlk]

Why do you want to know the distance? The receiver does not care about it as long as the signal is above a threshold. More gain in the receiver will allow it to work over a greater distance.