I have been trying to build a wireless infrared channel.

The distance is only about 4-5cm

Problems:

Can't seem to get the Receiver to work, I have tried all 4 different connections of very simple setup and nothing. In some I get 5V and some I get 0V. However, for the one that I get 5V the signal goes to zero when I interrupt the channel with my finger. But I don’t get any square waves the signal looks like a DC signal.

I know I need an amplifier and a voltage comparator but first I need to get the communication running.

Also I have seen many circuits using a transimpedance amplifier (converts and amplifies current to voltage), why not simply put a resistor in series to the photodiode and amplify the voltage at the node of the resistor and photodiode (sfh203FA) connection. (See figure).

Attached Images

infrared.jpg (28.3 KB, 960 views)

 

the ambient light is blinding the phototrasistor.Put an IR filter on it so that it only gets IR light (the confuguraton where it got 0V wen you put your finger on is corect)

Its much easyer to use IR reciver modules its a 3 pin thingy that outputs positive wen it gets IR light that is the freq. that is tuned for (from 30kHz to 40khz)

You simply conect the power suply on the two pins and an LED whith an resistor.That way the LED shod light up wen it recivers IR light modulated at the rigt freq.

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Il give you shocking experience.

 

Phototrnasistors are slow, I want to use photodiodes coz they are fatser, I want in the range of megaherts.

Someone Electro wrote : "the ambient light is blinding the phototrasistor.Put an IR filter on it so that it only gets IR light (the confuguraton where it got 0V wen you put your finger on is corect)".

The photodiode (sfh203FA) I'm using already has IR filter.

 

Check it using visible light, you will almost certainly find it's very sensitive to visible light - IR filters reduce the sensitivity to visible light, not prevent it. You should modulate your IR beam, and AC couple your receiver.

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I will modulate it but first I want to receive the signal that I'm putting at the gate of the mosfet of the transmitter.

i.e I'm putting 1Mhz at the gate of the mosfet from signal generator and I just want to see a 1Mhz signal at the receiver. Then I'll put amplifier and voltage comparator, modulators etc..

what u mean by : "AC couple your receiver".

Can u tell me which of the 4 configuration of receivers from the attachment, is the correct one?

 

I have put a black plastic tube joining LED emitter and detector and I still get a 5V look alike a dc voltage..when I squeeze the tube in the middle the signal goes to 0V.

 

It is kind of working now.

At these resistor values I get an amplitude of:

468 Ohms - 0.15V
384 Ohms - 0.12V
1K Ohms - 0.31V
1.5K Ohms - 0.61V
3.3K Ohms - 1V
5.1K Ohms - 1.5
10K - Looks like a sine square wave
100K goes flat

For any of the values above when frequency is increased the square shape shrinks and become to look like a triangle, and loose amplitude and width. (Means the signal is not stable).

Now is when I'm confused, do I try to amplify the 1.5V or amplify the current?

How do I make the signal stable? i.e. for the signal not to loose shape and amplitude.

Now, do I just connect an amplifier or voltage comparator?

Attached Images

infra.jpg (4.5 KB, 925 views)

 

meaby the photodiode cant switch that fast.

use an simple Op amp amplefyer

__________________
Il give you shocking experience.

 

You appear to be modulating your signal at 1MHz. Thats why you see a constant signal - the frequency response of your components must not be high enough. Change your modulation to around the 30kHz range and you should see a signal. You could try pointing a IR remote at your receiver and see if you get a signal from that.
HTH

Mike.

 

why do you use such an high freq.?

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Il give you shocking experience.

 

The photodiode I'm using can switch on and off at 5ns

It does work at 30 KHz very well.

So how do I make it work for 1 MHz?

It is no use connecting an amplifier or a voltage comparator at this stage, coz the signal looses shape and width above 1MHz, I have to make it work for 1 or 2 MHz first, then connect an amplifier or and a voltage comparator.

 

why not use 30kHz?It works fine at that freq.

why do you want to have it at 1 Mhz?

__________________
Il give you shocking experience.

 

Use capaitors to couple the stages of the receiver amplifier, for a start you should have a capacitor coupling the signal from the photodiode.

You're using a 1MHz modulation signal, so you only need to pass 1MHz, DC coupling it is only swamping the reciever with visible light. Your receiver should also incorporate a high pass filter, again you only need to pass 1MHz signals.

Either of the right hand ones, depending which you want.

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Actually I need to pass an FSK signal (1 MHz and 2 MHz).

Would it be possible to just connect a voltage comparator and maybe with a feedback might prevent the signal from changing shape and width when increasing frequency.

 

Fairly obviously a high pass filter will pass 2MHz as well as 1MHz!.

However, you don't need to have a high pass filter as high as that, just high enough to remove all the low frequency rubbish - in particular 50/60Hz modulation from lights.

1MHz to 2MHz seems rather a large chnage for FSK?, seems to me to introduce more difficulties for no good reason?.

Don't use such a high frequency, it's pointless using a 1MHz FSK shift - try using something like 1MHz and 1.1MHz (or even less).

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk