plz reply

 

I doubt that that's your problem. It feels like we're kinda stumbling around in the dark right now, what do you wish to accomplish by connecting a LED, are you simply trying to get a feel for the output signal?

If you are sure you have the correct signal from the board try to find some specifics, what is the minimum and maximum signal levels? Or better yet, if it's coming from an IC what is the Part code?

 

for now, yes, all i want is the feel of the signal, but once detected i want to connect it to my own decoder (a PIC micro), like i said, i want to use it as an Rx\Tx module, i'm only intereste in the RF reception part of the circuitry.

The IC is a decoder, so i'm not getting anything from the chip, i'm high-jacking the wire that goes to the input pin of the decoder.

The datasheet is in chinese :S, but gives the pinouts so i was able to identify the signal pin, and it gives an example of an RF receiver in which the chip can be implemented.

But, the circuit that i submited earlier (op-amp), is it correct?

 

If the voltage you measured is 0.2V it's either an analog signal or fast switching digital signal, either way the LED isn't going to tell you anything, I'm assuming you're doing this on a budget so try the following, strip an old mic and add some wire clips where the mic part use to be and download the following:

Visual Analyser 2009 HR

It turns your sound card into a makeshift oscilloscope, the line in port works better than the mic port and try not to over voltage it, 5v should be no problem but you don't want to blow your sound card, mine is fine but it's always possible. Hook up your circuit and you'll be able to see what's going on inside

 

SWEET! well now it's almost midnight, i'll try it tomorrow morning

but you didn't say if my op-amp circuit was correct, i might need it some other time

 

ok, i tried it; when i first push the transmitter's button, i get jump for -47dB to -27dB and back to -47
when i release it, the same thing happens
what do i conclude? how does that translate in volts? can i pick it up with a microcontroller?

i'm using about 1.5 meters of audio cable (high quality)

 

So it only gives you two states? one when the button is pressed and one when it isn't? (you can calibrate visual analyzer to give you voltage instead, play around with the settings, it's been a while since I've used it)

 

IT FINALLY WORKED!!!
ssssssss, well it worked because, i soldered my wire on the wrong pin o.o
Damn those chinese circuit boards!!!! :P

but a couple of days ago, you said that my transistor circuit can detect 3.4v as min
how did you calculate that??? it will help for my signal amp

and thanks alot for your help and that awesome piece of software

 

For a transistor to operate in the active region your base-emitter voltage should be about 0.7, this is kept relatively constant by the transistor for a large range of current, more is ok but less usually won't work, when this voltage is reached the transistor starts to amplify the current through the base. The same holds for a diode, it needs 0.7v as well, transistors and diodes are closely related, while a diode has one pn junction an npn transistor has two. Since a led is simply a special kind of diode it has the same characteristics but requires about 2v to turn on.

If you look at your circuit from your input pin it falls over 2 base-emitter junctions and a single diode so you simply add these together which gives you (0.7 + 0.7 + 2 = 3.4), note you could have used a single transistor. When a junction voltage is exceeded with a reasonable amount the device basically becomes a short which means that if you got your led to turn on it would effectively short your power supply blowing it instantly (presuming it wasn't just given a quick pulse). You need to include a resistor in series with your led to prevent this.

Most led's opperate best at around 20mA, presuming a 5v supply and 2v drop across the led you need a resistor of ((5-2)/0.02=150).

Since a transistor base-emitter junction is just a diode it has the same effect, if you simply hook it up to your supply it'll short out your supply so you need a resistor as well (between your amp input and the base of the transistor). Presuming your circuit we need to limit the current a bit, your circuit has an absurd amount of gain so just limit the current with anything you have at hand, not real reason to calculate it, a 1k should be fine.