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simple question~

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cool

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anyone tell me how a simple IR detector and a IR sensitive transistor work?? will they form the same frequency pulses that they received? :oops:
 
cool,

Here is a very simple IR detecting circuit with a description.
http://www.hut.fi/Misc/Electronics/circuits/irdetector.html

Basically the IR detection is done through a special transistor (IR sensitive one) . This picks up the IR signal at the base of the transistor and switches the transistor on. (ie: lets current flow through the transistor from the collector to the emitter.) Because of the way the circuit is constructed this switching of the IR transistor allows current to flow through the rest of the circuit. In the case of the circuit above, it turns an LED on and off as IR is received. But you could drive anything with it.

You can make a light sensitive transistor by cutting the top off the type that is in a small metal can, so that you can see the silicon. I'm not sure though however if that would work for IR frequencies. But if you want to have a bit of a play, it would be interesting to find out.

Now, 'will they form the same frequency pulses they receive?' Yes, most deffinitely! It is with these pulses that you encode your circuits. There will be a small amount of delay in the form of transmission delay, propagation delay etc. but not enough for you to really worry about.

I hope this was of some help to you. Post more questions if you have them.
 

cool

New Member
THanks~ :D
Will the IR modulator filter the input ir signal which is at its center freq?

as I would like to set a carrier freeq (40kHz) and a modulation freq(250Hz) for the ir signals in the tx part by using 555~ if the ir modulator can filter the signal wihich is at its center freq(40kHz), then i can only do the demodulation of the modulation freq(250Hz)! is it right?? :wink:
 

Nigel Goodwin

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cool said:
THanks~ :D
Will the IR modulator filter the input ir signal which is at its center freq?

as I would like to set a carrier freeq (40kHz) and a modulation freq(250Hz) for the ir signals in the tx part by using 555~ if the ir modulator can filter the signal wihich is at its center freq(40kHz), then i can only do the demodulation of the modulation freq(250Hz)! is it right?? :wink:
What you need is an IR receiver IC, they are three pin devices that give a logic output from a 40KHz (depending on device) modulated IR signal. Have a look at my PIC tutorials at http://www.winpicprog.co.uk, which use these in the IR tutorial. Even if you are not planning using PIC's, the hardware pages showing how to connect them will be useful, as will the descriptions of IR remote control.

Basically, you pulse the transmitter at 40KHz, you then turn this on and off at a lower frequency (ususally using some kind of digital coding), in your case 250Hz. The output from the receiver IC has already removed the 40KHz carrier, all you get out is a representation of the original low frequency modulation. If you are sending a simple 250Hz tone, you could use a PLL (like an LM567) to detect the tone.

A big problem though is that anything producing a similar frequency could trigger the PLL - which is why digital coding is normally used, it makes it reliable, and allows you to have more than one IR device in the same room.

It's a LOT! easier to do this with either PIC's or specific IC's, Holtek make chips for this exact purpose, and they are remarkably cheap (cheaper than using many PIC's in fact).
 
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