Infared led question ?

What sort of range could be achieved with this emitter and detector if it was used in a simple break beam across a doorway?

http://www.rapidonline.com/Electron...s/High-power-infrared-source-and-sensor/29398

Hard to say, only experience would tell since they aren't.

If it isn't sufficient optics can be added to extend the range of these things, those 0.99 card type fresnel magnifing lenses work fairly well but they've got about a 3 - 4" focal length.

Probably 3 or 4 metres. These things are commonly used in TV remote controls. If long distance, watch for reflections off walls. Performance will depend on the sensitivity of the photo-diode amplifier, ambient light and your optics.

3 or 4 meters might be attainable but you've got to understand that TV remotes pulse their IR emitters under high current / low duty cycle conditions.

I doubt the doorway the op mentioned is going to be more than 3 to 4 meters wide.... so for his application it should work if properly shielded from ambient and any other source of unwanted light...... of course, he could modulate it to prevent ambient light from triggering it and add optics to collimate the beam and get more "precision" from it....

B. Morse

Can anyone recommend me a simple break beam circuit for use with the parts i linked to that which would be ok for 1.5 meters range indoors ?

Does operating the emitter/detector infa led at a high freq increase its range of detection ?

Transmitter:-
The output of any LED, unless it has some internal circuitry, is affectivly determined by the total power it is capable of disipating. Its brightnes, mcd output, is a function of its chemistry and the current flowing through it.

If your LED is rated for say 50ma continuous current you will probably find that it will be quite happy with 200ma for 1/4 of the time (25% duty cycle square wave) provided you dont exceed some max pulse width criteria. Its all about how much energy is disipated over what period.

Now that is a huge generalisation rather than a working example but the spec sheet for your led will tell you what you need to know.

The advantage is that when its on it will be 'almost' 4 times brighter = bigger potential range in your case.

Detector:-
If you try to use some 'level' you will have to compensate one way or another for ambiant conditions and worse still the change in level will have to be significantly bigger than anything you may expect to see in the backround noise.

On the other hand if your detector is 'tuned' to isolate a spaciffic freequency and you avoid freequencys that may occur naturally, then even a tiny signal can be relyably amplified.

The upshot is ...
Modulate your emitter to maximise its output and tune your detectior to exclude as much as you can that isnt at that freequency.

I cant give you a circuit, because I am not good enough at this to design a working notch filter.

I have utilised IR in the past but always used detectiors with built in electronics that detect and demodulate for me or chips spaciffically designed to do the job with IR diodes as an input.

Hope this gets you going with targeted searches.
Al

The detector in that pair already contains an IR filter so you're a step ahead fighitng ambient light.

Simple IR emitter/detector pairs are dirt cheap to begin with, why not just grab a set and see what you can get out of them? If they don't provide enough distance place a collimating lens in front of the detector at or very near the focal length. This not only increases the sensitivity since more of the light is gathered then conecentrted on the detector but also narrows the detection coverage angle by quite a bit too, further helping to guard against ambient light.

Back in my days we called this sort of thing experimentation and we always tended to learn something from it.