object detection for 4 meters distance

hi,

anyone can give suggestion about what i should use for making detector object for 4 meters away?. currently i use a simple photo diode (infrared receiver and transmitter), it can reach around 2 meters away but it seems like very low signal. i need inline detection object, for example in four meters i put transmitter at one side and put receiver in another side, their installed should at the same position, so if object passing in the between, it can detect it.

thanks

Have you tried using lenses?

Problem with lenses is that they don't work at IR, glass is opaque to IR. Mirrors on the other hand (front silvered) will work but these are expensive.

You need more than a simple photo detector, you need the sort with a built in amplifier like they use in remote control receivers. These work with modulated IR, normally in the 32 to 38KHz region so you need to switch your IR emitter on and off at that frequency. Final make the IR beam stronger by using the smallest resistor the diode will stand and maybe using more than one of them.

Problem with lenses is that they don't work at IR, glass is opaque to IR.

That's true at longer wavelengths, but for the sort of wavelengths used by IR LEDs, glass is usually OK (otherwise, how would we check our IR remotes are working with our glass-lensed digital cameras?).

You just have to adjust the focus away from where it is for visible light.

OK yes your right. As you say focusing is not so easy though.

How about ultrasound? One transmitter and a receiver 4m away.

How about visible? I do stuff like this with a little $2 laser diode.

Yes, I was going to suggest the same. I use a laser and photodiode in one of my projects. In theory you can set up a beam interrupt detector ( laser pointing directly at photodiode ) as far as the laser can be aimed. If your project doesn't need the switching speed of the photodiode ( 0.00000001 seconds ), I'd recommend using a phototransistor ( 0.00001 seconds ), a bit easier to incorporate into a circuit and more sensitive. In addition, you may be able to decrease the sensitivity of alignment if you get a laser diode module with adjustable focus meaning you can widen the beam slightly at the point of detection. If this results in too small a signal for detection, just use a more powerful laser or more lasers aimed at the phototransistor.