Large surface area infrared/optical phototransistors?

[astronomerroyal]

Hello,

Does anyone know of a phototransistor design with a surface area approx. that of a dime or nickel or even larger.

Why? I wish to detect a laser beam from some distance i.e. a laser tripwire. Currently I'm using an array of three of those standard 5mm(?) phototransistors, wired in parallel. The laser still has to fall exactly on one of them, but having three helps somewhat. In this case the problem is that the 'packing efficiency' is not very high. In the field it's very difficult to get things lined up so precisely. I considered putting slightly opaque plastic over the phototransistors to diffuse the incoming light, so that it fell more evenly. in this case the problem is light loss.

I took a look on mouser and digikey for large phototransistors - utter failure. I don't really know what to search for.

I actually want to find two types; sensitive to 650nm and 980nm. Finally, I considered CdS cells, but they're too slow.

Any help greatly appreciated.

 

[crutschow]

You might try a lens, such as a magnifying glass, or parabolic reflector (like a flashlight reflector) to direct the light from a larger area to the detector. That way the detection area is equal to the size of the lens or reflector. You get only a small loss in light from that.

 

[astronomerroyal]

Hi,

My laser beam is collimated and is only about 3mm in diameter. To decollimate that, or diverge the beam would require similarly size optics/lens/reflector, right? Or a diffuse scattering surface.

Just having a large detector area would be a simpler solution. I wonder if there exist pre-assembled arrays of many closely packed surface mount phototransistors. If they're separated by less than the width of the beam then it would work well.

'phototransistor array' returns nothing on mouser and one thing on digikey, but that's just three not-very-closely-packed detectors in a line.

 

[crutschow]

Not sure why you need to decollimate the beam. Anywhere the beam strikes a lens or reflector will direct the beam to the focus where the detector is, no?

 

[on1aag]

Hi Astronomerroyal,

I think I've seen something like that in infrared
movement detectors.

Check this out:

**broken link removed**

If these are still too small you can try using a solar cell.

on1aag.

 

[Mikebits]

He did say 650nm and 980nm. Solar is pretty broadbanded. The other devices you mention are meant for heat detection.

 

[crutschow]

The solar cell sounds like a good idea for a large area detector. It would be slow but for a tripwire, it doesn't have to be fast.

 

[kchriste]

The problem you are going to find with a larger sensor such as a solar cell (massive photo-diode) is that the tiny dot created by the laser will be easily swamped by ambient light. This "solar cell" is smaller than a dime, but still larger than a regular photo diode:
https://www.clare.com/home/pdfs.nsf/www/CPC1822.pdf/$file/CPC1822.pdf
But I think you'll just have to find a way to aim your laser better.

 

[astronomerroyal]

Crutschow, sorry, I think I misinterpreted your original post. However, I'm still not sure about a lense/reflector. My laser is essentially a 'pencil beam' - a lense merely redirects it. *If* the axis of the lense is pointing parallel to the laser beam, then I suppose the beam is directed to the lense's focus. However, if the lense axis is not perfectly aligned, it merely deflects the laser to the side of the phototransistor (which is fixed at the lense's focus). While the lense addresses the 'area problem', it introduces (or transforms it into) an 'alignment problem.'

In the field, getting the laser to hit a 5cm diameter lense should be easy. turning the lense so that it's pointing at the laser is the problem - however i think it's still a net improvement because I use those highly adjustable ballhead mounts that photographers use.

As for speed, ideally I'd like something that responds within about 1E-5 seconds, since that's the timescale on which my microcontroller checks the detectors. The tripwire's planned to be used with rapid things as well.

Solar cells are an interesting suggestion, although I'm really looking for a resistive/conductive type detector - it's part of a voltage divider.

 

[Blueteeth]

Hmm, a 10us response time is well within photodiodes range (normally nanoseconds), but of course as you pointed out, large area photodiodes are hard to find - not to mention expensive!. Then there phototransistors. Huge gain, but probably maybe a bit slow.

I was also thinking of modulation, until I saw the required response time, sure it would increase the sensitivity no end since you coudl filter out ambient light and all other frequencies except yours, but for 10us...you would have to modulate at a very high frequency. Then again, the delay *should* be fixed, so for measurement purposes this won't be a problem, just do it in post processing. All depends on the shortest period you're measuring.

I think others have covered the lens idea pretty well. You must remember that a convex lens will focus a large area, to a small one. This means if you move your beam half way across your lens, or at an angle (attempting to align), say 4cm, the focal point will only move a few mm. Coupled with a a pseudo parabolic reflector, the type you see used in flashlights, I wouldn't have thought you would have a problem.

I just thought....a cone shaped reflector, long and thin, with a convex lens at one end, and your photodiode at the other. Should sort out the alignment problem, with minimal light loss.

For a 'small' detector, nulike the cone idea above, instead of a specular reflector (like a mirror, shiney) use a lambertian reflector....white paper. Again, sacrificing sensitivity for alignment simplicity. So you wouldn't diffuse the beam as it enters, firstly it gets focused more or less on the detector, with the surrounding white reflector picking up any misalignment and scattering it around the inside of the reciever. A few carefully placed photodiodes around the box (not in the same place) might give a half decent signal?

You still have to deal with natural light though. Ambient light would be DC, but also anything that moves will produce a changing signal. Perhaps in that case we can assume the laser 'swamps' the reciever *when* it is aligned. So looking for a large sudden change would just be a case of using a comparator, with hysteresis. - Like a dataslicer, the kind used in RF recievers to detect changing signals, whilst 'tracking' a slow moving DC component (changing ambient light in your case).

Hm, I don't have much experience with this. Another simple solution would be to widen your laser Or to help you align one of those reflector strips that you see on cyclists. They light up nicely even when a laser isn't 'directly' on them.

I hope that gives you some ideas, I'm just waffling on. If its uselses ignore it, but if you want some dirty diagrams, I'd be happy to knock up something in mspaint.

Blueteeth

 

[astronomerroyal]

I appreciate your brainstorming.

I think I'll try the basic lense idea. A short focal length one would be best I imagine. If there are any residual problems I may add your ideas to 'corale' the light onto the detector. I believe the eyes of trilobites were like that - a lense and a reflective funnel. Maybe it was a modern day insect. At one point I even considered using fibre optics to guide the light onto the detector - but I dismissed the idea because I thought I could just go out and buy a larger detector. But apparently it's not that easy.

Aside: Yes, ambient DC light is the enemy. The first version of this device I made used the 'missing pulse detector' from Mims' book. No microcontrollers. Amazingly it worked very well. I pulsed the laser, and fed the detector's state through a differentiator to filter out the DC light. That was then. Now it's all microcontrollers, so much so that I'd probably implement the differentiator in software rather than hardware.

cheers.

 

[Torben]

I'd try indirectly detecting the beam. i.e. not targeting the sensor, but something else which would diffuse the beam and glow fairly brightly--say, a cotton ball (for a quick test) or preferably a small chunk of translucent white plastic--some plastics appear to glow from within when struck by a laser. Then just stick the phototransistor to the plastic. Set properly it should be able to trigger only on the laser if you bury the whole lot in a dark box at the end of an opaque, matte black tube.

Just a thought. I don't know how much room you have for the whole tube assembly.


Torben

 

[Blueteeth]

Good point torben!

If you've ever shone a laser onto a diffuse milky white LED housing, you'll see it. Even ping pong balls work quite well. Nice and simple that is. Again a lens at the front would narrow the viewing angle even further, but might over kill.

BT