Alt-BEAM Archive

Message #01021



To: sathe_dilip@bah.com
From: "George Rix" rix.g@bmts.com
Date: Fri, 26 Feb 1999 07:43:02 -0500
Subject: [alt-beam] Re: photodiode - sensitivity


>> The nearest dollar store is VERY far away, unfortunately. Our local one =
went
>> bankrupt for some reason or other. It's a bicore photovore=8Ba photovore w=
ith
>> two motors (duh) that's driven by a single bicore, with the photodiodes
>> controlling the direction (again, duh). I think I got it down pat now,
>> though.
>
> I meant is there a web page for the circuit you are using. Is it the
> one being discussed over last week between (I think) Justin, Sean, Wilf
> et al?
>
>>From my earlier experiments (non-BEAM), I remember something that may be
> useful. But you will have to experiment yourself as my lab set-up is
> not complete yet.
>
> If instead of photo diodes, you use photo transistors (3 legged animal),
> you can get some gain out of the transistor. You tie the base to
> collector using a high value resistor (try megs) and then use the
> emitter & collector where you would connect the photo diode. You will
> have to take care of the polarity depending on the PNP/NPN transistor &
> where you are connecting it in the circuit. Now if you say I do not
> have photo transistors, I have had some success using metal can
> transistors (TO-18) packages of the small signal variety. Just grind
> off the top portion of the can and you expose the junction to light.
Don't have them either. Just the photodiodes. I removed one of the resistor=
s
and rewired it to include just one, and unless I remove both the PDs, both
the LEDs (I don't have my motors hooked up yet) go on. When I remove the
PDs, They flash properly.
No metal can transistors =3D)
This is getting kind of pathetic, isn't it =3D)
I wish I knew what was up with it. I'm gonna try a bunch of other stuff.
I'll let you know if I can get it to work.
> Another variation that you could try is using a photo diode to bias an
> ordinary transistor (The ones used in SEs are fine). Try connecting the
> photo diode between collector and base - with a high value resistor in
> series if necessary. Then use the emitter & collector of the transistor
> where you would have connected the photo diode. Again ensure the
> correct polarities (the photo diode should be reverse biased with the
> available voltage).
What kind of transistor? Any 3904 or 3906?
I have a bunch of others, but those are the only ones I've got two or more
of. Oh, wait, you said the ones used in SEs are fine. Well, I'll give it a
shot! I'll write back soon, I hope.
> Then again, these experiments may not be suitable for the photovore
> circuit you are working on. Going back to lenses. You could try a toy
> store for some cheap binoculars or Viewmaster type of toy. I have also
> seen some key chains that you get at tourist places. These have a few
> pictures/transparencies that you can view by rotating a small disk. The
> lenses in these are ideal as the focal length & size is small enough.
Well, I checked the nearest toy, hobby, craft and buck stuff stores, and no=
t
a single lens in anything under 12 bucks. I'm not spending more on the
lenses than I am on the rest of the circuit =3D)
> If you try any of the suggestions pl. let me know how/whether they
> worked. Hope this helps.
Signing off,
Rob Rix

'If anything can possibly go wrong, it probably already has'-Murphy's law

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