piclist 2000\06\06\181513a >
Thread: Noise in phototransistors/photodiodes/light
face picon face BY : Sean Breheny email (remove spam text)

Thanks to all who responded to this question. As usual, I didn't make
myself clear enough, so many of the proposed solutions are not applicable
in this case. Let me explain.

I am a model rocket enthusiast (as are many fellow piclisters, as I have
seen in previous threads), and for a long time I have wanted to try a
project where some kind of control was added to a model rocket. The most
obvious thing (of any significant challenge level) is to try to get the
rocket to hit or pass near to a target suspended in the air (by several
helium balloons).

So, what I have decided to do is to make the target consist of an array of
IR leds, pulsed any way we want (however is best). A four-channel receiver
goes into the rocket's nose and by placing light baffels in the right
positions, the onboard PIC (yes, it involves a PIC ;-) can get a rough idea
of its angle relative to the target by seeing which of the four receivers
is receiving light from the target. Two micro servos would then operate
control fins in an attempt to guide it to the target.

Because of the large number of variables, I can't guarantee the orientation
of the rocket with respect to the target, so I can't collimate the beam
from the target or put all the LEDs pointing in the same direction, etc. In
addition, because of size constraints in the rocket(four receivers have to
fit in a 1.8 inch diameter tube), I don't think it is feasible to use
lenses on the receiver photodiodes/phototransistors.

As I said before, I was able to get the receiver to detect the target at
about 100 feet (perhaps a bit more), so I don't need a really huge
improvement (about a factor of 8 or so in signal to noise ratio should
probably do it).

So, as you can see, this is not a communications system, and I can't put
the receiver and transmitter in tubes with lenses ;-)

I haven't exhausted all my own ideas on this yet, but I thought I would ask
to save myself the trouble if someone had a really good idea. Please do
give me any suggestions you have.

By the way, I have been working on this for quite some time, but I
hesitated to post it for fear that people would think I was trying to work
on a real guided missile. Rest assured, it carries no explosive, has a
ceiling of about 300 feet, and it's guidance system is not practical for
military applications.


At 12:30 AM 6/6/00 -0700, Arthur Brown wrote:
>Try putting tx & rx in a tube so as to shade from sunlight make sure you use
>lens with the right focal lenth.
>Firm I worked for used this over 250ft.
>Regards Art

| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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