I'm trying to make a battery-operated, microcontroller-controlled,
short-range, laser range-finder. It needs to be very light (weight-wise).
Does anyone know of a reliable method of detecting reflected green laser
light from about 50 feet from a grassy surface in an outdoor environment?
The grass seems to reflect a sizeable amount of light back to the source
(at
least by eyballing it). It seems a detector should be able to
differentiate
the presence or absence of the laser light. The green laser light seems
to
reflect better than red on a grassy field. I have a very small,
light-weight green laser module which is suitable to my application but
now
I need to develop a reflected light detection system.
I have tried a phototransistor (Panasonic PNZ121S0R) but it didn't seem to
pick up any changes at about 10 feet onto a fairly glossy hardwood surface
(wood panelling). When I try the laser directly into the phototransistor
over about 10 feet distance, it worked very well, though. I had the
collector pulled up to 4.5VDC (3 AA cells) and the emitter grounded. The
collector showed about 4.5VDC with no laser light and about 0.15VDC with
direct laser light. With the reflection, though, there was no change at
the
collector from the 4.5VDC. I verified the detector was aligned with the
reflected light. I even tried a small straw as an ambient light reducer
but
no change.
I then tried a photoresistor. It seems to be a little more responsive.
With just an ohmmeter across the leads, it showed a quiescent resistance
of
about 100K and when it sees the reflected light, it drops to about 85K or
90K. This was inside in a flourescent light environment, though. I did
use
a guide tube (straw) with the photoresistor and it helped significantly in
reducing the ambient light. I thought I could use the photoresistor in a
comparator circuit to set a range of operation.
Any thoughts or experience would be appreciated.
Thanks.
Dave
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