sci.electronics.basics

> "whit3rd" wrote in message
> news:5d660049-1d1d-45e3-bf72-0dd5ca4f1b39@...
> On Mar 14, 9:36 pm, Robert Barr wrote:
>> Anyhow, what I have in mind is a low-cost timing setup, used to measure
>> the time between two events -- both of which would be the interruption
>> of a light beam. Extreme accuracy isn't necessary
>
> Outdoors optoelectronics can be difficult, but does it
> really have to be optical? Could you use a taut thread
> holding a springloaded switch open? Run through
> the thread, it snaps and the switch closes.


You could do that as well, but getting everything just right might be
painful. I personally like the idea of using lasers, since they won't break
when people trip on them.

My experiments last night with a $5 laser pointer show that a photodiode and
a laser pointer gives me a huge voltage difference with the posted circuit,
which is very easy to deal with. Right under a fluorescent light with 2
tubes, the opamp outputs about 200mV. If I point the laser at it, I get 2
volts. This is with a 9V supply. I have not taken it outside, however, so I
don't really know how sunlight will affect it.

I suspect that if he uses a tunnel made out of non-reflective material
(actually, a piece of hose or pipe as the tunnel would work well), then the
ambient light should not affect the photodiode at all.

On the other hand, one point to consider is the life of the laser pointer
cartridge. Cheap laser pointers don't generally have a feedback pin, and may
fade after a few hours of use. I'd say to limit the current into the laser
cartridge to the minimum required using a constant current scheme of some
kind.

Regards,
Bob Monsen