AVR-Chat

Dave VanHorn wrote:
> At 09:23 PM 3/10/2004 +0000, William Nachefski wrote:
>> Wow!  what a response.  Actually my tolerances are quite large, + or
>>  - say 10 m/sec.  As for placement of the sensors I have no set
>> length yet, I just threw 1" as an arbitrary number.  What distance
>> would be best?  I do intend to condition the signal of the sensors
>> before the uC.  As for accurate distance between them I was going to
>> use a dial caliper.  Is that accurate enough, obviously the larger
>> the distance apart the less important it becomes.  I think I will go
>> with 10 centimeters apart.
>
> Your error will be mostly in the response time of the two
> phototransistors, and in the problem of determining exactly when the
> beam is to be called "blocked".
> 10% drop in light, or 90%, and what is the gain of this
> phototransistor today, at this temperature, anyway?
>
> A photodiode, amplified by a current to voltage converter, is more
> repeatable.  Don't let that scare you, a current to voltage converter
> is just a simple op-amp circuit. Of course the op-amp you use also
> has to be fast enough to play this game..
>
> A tip: Make the IR light sources LOUD, and make the optosensors
> relatively deaf.


If using the same kind of phototransistor and light source, the
environmental error will be common to both, including the time response of
the thing.  The first will sense the blocker object (projectile) the same
exactly way as the second, so, a lazy sensor will answer late the same way
as the second.  The time delay between both sensors will be the same, no
matter temperature, air pressure or anything else.  I am considering two
exactly the same sensors and light source, also physical installations.

It would be different if only one sensor is to be used.  The sensor could
measure the darkness period of time, in true it would be measuring how long
the "no-light" situation takes in microseconds. Knowing the projectile size
it would be easy to calculate speed. This idea is no good, since due
environmental conditions a phototransistor could conduct faster than cut...




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