AVR-Chat

You can use the same emitter/receiver pair for both beams. I'm no  
Michel Asciio, but I'll give it a shot:

-----/-----\-----
-----u-----^-----

/ and \ represent mirrors (front surface?).
u is a receiver.
^ is an emitter (laser diode?).

With a bright enough source, this mirror pattern could continue down  
the tube, so you could get multiple velocity readings.

You can also use one emitter/receiver and no mirrors to measure the  
velocity. The length of the projectile is known, so you only need the  
dark/light time to determine the velocity.

Another thought: at the target, use a sandwich of three pieces of foil,  
insulated from each other. When the outer two pieces make contact,  
start a counter, and when the inner two pieces make, stop the counter.  
I like the optical ideas better.

Regards,
   JJ


On Friday, Mar 12, 2004, at 22:14 US/Eastern, LightYearCS wrote:

> Take a look at this line of parts for working with photodiodes:
>
> http://www.maxim-ic.com/quick_view2.cfm?qv_pk=3760&ln=
>
>
> -----Original Message-----
> From: Dave VanHorn [mailto:dvanhorn@cedar.net]
> Sent: Wednesday, March 10, 2004 1:37 PM
> Subject: Re: [AVR-Chat] Re: Speed Trap
>
> 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.
>
>
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