how to make an optical sensor work?

[John Rowell]

Hi,

I've been trying to figure out how to use a phototransistor to send a signal
to a TTL circuit when the light level changes momentarily. The idea is the
phototransistor is placed outdoors facing upward. When a projectile passes
overhead quickly a signal should be sent to a circuit to start the timer. So
how would I filter out ambient light and make the circuit only react to a
quick change in light intensity? Any ideas?

Thanks,

John

 

[MikeM]

John Rowell wrote:

Hi,

I've been trying to figure out how to use a phototransistor to send a signal
to a TTL circuit when the light level changes momentarily. The idea is the
phototransistor is placed outdoors facing upward. When a projectile passes
overhead quickly a signal should be sent to a circuit to start the timer.

How much of the phototransistor's field-of-view does the projectile
cover as it flys by? Does the projectile pass close or far away?

Are there some optics in front of the photoTransistor which narrow
the field of view?

If the projectile is something small like a bullet, you will need some
fancy optics.

So how would I filter out ambient light and make the circuit only
react to a quick change in light intensity? Any ideas?

This is easy. Use an opamp to get a dc gain of 1000 or so, but
capacitively couple it to the phototransistor. The capacitor
passes the high frequency signals caused by the projectile wizzing
by, but blocks the slowly varying changes in intensity, like clouds
going by.

MikeM

 

[budgie]

On Wed, 07 Apr 2004 02:16:53 GMT, "John Rowell" <johnrowell@earthlink.net>
wrote:

Hi,

I've been trying to figure out how to use a phototransistor to send a signal
to a TTL circuit when the light level changes momentarily. The idea is the
phototransistor is placed outdoors facing upward. When a projectile passes
overhead quickly a signal should be sent to a circuit to start the timer. So
how would I filter out ambient light and make the circuit only react to a
quick change in light intensity? Any ideas?

Use a second phototransistor and place them in a bridge configuration (or
biasing a long-tailed pair).