Better understanding of IR collectors or phototransistors

We are looking to focus an IR laser on a phototransistor to use as a security device. As I understand it, the PT acts no different than any other transistor so the amount of IR it received should not be as much a problem as the load it is running? IE, how much can they handle without failure?

My thought was if they act as a transistor and if they conduct when exposed to IR, if the load through the transistor is limited, they should not fail?


For calculation example, I am looking at a 10mW laser, focused to an 8" spot at 500ft distance. That gives a watt density that is ridiculously small and might be very hard to see with a photocell. Like wise, a 250mW laser is just .77mW/cm

Are my calculations just off?

Have you considered that ambient IR may be a problem? Are you proposing to use modulated IR (e.g. at 38kHz, as per TV remotes)?

We would eventually modulate and add on to reduce nuisance trips. Also plan to shield the IR sensor from ambient light in a black tube.

I am considering a photo resistor since they are stupid simple to work with but not sure if they are fast enough. They might actually be just slow enough to reduce nuisance trips by default. Regardless, I just need to start the send/receive process and build on from there. Probably go through multiple revisions.

i played w/ a similar project, but i used a standard red laser, i used a photoresistor, and i did add a small delay (debounce) before activation. (birds and/or bats would fly in the beam
and give me a false trigger. i did also put tubing over the resistor.