laser trip wire thing???

[tech-freak-wanabe]

i have been wanting to make a laster trip wire using a small laser pointer. i was thinking ov using a photoresistor hooked up to a dark activated circuit and that the laser would be able yo keep the resitor "off" and when it is blocked it would turn it "on". maby a "hood" over the resistor would help. would this work??

 

[dknguyen]

Yes

Sounds good to me.

I would use a photo-transistor (or a photo-diode, they are very similar) instead of a photo-resistor to make it have more off/on than low/high resistance. I think they are also more sensitive than a photo-resistor. According to Wikki the photo-transistor is much more sensitive than the photo-diode. Also make sure that the diode is best at detect the wavelength emitted by the laser. You might want to see if you can't find a photo-receiver with a collimating lens to make the device less omni-directional and increase directionality/sensitivity in the direction of the laser pointer. You may also want to filter the photo-receiver so that it only sees the laser light.

https://en.wikipedia.org/wiki/Phototransistor

You can use the photo-receiver with pull-up or pull-down resistors to make it dark-activated or light-activated. (I keep forgetting which is which, dark-activated means if the receiver isn't receiving anything (ie. the laser is blocked) then the output is low right?) The only other thing that is really needed is maybe a capacitor to filter noise from the output and make it stable.

YOu can choose to stick the laser and receiver in two separate units opposite of each other, or you can stick them in the same unit with a mirror on the opposite end to reflect the light back. If you choose the mirror way, you have to make sure the laser's angle will reflect off the mirror to hit the photo-receiver. And this angle will be different depending on the "width" of the tripwire.

You shouldn't have any problems from what I can tell.

 

[tech-freak-wanabe]

i think i can manadge once i do some googleing

 

[dknguyen]

Regualtor

Did I mention that you also need a voltage regulator for 5V (probably) so you can safley read the high/low output into a microcontroller or computer or whatever.

 

[HarveyH42]

I built this on a breadboard a while back, but never got around to finishing it up. I wanted to add something to indicate when the laser was lined up with the photodiode, and drive circuit for cheap pocket laser. I think mostly that I want to modulate the laser, so shielding out ambient light wouldn't be such an issue, since this was going to be used outdoors (cat detector).

Anyway, should help you out some