food for thought

[casrip1@gmx.com]

this notion has been bugging me since forever and i like to think of it as (sort of) a paradox...

 

take a high intensity laser pointer, hook it up with a light sensitive switch, and program it so that when the light sensitive switch is hit with light, the circuit is open and the laser will turn off. now go in a dark room, and point the laser pointer (which should now be nice and bright) on the light sensitive switch.

 

it sort of creates a paradoxical event because if the laser is on, the switch is told that there is no need for the laser to be on, but as soon as it would turn off the light sensitive switch would close the circuit, turning the laser back on. thus creating a cycle of continuous repetition; an infinite loop, if you will...

 

what do you guys think world happen? and how would it look; would the laser be on like it normally would? or will it flicker? or will it be stable at a lower intensity?

 

i would test it out but i don't really know how to work with a light sensitive switch (i believe it requires some sort of circuitry)

Edited July 11, 2013 by casrip1@gmx.com

[EdEarl]

This should make it easy.

http://www.ebay.com/itm/Velleman-Light-Sensitive-Switch-Kit-MK125-/261229799098

[krash661]

you probably can go to radio shack and build your own

but you need the schematics.

i have no clue if it will be cheaper, but building components is fun.

[casrip1@gmx.com]

lol you guys are missing the point here, the idea is to brainstorm what would happen. you gotta try and process this without the actual experiment (doing mental experiments to determine the outcome )

[EdEarl]

Oh, one with a relay will cycle on an off every few ms, depending on speed of the relay. Electronic only would cycle faster and appear to be a dim laser.

[Delta1212]

Yeah, it'd just flicker, though probably too fast to be visible. The laser would turn on. The photons would travel to the sensor. The sensor sends a signal to turn off the laser. The last photons emitted by the laser travel to the sensor. The sensor stops receiving photons. The sensor sends a signal back to the laser to turn on again.

[Enthalpy]

This depends fully on the detailed time behaviour of all components in the loop.

 

What you describe - in this case with light, others use only electricity, or hydraulics... - is a feedback loop, which is commonly used in servomecanisms. For instance all control surfaces of an aeroplane, like flaps etc., are moved by actuators because the pilot's muscles wouldn't suffice, and their position follows the steering stick thanks to such a feedback, which compares the input by the pilot with the flaps' position measured by a sensor.

 

Very complete theories exist to tell if the loop is stable, in which case the output follows the input, or unstable, when the output "flickers". Rather easy for electronics or hydraulics, they become more cumbersome if aerodynamics is involved.

 

In the case you suggest, a laser diode "can" be controlled to emit an intermediate power of light (not easy), making stability possible - this depends on details of the loop. If the laser control is only on/off, then straight stability is impossible and the light will flicker; though, in some uses, you may be interested in the mean power of this flicker, with the proper loop design, this mean power can be controlled by the input. Such an operation is called sigma-delta in electronics (for add-subtract) and used for metrology and audio.

[studiot]

The described system is quite interesting and is not a simple feedback system.

 

It is, in fact, a form of multivibrator, one view of which is that it is not one but two cross coupled feedback systems.

 

With the right time constants you can make this system into a chaotic one.

 

go well

[swansont]

Done it with a regular light. The system oscillates at whatever frequency the delay of the electronics will dictate.