I need a circuit which gets activated on making an obstruction between an infra red led and infra red detector ?

Its pretty much like a laser trip wire,this detector keeps detecting the rays from the led until an obstruction is made and thats when the signal must be triggered in order to sound a buzzer....pls specify the transistor which is to be used and if you are that kind pls suggest a schematic or atleast an instructable which could be helpful..

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This might sound like a dumb question, but how big is the obstruction?

I mean how long is the path between the IRLED and the detector?  Is it a few meters? Or a few millimeters?

The reason I ask is because it is somewhat tricky (but not impossible) to get the light from an IR LED to sort of  "throw" and be easily detected across a distance greater than a centimeter or so.

If it happens to be the case that you need to throw the light a few meters or so, then the best way to detect it would be using one of those little receiver modules (and also transmit with modulated IR like from an TV remote)
http://www.google.com/search?hl=en&tbm=isch&q=ir+receiver+module

It seems like I was telling the story of IR receiver modules, just a few days ago, here:
https://www.instructables.com/answers/in-what-way-we-can-convert-dc-voltage-from-batter/?comments=all#C59LUBDH7URBDYW

If it happens that the light path is just a few millimeters,  I recently built a thing that uses the optical encoder wheel from an old computer mouse, and you can see how I wired up the IRLED and the photodiode on that, in this diagram here:
https://www.instructables.com/files/orig/F55/6408/H6MF2Y3F/F556408H6MF2Y3F.jpg
I am guessing you need just the first stage or so in that diagram, the circuit including the IR LED, the photodiode, and that first 555 IC.
crank-driven-mp3-circuit-diagram.jpgmouse-encoder-crank-pulley-mechanism.jpg
Adarsh_tronix (author)  Jack A Lopez5 years ago
Jack,the distance would be just a few mms and the pc mouse thing may work cuz I've got plent of broken mice with me.
mrmerino5 years ago
Here. You'll have to look at the specs of the phototransistor to see if you might even need that empty resistor at the top. You might not even need that. I think that should pretty much be it, you can just use something like a 2222 or general purpose switching transistor to create the Darlington pair bit. You can get those anywhere.

Wait a little bit before you you try this, at least until someone more experienced than me says that my circuit will set your house on fire. I'm pretty good with that stuff, but just be cautious.
Bi-Stable555.jpg
EDIT: I did some more research and I don't think you even need that resistor in front of the phototransistor.
Hmm, You'll definitely need to invert the input, otherwise it'll be always on. I don't know why I didn't catch that yesterday. At least we know that nobody's really reading this one. It's retardedly hard o research, but I think they have pnp phototransistors.
Adarsh_tronix (author)  mrmerino5 years ago
mrmerino,your schematics really good and thanks for mentioning bout the resistor....
I think I'll try the direct method first i.e,make the buzzer buzz when the receptor senses the IR,meanwhile I'll search for a pnp phototransistor and accomplish what I actually want..
mrmerino5 years ago
If you want the buzzer to stay on even after the beam is unbroken, you might look into using a 555 timer as a latch. use the photo-transistor to turn on a regular transistor with allows power to the latch. I'm gonna go find a schematic.
Adarsh_tronix (author)  mrmerino5 years ago
yeah,I do want the buzzer to stay on.....and u've just helped me...thanks
Adarsh_tronix (author) 5 years ago
Thanks for all your help(atleast I am gathering something)....and Jack,the distance would be just a few mms and the pc mouse thing may work cuz I've got plent of broken mice with me.
rickharris5 years ago
https://www.instructables.com/id/Chronograph-for-Air-Cannon/

google search for light gate