Help Laser Trip wire

[chrisg44]

Hi, Im looking to make a laser trip wire so that when my dog jumps on my bed and im not home, a 12v speaker is activated to try and scare her off, i only want power out when the beam is crossed. I saw a youtube video where a guy shows his tripwire and how when he moves his hand in front of the beam, the led comes on, and when his hand passes though the led is off. only problem is, his breadboard is at an angle and i cant read schematic he put up. I tried doing what his looked like to no avail.

i have photoresistor, resistor, transistor, but confused how everything goes on breadboard, and have had no luck in two hours of google searching.

please somebody help me.

 

[tomemmerson]

you can buy things like that from lots of toy shops. just instead of using the built-in speaker you hook it up to a bigger one

 

[gtr_stitch]

Just helped another guy...

I just saw another post like this, except he wanted power constant once the beam was crossed. I can show you both circuits cause crossing the beam can be quick, and you may want something to stay on just a little longer...

The circuit with the transistor is what the video was showing. Let's say the value of R1 is 10K, and the value of the photoresistor (ldr) with beam on is 1K, without beam is 22K. While the beam is on the LDR, power is going through the LDR and not R1, thus the transistor has no power. Once the beam is broken, power now goes through R1, switching on the transistor and whatever you have on the output. The transistor is only on when the beam is broken. This also makes a nifty night light. If you wanted something to turn on for a moment or two, you could add a 555 at the output (in monostable) and it could supply power to a siren or buzzer for however long you wish.

The circuit with the SCR acts more like a tripped alarm that stays on until you reset it. Same basic principles apply.

Hope this helps!