Hi. I decided to start a small project, which is my second, that is slightly more complicated than the previous.

What im planning to make is a security system on my stairs so that when my little sister tries to go up all by herself, it starts beeping. To get pass, one has to press a button that is out of her reach, and it gives one about 5 seconds to go unnoticed. It has to use very little power because the only source its going to have are a few batteries.

Now as i am completely new to the electronics market, and i have a few questions to you.

1.I know there a ton of diffrent techniques to detect if there is an obstacle between 2 detectors, but what is the cheapest, most efficient in energy usage and smallest one i can buy? I was trying to look on some sites but i didnt really know what im looking for because all the components have their unique names that i dont know of.

2. Do you think its better to make a circut out of raw components(which i would prefer, becuase its really cool) or get arduino and program everything onto a chip?
1) The cheapest method would be a laser or LED with lens and a photodiode. If you want it to be invisible to the naked eye, an IR LED would serve you well.
2) It sounds to me like an Arduino would be massive overkill for this project. A 555 or two along with a transistor latch would be all you really needed.

Good luck; I hope to see progress reports on this project, and don't hesitate to ask any questions as you go.
My worry with the LED/photodiode would be all the ambient light; IR might work better in that case, but I don't know. When I was building robots in college I used a sonic range finder which was pretty neat. Just calibrate it to know the correct distance and when it's less than that you could beep. I think the laser idea might work better, though.
One way to work around the fluctuating ambient light would be to use a component that is designed to handle it for you - I'd recommend a TSOP2438 (or equivalent) IR receiver module. This is designed to be used with remote control circuits and detects, amplifies and filters a 38kHz infrared signal providing a clean logic output.

Your sender would need to send a periodic burst of 38kHz IR (you can't use a continuous 38kHz signal as that would affect the receiver module's automatic gain control - it needs to be in bursts). One way to simulate this during development would be to tape down a button on a regular IR remote control. The receiver circuit would then just need to make sure that it receives a regular stream of pulses from the IR receiver - if the pulses are interrupted for a certain duration then the alarm can be made to sound.

The whole thing could be put together with some 555 timer ICs but it may be easier to protoype with an Arduino. One thing to watch out for is that those IR modules are not particularly directional and will be triggered by IR reflecting from nearby surfaces. A paper tube could be mounted in front of the receiver to block these reflections and make sure only signals coming directly from the IR LED are received.
  
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