Hi! I'm new to the board and to arduino in general so please excuse my ignorance on the topic.
I'm trying to make a device using arduino that will be a stationary sensor which detects when a specific object falls past it and triggers a noise alert.
I've looked into ultrasonic possibilities but this may pick up if the object is simply nearby, I need it to only take effect when the object passes through the sensor. Kind of like breaking a beam if that makes sense, but specific to the one device and not just any motion.
If someone can let me know of any components that would do the job it would be much appreciated.
Thanks!
It may help if you can give a bit more information.
Can you put the arduino inside the object that you want to detect moving?
How big is the object, and how big the area in which it might fall?
Are you describing something like a ball in a drainpipe (very closely constrained) or a ball in a baseball stadium (tiny object compared to area in which it can fall).
GB
How big is the object? How fast will it be moving when it passes the sensor? How close will it be?
I'm thinking myabe RFID to confirm that the object is "near", with a laser and phototransistor to get a more precise indication of its position.
The idea is that there will be a sensor on a wall in a small shop, and products in the shop will be fitted with a device on the packaging which detects when the items drop to the floor. So the area is perhaps 20 feet by 30 or something in that region.
This is for a uni project so is just a prototype so can be pretty and big and clunky as long as it meets the function.
Thanks for your replies :o)
apologies for the odd smiley there lol.
The sensor on the wall is to be placed near the floor, which detects when products carrying the device will drop past the sensor. I am unsure what from these devices would need to take which is what I needed help with as am totally new to this. In theory the devices would have to be small to fit within carding, but as I said it is just a prototype to can be as big as necessary. The products onto which the devices would be put are small accessories such as jewelry and hair products. As I said the area of detection would need to be the entire floor space in a 20/30 ft room approx.
The theoretical scenario is basically every item in this shop is fitted with the device to alert staff when items drop on the floor to avoid damage to the items.
I hope that is a little clearer...
Thanks.
Also I think i'm allowed to use two arduinos that do the job combined if that's any help...
Let me replay to see if I have understood.
Ideally, the 'tag' needs to be about the size of a price tag, but it can be bigger for the prototype.
There are hundreds or thousands of things in the detection area.
People (and other stuff) will be moving through the same area frequently.
The need is to detect something which has fallen or is falling on the floor.
Ideally, the 'tag' needs to be relatively cheap to mass produce, but that isn't a requirement for the prototype.
The system needs to be made with one or two Arduino's.
An 'easy' way to do it is put a some intelligence on every product tag which can detect itself falling, and 'scream' out to a sensor.
Modern laptops have that capability. They use accelerometers which detects falling so that hard drive heads are made safe.
If you have access to an electronics lab, you could make a neat prototype.
If not, you could use an Aduino+Accelerometer+communication mechanism.
Sparkfun sell acceleromers on breakout boards.
I'd use a low power radio transcievers for comms. Maybe something like this nRF24L01 low power radio.
[edit]To make it look more feasible to the person looking at it, you might use an Arduino nano, or Arduino Pro Mini because an Arduino is going to look huge.[/edit]
HTH
GB
Let me just check i understand. So you're suggesting an accelerometer would be on the product tag, which would be wirelessly connected to an arduino which is in the role of the wall sensor which is programmed to detect the accelerometers falling?
Would making an IR beam work?
If you are able to have the 'object' fall between 2 points in space (through a IR line, between a reflector and an emitter) and only that object will go through that area, you should be set.
If you need to identify objects, or discern which ones will cause a trigger and which wont, much more logic or hardware will be required.
If you can explain better as to what you want to happen and the physical constraints, your answer quality will improve drastically. Oh, and drawings help too.
Let me just check i understand. So you're suggesting an accelerometer would be on the product tag, which would be wirelessly connected to an arduino which is in the role of the wall sensor which is programmed to detect the accelerometers falling?
Not quite.
An accelerometer has no intelligence, it's just a 'dumb' piece of electronics which yields a value which is the acceleration is experiencing/measuring.
This value needs to be processed and communicated, and that could be an Arduino in your prototype.
I assume you'd like to keep it small, and low power, so there is no need to have a device that attracts the humans attention in the tag. That could be elsewhere.
All the 'tag' needs to do is notify the external system, and the external system (2nd Arduino) can do whatever it needs to do to alert the humans.
So tags are an accelerometer+Arduino+communication to a 2nd system which is an Arduino+communication+way to alert humans (and probably other stuff)
Is that clearer?
I think I've done enough of your homework for now ... 
GB
Right, think i'm following you there.
I appreciate your time and advice.
One more query if anyone can help...
I had a look at the Triple Axis Accelerometer Breakout - MMA7260Q but it said something about possibly needing more hardware to convert the analog to a usable digital signal but that many uCs have a built in converter.
Can you confirm whether this accelerometer would be usable with a Duemilanove or if another arduino will be more appropriate.
I think they are only saying it needs analogue-to-digital converters (ADC).
Arduino's have 6 or more, so it should be fine.
When the 'tag' is falling, the acceleration will change by 1G, which it says is about 0.8V. This is pretty definite when the ADC can detect 0.005 V change.
HTH
GB