Hello, i wanna create an automatic trash sorting machine, the important part is the metal detecting, I wanna use a proximity inductive sensor but I don't know which one will be best? also if I should choose an NPN or PNP? most of the sensors are of 4mm detecting distance, is it enough? in the project I ll use an ultrasonic and an LDR sensor as well plus the proximity inductive sensor, is an Arduino uno enough or should I get the Mega? this is my first post so I apologize for the too many questions, thanks in advance.
As the designer of the garbage sorting machine that you haven't specified, that is totally your decision.
Depends on the rest of the circuit.
the important part is the metal detecting
What sort of metal?
Have you figured out how the machinery will present bits of trash to the detector? That might be the difficult part of the project.
Probably not.
The best form of metal detector for this sort of thing is a two, or three, coil magnetometer arrangement, where the waste has to pass through the coils to register. I am not sure if these are commercial available, you might have to make your own.
These are designed for rotary encoders by counting the passing teeth of a sprocket wheel.
A metal detector for treasure hunters may be a good starting point.
Hi,
Google;
conveyor belt metal detector
What are you going to do when you detect a piece of metal?
Stop the belt.
Have a pusher shove the metal ladened load off the belt?
Pick the metal up with an electro-magnet?
Tom.. 
Well back in the 60's in my first job we used to use them as replacement for reed / magnetic switches for extra reliability.
Then one wakes week break, there was just a skeleton staff in the building and all the development engineers were away. I was given the project of seeing if they could be used for measuring frequency of rotating objects, and what were the limitations. When they came back my boss was impressed as to how I had investigated not only counting teeth, but counting a single rod and a single hole.
Yes that might be a good start, but this same company made some of the first metal detectors, so I have lots of experience with large 3 coil detectors. Out first product was for measuring stainless steel strands from meshes that got into pharmaceuticals. We had a large tub of "the contraceptive pill" to test with. Then it used to be used to prop the door open.
Stainless steel is quite hard to detect.
can you tell me how the rest of the circuit will determine if I should get npn or pnp, I just spoke with a seller and we ordered a npn sensor, it will arrive in 3 days, for the trash part, the machine will be like any trash can just bigger, there will be an opening where the trash will be thrown, then there will be like a small box where the sensors are, the trash arrives there, the inductif sensor detects if it s metal, in this case a servo motor will move the box with a specified angle and place the metal into there place, for the paper and plastic process, we ll use an LDR sensor to try to differentiate between these 2.
The hardware side of this project - the mechanical bits ; you need to get that sorted first .that might include trying a range of sensors to see what works . Deciding on a UNO or Mega or Npn /pnp are small decisions that can wait
You design the circuit to accommodate either the sensor PNP high side output or NPN low side output. For an Arduino input, an NPN open collector output is convenient.
Can you post the data sheet to this sensor or the vendor and their part number, along with how you propose to wire it up? That is post a schematic.
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