Hi,
My friend is working on a project whereby he needs to detect a falling object before it hits the ground and measure its velocity before it lands.
This is to be done using proximity sensors that are made out of common components (namely Cds Light sensitive resistor, photodiode and MCP6004 op-amp), a push switch button is also available.
Any ideas on how we should go about this?
I am thinking of using the push switch to detect the time the object falls, but am otherwise slightly lost.
Any help would be greatly appreciated.
Google "photogate arduino" for lots of examples.
Hi,
Opto-transistors would be more accurate and not much different in price and availability.
You may not need the opamp.
Tom... 
Supergiant:
I am thinking of using the push switch to detect the time the object falls, but am otherwise slightly lost.
How will the push button detect a falling object while it is still in the air? I guess you could let the object fall on the button thereby pressing it, but that would only detect it after it hit.
TomGeorge:
Opto-transistors would be more accurate
And depending on the size of the object, a pre-packaged gate thingy like this with the source and the transistor in one item may do the trick.
I also just saw in another thread, that adafruit has these which are nicely packaged and have the source / receiver separate.
What will the object be? Include dimensions and mass.
Will it always fall how close to exactly the same?
Will it be conductive? Metal? Charged?
A metal bearing of known weight falling could induce current flow in a coil analogous to the velocity of the bearing if a magnet is also present and aligned.
A metal bearing falling through a light beam would have to fall on the same path every time. Off to the side a bit and you get some error.
Regarding Optos- I doubt a Cds cell will be fast enough for many applications.
While a micro may suffice for timing - depending on the measured distance and accuracy required, you may be better using an oscillator, counter, and gating the start.stop of the counter.
10MHz clock will give you 100nS temporal resolution, say over a 100mm distance would be pretty good!
or when Using a micro directly, you need to use the on-chip hardware counter-timers.
They probably want you to determine how long it took to fall some measured height from a drop but that is only very very likely.
What was taught in class so far is the big clue. Half the class may not have taken notes or paid attention and has to get someone else to bail them out, tomorrows management material.