Hi I am new to Arduino and have an idea for a project I want to do with it. My idea requires 100 motors running independent of each other. Is this possible to do with Arduino?
Well.....maybe. An Arduino only has ~18 I/O pins so you could control 18 motors directly, or you can use an external multiplexer/logic to control more, or you could use multiple Arduinos that communicate with each other, each Arduino controlling ~16 motors (let's save 2 wires for communication).
More importantly, where are you going to get the power to turn 100 motors? How much current/voltage does each motor require?
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The Quick Shield: breakout all 28 pins to quick-connect terminals
It's all very theory based at the moment. I'm not sure how much power they would need. What kind of motor and power requirements would I need for 1 motor to pull roughly 0.035kg
A small hobby motor looks like it would suffice. Their torques are often rated as oz-in. or g-cm, both measures of force at some radius (hence torque). How far away from the motor shaft will you be pulling your 35gm? That determines the "cm".
Maybe something like this?
You are probably looking at 100mA-200mA per motor while pulling this kind of weight.
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, light sensor, potentiometers, pushbuttons
So each motor would be pulling that weight at a max of 3 meters away on a string. Would I need any kind of a motor shield or anything.
Thanks for all your help
Yes, you will absolutely need some method of converting digital outputs of the Arduino to the "high-current" values needed by the motors. There are lots of examples of motor driving on the playground:
http://www.arduino.cc/playground/Main/InterfacingWithHardware#Physical_Mechanical
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Need a custom shield? Let us design and build one for you.
Here is how to drive small motors with transistors and relays:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html
Do you need the motor to change direction as that makes the control more complex.
If you are an absolute beginner then driving 100 motors is going to quite a challenge.
...makes the control more complex
...makes it A LITTLE BIT more complex
(though makes it 3 times as expensive 
)
Thanks for all the advice guys. Yes the motors will have to drive in both directions. I am not a total beginner but not far off. Ive played around with leds, servos, proximity sensors, buttons, old joysticks et al.