Once you hack a servo for continuous rotation (typical in many two drive wheel robots) it is difficult if not impossible to achieve accurate speed & position control without feedback. Just about the same problem you have with a DC motor. I have used some of the Nubotics Wheel Watcher encoder kits years ago and wondered about any other recent low-cost options that might be new. The nubotics kits are available in the US from www.acroname.com and in the UK from www.totalrobots.com. They just attach to the top of a standard RC servo or some geared DC motors and work well, but wind up costing as much as the typical servo or geared DC motor. That may still be the best option. If anyone knows of other options let me know. I need quite a few, so I am really looking for an easy off-the-shelf kit solution. I have seen a few servos that have this built-in, but they are very expensive. Some images of the Nubotics encoders are shown below. The last one is magnetic and  replaces the pot inside the servo and can add I2C feedback and servo control. It is new and I have not used it.

Is it just me, or are you constantly bumping this topic for some reason...? :S

I added some info on the new internal magnetic encoder that I just found and fixed a couple typos. I ran the Beta and it locked up in edit mode, so it took a couple trys to get it right and size the images. Does it send out email every time I edit an existing post?

Haha, no... I just thought I was going crazy. It's all good.

Someone told me about this low-cost encoder from www.pololu.com . The problem I see with it, is that you have to use their geared DC motor and wheel setup. Another nice thing about servos is the built-in motor driver circuit setup.

I'm not sure if it's particularly useful for a quantity solution, but to just experiment you could disassemble an ink-jet printer (often free from disgruntled owners or cheap at thrift shops, Goodwill stores, computer recyclers, etc)   Usually the carriage has a linear encoder, and the paper drive has a rotary encoder attached to a drive roller.  The paper encoder may use linear rather than digital outputs from the sensor - that's how you get 9600 dpi resolution from an encoder with fewer lines per inch.   You could square up the edges with comparators.   Both are quadrature encoders since bidirectional motion with fast turnaround is needed.   Some printers have additional motors for a document feeder, maybe a service station (where the ink cartridges go for storage, cleaning, etc) and other functions.  These tend to need less resolution, but may have encoders built right onto the motor.  May or may not be quadrature encoders.    Most printers use DC motors, with PWM drive controlled by servo algorithms run by the processor that runs the printer.    Outputs to the motors are often H-bridges run from a higher voltage - do be careful.

SInce printers are produced by the millions, small DC motors with attached encoders ought to be cheap if you can locate the right supplier.

Given the $29 and $39 inkjet printer deals you see sometimes, I guess you could even buy a truckload, take out the motor and encoder, throw away everything else, and wind up with less in it. It would serve them right for the crazy business model of giving away printers and charging big bucks for that tiny amount of ink!

You might even wind up making money if you sold all of the ink cartridges on eBay.

The printer idea also made me also think of this: The roomba and iCreate have motors with feedback. I wonder if you could find those someplace? I may have to bust one open.

I checked at some of the typical robotics parts places and it looks like the geared DC motor with built-in encoder option is still in the $35 to $45 range and above. At least for the motors they sell there and you would still need to add an H bridge driver module. Here is what I found:

http://www.roboticsconnection.com- I have used this one in another project and thought it was nice for the money.

http://www.lynxmotion.com- have to add encoder module to back of motor

http://www.robotelectronics.com

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As long as you are just talking about digital input pins, the mbed pins are 5V tolerant.

Analog in is 3.3V full scale so it would need a circuit to drop down the voltage a bit.

See bottom of this page http://mbed.org/handbook/mbed-NXP-LPC1768

Thanks for you fast respond. There are pulses that change by different frequencies.So if the max current of the mbed is 40mA should I put a resistor higher than 42.5 ohms?

Once Again Thanks for your feedback.