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Optoisolators and Dual Supply Circuit Logic
To protect the mBed from the necessary external power supplies, we used an optoisolator (MCT6) chip to bridge them. An optoisolator is basically a relay that uses photodiodes instead of mechanical contacts to switch back and forth.
We needed one because the 3.3V regulated Vout was not enough to control a servo motor, which requires 4-6V. In addition, the 5.0V USB out cannot function at the same time as the mBed. Either you are controlling your electronics with the mBed or powering them with 5.0V. This is not ideal.
We found this MCT6 datasheet to be helpful. The MCT6 chip contains 2 optoisolators. Here is how to connect the first one: 1. Connect your output wire from the mBed to pin 1. 2. Connect pin 2 to mBed ground with a small valued (1k) resistor. 3. Connect pin 7 to external DC ground with a large resistor (10k). 4. Connect pin 8 to external DC Vcc and to the input signal wire of whatever you need to control.
Figure 1: The MCT6 optoisolators used in a circuit.