Although there has been several attempts at onboard speech recognition on the Arduino platform, the most successful systems need an external speech processor due to the computing effort required. One example of this has been demonstrated by Chanud Yasanayake who uses a PC running Bitvoicer speech recognition software, which can take care of the voice recognition then send commands via USB to an Arduino or compatible board.
This can then be used to control external circuitry for the final purpose - in this case a handheld gaming console. The circuitry uses relays to electrically isolate the Arduino from the console's control buttons, however for lower-current circuitry perhaps optocouplers could be used instead. Nevertheless the following video offers a demonstration of the system at work:
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