BillO:
... I would recommend a Darlington arrangement which is basically 2 transistors, althogh you can buy power Dalingtons in a single device.See the attached schematic. It will handle a motor that draws up to 10A and is pretty cheap.
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The flyback diode is on the wrong place in that schematic, it doesn't protect the transistor when the motor switches off. For that, you need a flyback diode connected in parallel with the motor.
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IMO darlingtons should almost never be used, because they have high saturation voltage. If the motor draws a substantial current, this results in a lot of power being dissipated in the darlington, so it will get hot and need a heatsink. A power mosfet is a much better solution in such a case. If the current is not so high (say 500mA or less), then a single transistor will do the job more efficiently and with less heat generation than a darlington, especially if the transistor is a high gain low saturation type such as the ZTX851.