Well since we don't know the current rating of the motor it is hard to be specific. In the motor at 6V mode the drop across the transistor will be about 6 volts. If the motor draws 300 ma you will be dissapating 1.8 Watts in the transistor...thus you want to find something in a TO-220 type package (or larger) that you can attach a heatsink to. If you used a SOT-223 type package you would need to make the copper of the curcuit board about 1 inch² or more for a heat sink.

Do not confuse this transistor and its needed ratings with those of the on/off switch on the existing logic board. There the transistor operates in a fully saturated mode and most likely operates with a drop of less than 0.5 volts. Thus a small TO-92 or SOT-23 transistor may be an adequate solution for the switch....but not on your dongle as the voltage dropper !!

Personally I would look for a transistor with a h_FE of 300 or more. If you cannot find a suitable device then you may want to look at a device packaged as a darlington. Then gains up to 3000-5000 can be found. Note that if you use a darlington in Kai's circuit then the 1N4148 V_be compensation diode would want to be replaced with a pair of 1N4148's in series due to the double V_be drop of a darlingtion.

The major reason that a high gain transistor is needed in Kai's circuit is that current variation in the motor draw translates to variation in the base current of the transistor. If the current through the voltage divider resistors on the base of the transistor is of similar magnitude as the base current variation then the divider will no longer work as a 2::1 divider. Ideally the current through this divider should be at least 10 or 20 times higher than the base current that the transistor will require. If you had a transistor with a current gain of say 30 and a motor draw of 300 ma then the base current would be on the order of 10 ma. In this case then you would be looking at having to change the divider resistors to values of 50 ohms.

If you have access to DigiKey why don't you look at the ZETEX device listed as DigiKey part number FZT493CT-ND at http://www.digikey.com.

Michael Karas