A convenient rule of thumb is that a BJT needs a base current of about 1/10 the load current in order to function as a reliable switch - to saturate and conduct fully. In practice you can often get 1/20 or 1/50th, but you start with 1/10 as absolutely certain to work.
So is it stopping because the battery runs down? There's not a lot that can be done about that except to use a higher capacity cell. Starting at a higher voltage (alkaline cell, for example) might buy you a longer run time.Thanks Wayneh, that is super helpful! Using those values, I was able to get it running and now I am tweaking it with potentiometers. It consistently stops running after a few hours. It is a super low speed, low torque motor (~30 RPM) which is what I want since it is just a conversation piece. I am not sure if it stops due to an electrical issue or a mechanical issue since it has very little "reserve" power, but now that it runs I will keep tweaking the resistance to see if I can find the ultimate sweet spot.
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by Jake Hertz