I was thinking lately of taking a few battery-operated devices and converting them such that they could run off other energy sources (like outlets). But, I was wondering what concepts should be taken into consideration? For example, I have a fan that runs off of two D 1.5V batteries; this is all I know. I do not know the current draw from the batteries (and consequentially the power limitations of the fan's internal circuitry). I know the limitations of the batteries, in terms of mA-h, but that is all and gives you little information. I was thinking perhaps attempting to come up with a Thevenin Eq. circuit and going from there, but with the electric motor to turn the fan I am not sure where to start. --I was thinking 5v from an arduino, and then a transistor for current drive. Although, I am not sure how much current. Or perhaps I want to run a small DC motor that I have. The only information on the datasheet is that it's 1.5-3.0V motor with 300mA draw at no load. What about when there is a load? Power dissipation limitations? Do you just arbitrarily design a circuit, perhaps a transistor, with an output of ~500mA at 2V? Similarly, I was looking at converting a laser-pointer. The device uses three 1.5V batteries and the specifications are ~<5mW. This gives me a starting point much better than the other two projects mentioned above, but still leaves some uncertainty. Is this circuit as simple as using the power limitations of 5mW, and voltage of 4.5V, designing a circuit that will supply ~1.11mA? I have never seen an LED that draws such low current. I am fully able to work through problems with the constraints and requirements given, but how to do you figure out such constraints to begin with?