I thought I'd do this as a tip after Geir Andersen posted on the circuit redesign for his UAV. The need was to maintain a supply long enough on shutdown to flush data through to an sd card and to prevent loss or corruption. He started out on the right track with a big ass capacitor which would supply the circuit for a time on shutdown. Long enough hopefully to prevent data loss.
The circuit appeared to work in theory on a breadboard however when moved to the UAV with all its power consumption the capacitor depleted too quickly. Here is where a little backward thinking comes handy. We all know that with a little current a transistor can drive a much larger current. It is also a switch. So by using a transistor in this situation we can extend the usefullness of our capacitor.
I don't know how much current the UAV requires in operation but say it was 500 mA. Any capacitor is going to drain real fast supplying that kind of current. But if you connect your transistor say with a gain of 100 to the capacitor then you will only draw 5ma till it's discharged. A big difference and it will most likely buy you enough time to write the remaining data to the sd card.
This may come useful in other applications. Say you don't like your robot stopping instantly you could employ this method to make it slow to a halt instead of instantly.Just experiment with capacitor size until your happy. I can't think of any other examples but I hope there is something to learn here.
I have shot 3 videos illustrating the concept. The first video shows our load current and how it remains connected for a time after shutdown. The second one indicates how little base-emitter current is required to hold the transistor on. And the last one shows how fast the capacitor drains when it is supplying all the current for the load.
The circuit diagram I drew by hand. Don't laugh but it is faster for me this way.