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stop a capacitor charging when full

i a ma a complete beginner and i am reading a book about beam bots (mostly) and i had an idea for a beam bot which probably wont work and i might not build but i decided to try and draw a schematic anyway. the idea is it goes towards light and when there is lot of light it starts to charge a very large capacitor before going back to following light. when it detects vibration(can you get vibration sensors?) it uses the power from the large capacitor  to run until there is a lot less light ( ie it is under something) and stay until the vibration has gone. so like real small bugs it gets food until a predator comes near it then hides 

my problem: charging the large capacitor and knowing when it is full then after the fleeing process knowing that it needs to be charge a gain could anyone please help?

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thanks guys theres a lot for me to think about here .didn't expect so many answers :)

I think you might be expecting too much from a capacitor, and instead you should use rechargable batteries.

How about charging the capacitor through a resistor and monitoring the capacitor voltage with an adc (or comparator)

I was going to suggest checking out the BEAM baskbot circuit but I can't find it anymore. Found the forum posts but not the schematic. I will dig that up when I get home. It does essentially what you're looking to do (stop in light, fill a cap, and then go about its business). It might give you some ideas.

I don't have the electronic format either. Somewhere... somewhere... is a physical hardcopy. I will post it when I find it, sorry m8.

Look into a circuit called a http://en.wikipedia.org/wiki/Schmitt_trigger . You should be able to do a search for this term on google and find out how to design the circuit. They should be fairly easy to make with common available parts. Look at comparators as well.

Well you can use a voltage trigger (Solarbotics sells them) and wire it between your solar panel and a pull-down resistor. So lets say when your soar panel outputs 4V it triggers the voltage trigger and it lets the current flow into a capacitator that will be hooked up after the voltage trigger. Then you put another voltage trigger at the negative end of the capacitator that will trigger at 4V (or whatever voltage your solar panel outputs at a reasonable amount of light) so then you can interpret that voltage to stop the charging process. As for vibration sensors... i have no clue but this might be good: http://www.robotshop.com/ca/phidgets-vibration-sensor-2.html.