And I don't need the "Don't charge those batteries"-speech, I know the risks involved.
My arduino is monitoring the voltage on the battery as it is charging, and i would like to use the arduino to cut the power when the voltage has reached a specific value.
How would I implement a transistor switch, base-controlled by the arduino, into this circuit?
Or http://ruggedcircuits.com/html/circuit__17.html
It provide 3.3V , that exactly what you need for 2 AA, so you 'd not need disconnect them at all,
just keep under trickle charging current that drops to low level after batteries get charged.
That circuit is best controlled with a PNP transistor to do high-side switching before the regulator. However you'll need an NPN to shift logic levels up to whatever Vin is.
There is probably a much better way to control that constant current circuit directly at the sense terminal of the regulator.