In Reply to: ac/dc adapter and battery source posted by Todd on 08/22/01 at 4:51 PM:
You could use a 2 fets, one to allow the current from the battery through to the circuit when the jnct is biased on (with a high value resistor from your battery supply to the gate pin - 47k) and another fet from the gate of the first connected to the drain of the second and ground to the source. The gate of this fet would be connected to your psu, again with a high value resistor so that when you have a voltage on your psu it turns the fet on conducting the current on the gate of the first fet to ground and stops the battery current flowing through it.
You would have to add a diode in somewhere to stop the voltage from the output of the first fet turning the second fet on.
You would have to work out what sort of transistors you would need.