This is what it says on the Demeter site..
Power supply: 9 volts DC. Either external or internal. For internal power, use a 9 volt battery. To access, remove the four screws on the sides of the unit and pull apart. Note: battery must have at least 7 volts for unit to work! External power supply using mini plug, tip positive, 9 volt DC regulated (100 milliamp or more).
I guess I need a positive tip one then. Thanks.