This is a custom designed and made tester especially for working with a dollhouse or any 12 volt wiring job. It is both a continuity tester and a 12 volt tester. But even more, it can distinguish between AC and DC. If a DC circuit is revealed, the red tester lead will inticate which side of the circuit is positive. The continuity tester simply buzzes when a circuit is complete. The best part is: If one should accidently touch a live 12 volt line with the continuity switched on, the unit will not be damaged or blow any fuses. The unit has a self-contained 2450 battery that should last for several years without being replaced. This tester is very easy to use and extremely helpful for testing the wiring in your project. Previously, the only way to get one was to purchase a Master tool kit or by taking one of my classes on wiring a dollhouse. I now have them available for everyone to purchase as I now make them totally in-house.