That unit senses the ambient air temp with that little antenna like thing you see sticking up on the top left. You might be able to use it if you mount it inside the freezer, but then you still need to get power to and from it. I don't think there's any way to lengthen the sensor probe which would be the ideal solution. It might not be safe to mount this unit inside the freezer anyway due to the often wet or very damp conditions. All said and done, it would probably be better to go with the standard Johnson analog controller as most do. I prefer the digital models, but they cost a little more. I've also been looking for a cheap way to make a controller. I need a lot of them!