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This is great in theory, however if you turn the room light on, or want to control the photo-period different from the external light, you have a problem. This can be got over by cutting a large hole in the vivarium, just a little bigger than the thermostat, and fixing a perspex cover over it. You then make a little cradle up and fix it to the vivarium so that the thermostat is cradled and faces into the vivarium through the perspex, then the 'magic eye' sensors the internal light in the viv. Remember the thermostat only needs to be loosely supported as you may need to remove the thermostat and adjust the temperature.

Having the thermostat facing you the sensitivity control is on the bottom of the thermostat, turning the unit fully clockwise is setting the unit to it's less sensitive position, this means that you need less of a difference in the light intensity to operate the change over.

The magic eye part of the thermostat must be in-line with the light. By having the sensor above or below the centre line of the light may throw a shadow onto the sensor, and problems will occur because not enough light will reach the sensor.