Garden window freezes, need temp sensor to trigger heat lamp
I have a protruding garden window that unfortunately freezes very easily due its projection outside of the house.
I currently have a 150-watt Infrared Heat Lamp (actually a heat source for small reptiles) that works well here.
However, I want a timer that senses FREEZING TEMPERATURES (not a daily on/ off timer).
Is there such a device? Can somebody send me a link to a good source for this?
Thank you all in advice for your expertise.