Pictured below is a frost detection warning light system we have been putting together to be used in vineyards (though it could of course be used for any other temperature sensitive crops).
This device takes a 12VDC power input, and connects to a waterproof DS18B20 digital temperature sensor. On the output side are three 12V outputs which will be connected to three 2.4W LED lighting units (one red, one yellow, and one green) and mounted on towers attached to the vineyard posts so that they can be seen from up to 100 metres away. (In the photo above we are just using three individual LED bulbs of the relevant colours during testing and calibration.)
When the temperature is measured to be >+1 degree Celcius, the green light is on. When the temperature is from 0 to +1, the yellow light is on. When the temperature is between -1 and 0 the red light is on. When the temperature falls below -1 degree Celcius, the red light flashes.
Above you can see testing in progress. The bowl on the left contain water, salt, and ice (measured to be -5 degrees Celcius after half an hour in the freezer) and the bowl on the right just ice and water (measured to be just above 0 degrees Celcius). The red LED is in the middle of flashing to indicate that the measured temperature is below -1 degree Celcius.
We have made multiple identical units for this particular vineyard which will be distributed around the area making it possible for the vintner to know immediately where any pockets of frost are located.
If you need any kind of thermostat, low/high temperature warning system, or temperature datalogger, email email@example.com with details of your exact requirements.