A want to make a system with arduino to measure the temperature of many fridges and send SMS if temperature is over limits. I am wondering how to measure digital temperature without putting wire inside the fridge. One easy solution is if fridge has alarm for high temp (I can connect a wire on alarm and when it is on it will send SMS). What about if 1) fridge has not alarm, but has LCD temperature 2) fridge has not alarm and LCD
Probably the easiest way to do this is using a thermistor to measure the temperature directly. Since you don't want to put a wire inside the fridge, you'll have to find some way to interface the arduino with the existing fridge electronics. I can't tell you how exactly to do this, because every fridge is going to be different. The easiest way is if the fridge already has a built-in warning light for high temperature. Connect a digital input pin to detect when the light is on. If the circuitry is hidden, you could also use a photoresistor to detect when the light comes on. Interfacing directly with a microcontroller or LCD will be very difficult. At that point you are better off just putting a wire inside the fridge.