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

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    \$\begingroup\$ I would not connect wires to the fridge's alarm or LCD, you cannot be sure if it is mains isolated and therefore safe to touch. I think you cannot avoid using a wired or wireless temperature sensor to do this safely. \$\endgroup\$ – Bimpelrekkie May 4 '16 at 15:04
  • \$\begingroup\$ @FakeMoustache Thank you for your comment. I don't think alarm or lcd is using high voltage to be risky. How can I put the wire inside from the door, I think it will enter hot air, it needs isolation from outdoor temperature. Wireless will increase the cost. Imagine if there are 10 fridges \$\endgroup\$ – Jim May 4 '16 at 15:42
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    \$\begingroup\$ "I don't think alarm or lcd is using high voltage " You misunderstand what I'm saying. The electronics themselves run on a low voltage for sure. The thing is that for you to be able to safely connect something to it the electronics need to be mains isolated. If you don't know what that means Google it. It is essential for safety. If you want to electrocute yourself or others fine but don't say didn't warn you. \$\endgroup\$ – Bimpelrekkie May 4 '16 at 19:12
  • \$\begingroup\$ Doing this without having a wire going inside the fridge will be difficult. I'd avoid fiddling with the fridges existing electronics as that could result in unexpected side effects (not to mention voided warranty). How about a wireless radio? a little radio sits in the fridge and sends an alarm wirelessly to the arduino? \$\endgroup\$ – Sam May 5 '16 at 8:10

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.

  • \$\begingroup\$ Yes but if you put wire from the door there will be a space and will enter a bit hot air (the door will not close 100%). Have you imagine how to put the wire with thermistor inside? \$\endgroup\$ – Jim May 4 '16 at 15:50
  • \$\begingroup\$ The leakage will be negligible compared to the amount of heat escaping through the insulation and when the door is opened. If you use small wires, the seal around the door should not have any issues allowing the door to close and seal fully. Since the sensor carries little current, you can use as small of wires as possible without the risk of them breaking. I would choose 24ga or smaller and run them under the door seal on the hinged side of the door. \$\endgroup\$ – Robert Stiffler May 4 '16 at 15:56
  • \$\begingroup\$ @Dimitris I nthe same manner as the built in thermostat and light wire comes in, by making a hole and then use kind of a glue. \$\endgroup\$ – Marko Buršič May 4 '16 at 16:29
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    \$\begingroup\$ Just run a thermistor with thin gauge wire through the seal. The amount of "leakage" is laughable. You can get incredibly thin and flexible magnet wire (insulated) that will have no effect. \$\endgroup\$ – Joel Wigton May 4 '16 at 16:50
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    \$\begingroup\$ You can also buy flat cable material that would be much more rugged than magnet wire. Flat cables such as these: digikey.com/product-search/en/cable-assemblies/… would be easy to bond to the door frame and not impinge on the door seal at all. \$\endgroup\$ – Michael Karas May 4 '16 at 18:11

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