Temperature Switchfor fridge monitoring

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RodneyB

Joined Apr 28, 2012
697
I have a small fridge that I need to know when the temperature gets to 7 degrees. I will then connect it to a GSM module to send me a message. I was thinking of using the LM35 but got very confused at all the circuit on the internet.

Really looking for advice and direction
 

cmartinez

Joined Jan 17, 2007
7,024
I have a small fridge that I need to know when the temperature gets to 7 degrees. I will then connect it to a GSM module to send me a message. I was thinking of using the LM35 but got very confused at all the circuit on the internet.

Really looking for advice and direction
The challenge to your project is to keep it as simple as possible. You may start by using a digital thermometer connected to a small MCU, such as the DS1822. Frankly, I can't think of anything simpler (nor cheaper), since you'd need the MCU to interface it to the GSM module anyway.
 

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wayneh

Joined Sep 9, 2010
16,398
I don't disagree, but the LM35 is very easy to use also. The slight challenge here is that the low temperature will produce a low output voltage, and thus you need circuitry that senses close to the negative rail, ie. a rail-to-rail op-amp or a comparator. Once you use a comparator to switch, like a thermostat, at the trigger temperature, the output is digital, 0 or 1, on or off.
 

#12

Joined Nov 30, 2010
18,210
Here's an idea. Refrigerators are not freezers, so the spec must be in Centigrade. Use a Fahrenheit sensor to detect 44.6 F. That will get you a higher voltage for the set point.
 

cmartinez

Joined Jan 17, 2007
7,024
I don't disagree, but the LM35 is very easy to use also. The slight challenge here is that the low temperature will produce a low output voltage, and thus you need circuitry that senses close to the negative rail, ie. a rail-to-rail op-amp or a comparator. Once you use a comparator to switch, like a thermostat, at the trigger temperature, the output is digital, 0 or 1, on or off.
Yes, I hadn't considered an analog thermometer for this application, since I feel that the OP will probably end up having to use an MCU anyway... but maybe not. Your analog comparator idea is simpler, and does not involve programming. So now what's left to know is what kind of GSM module the OP is planning to use, and how it's supposed to be interfaced.
 

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cmartinez

Joined Jan 17, 2007
7,024
Here's an idea. Refrigerators are not freezers, so the spec must be in Centigrade. Use a Fahrenheit sensor to detect 44.6 F. That will get you a higher voltage for the set point.
The only sensor calibrated for Fahrenheit output I could find is the LM34. It's able to read 5°F to 300°F with a single supply. So yes, the set point would be higher than if he were to use an LM35, making the use of a comparator simpler.

EDIT: The thing is more than $6 dlls a pop and requires a minimum quantity of 1,000 at Digikey!
 

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wayneh

Joined Sep 9, 2010
16,398
Yeah, I wanted to use the LM34 for a project because of the higher sensitivity, but gave that up when I found how much rarer the piece is.

But the LM35 would be fine at 7°C. That's 70mV output and the LM339 would have no problem there. 7°F is a different story. I assumed we were in °C, as #12 noted also.
 

cmartinez

Joined Jan 17, 2007
7,024
I would us a TMP36 sensor, gives out 10mV /deg C, it works from -50 to +125C, gives out 0mV at -50C, 500mV at zero C, and therefore would give out 570mV at 7deg C, works on a 3V to 5V supply.
Well I'll be... this is the first time I've heard of that sensor, it's pretty affordable in Digikey. And it's got a very complete datasheet with example circuits and all.
 

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cmartinez

Joined Jan 17, 2007
7,024
DS1821 is a stand-alone 1 wire programmable thermostat. Set it and forget it.
Yes, it's a viable choice, but it requires programming to set it, and I'm not sure if the OP has the skills to do that.
Also, I couldn't find any available in Newark, Mouser nor Digikey.
 
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