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This is a simple alarm that I made so that it would go off when the temperature of a deep freezer or other similair appliance got above a certain temperature in degrees. I thought that this would help by giving you a little bit of indication of when it was time to move your food before it went all bad.

Step 1: Get Some Parts Together

Probably the most difficult and important step of any project is the selection of all the little bits. If you wanted to make this alarm you would need:

A Temperature Sensor -- I used one called TC622EAT from Microchip for this because it seemed ready made for this application. It simply needs an external resistor to set its trip temperature and it comes in a transistor like package perfect for putting at the end of some longer cord and run inside your freezer. I think most temperature sensors would work however, just as long as they operate in the right temperature range, and if they do not have the trip set feature, that just means that you have to get a little more creative ie add a relay or something like that.

A Resistor -- Just to set the temperature of the TC622. There is a little formula to figure out the resistance needed for the desired trip temperature in the datasheets on the sensor on the Microchip website.

An Op-Amp -- This temperature sensor can't output the current needed to switch on the alarm that I had for this, so an Op-Amp was used as a buffer so that the alarm would go off.

An Alarm -- Here I cheated, and used a window alarm that I bought from Canadian Tire for 1.50, after stripping some of the unnecessary parts from it.

Misc Cable and some batteries -- for the temperature sensor lead, I will use a piece of IDE cable, because its flat so it can go between the seal for the freezer better, and I have a lot of them lying around unused. the temperature sensor that I have needs about 4.5 volts minimum, so I will use 3 AA batteries.

Step 2: Put It All Together

Probably the easiest part. This part really depends on what you used for components, so I will won't go into a lot of details, just provide the schematic that I used.

The end of the sensor cable was done up with heat shrink tubing to make it really neat as it would be in the freezer. I got lazy with the op amp and the connections are just soldered right onto the pins.

The batteries are just taped together with some wire makeing them seriesed until I can find a battery clip or something like that for them.

Step 3: All Finished

Now that the alarm is complete, all that remains to do is to test it if you so desire, and to mount it in the fridge or freezer of your choice. I don't recommend putting it in a fridge because the temperature goes up so much when you open the door, that you might have it going off at you all the time.

Also, you don't really want to put the whole thing in a freezer because I believe that the cold will kill the batteries faster than normal.

Although the alarm isn't intensely loud, it should be loud enough that if you picked your temperatures right, you should have enough time when you hear it to do something about it before all your food goes bad. Maybe somebody unplugged it by accident or something easy like that.

I have done a few tests with the setup in the second photo. I haven't pulled the plug on the freezer, only moved the sensor in and out, and it worked as I hoped that it would.

This little project is easy and cheap, and I believe that it has the potential to save hundreds of dollars in spoiled food.
Is a LM368 okay for this circuit?
hey EJ, which op-amp did u use...? was cusrious cus of the 4.5v power supply, LM741 does work only with +12v to -12v
Just off the top of my head, I think that the op amp I used was mcp6004, from the microchip company, which has a supply voltage of between 1.8 and5.5 volts
Thanks - I ended up with a Darlington Transistor between the sensor and the relay and it is all working fine. I am using this to limit the run time of a standby generator - will only come on during a power outage if the freezer temperature is above 23 F. This is to conserve fuel - otherwise it would run 24/7 and run out of fuel if I were out of town. Thanks again - appreciate your sharing.
this is a FANTASTIC idea for my work! i think i'll make one and show it to the owner. we had our fridges go out once and they got up to 62F before anyone noticed they were broke! hundereds of dollars of food wasted! this is BRILLIANT!
Thanks, its nice to know that you have an idea thats useful. Drop me a line if I can be any help with your design. PS: might be interesting to note that you can get samples of the temperature sensors from the manufacturer
Hi, there - I am an absolute novice and want to play with your circuit - I have a 741 8 pin op amp and am not sure from your schematic, which pins are connected. Will Microchip furnish samples to just an individual? Thanks for the idea - if I get it working the way I want, will send you the details of how I am using it
if the info I found on 741 opamps is right then: on a 741 op amp pin 7 would go to the + side pin 4 would go to the - side pin 3 would be the output of the temp sensor pin2 would be hooked to pin 6 and that all would go to the sensor you should theoretically be able to check the manufacturers datasheet for the IC and pull the pin numbers from that. Hope this helps
<sup>sorry, in the last pin-out line, I mean alarm, not sensor</sup><br/>
Thanks - ordered the sensor today - will let you know how I do with it.
ok - got the sensor - wired it with 91K resistance and powered it with 12 vdc. Breadboarded it and all worked fine-reality check - was using the high out to operate a 12v relay. However coil resistance was 160 ohm which if I figured correctly, caused a draw of 75 ma. But the sensor is rated for a max of 600 microamps so damaged it. (High out is now only 4v but when it warms up, low out is 12v) I need a 12 v relay whose contacts can handle 10 amps is the problem. Any suggestions? Thanks
did you not buffer the output of the sensor?
No I did not have any buffer. Went directly from the sensor output to one side of the relay coil. Being dumb about this stuff, I really don't know how to buffer it. Seems like I need a current amplifier between the sensor output and the relay coil.Thanks
In my circuit diagram, the op amp is what is doing the buffering the way it is hooked up there makes the output of it to be the same as the input, but it can supply more current than the sensor
kudos man

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