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Make a Freezer Alarm Using a Power Op Amp with Zero Power Consumption

Editor’s note: This guest blog is courtesy of Texas Instruments from authors on TIs Precision Hub

Warning : This construction article uses dihydrogen monoxide. Make sure you have researched and understood the dangers associated with this substance before proceeding.

When your home’s deep freezer, full of food, experiences an AC power failure while you are out for an extended time, its contents can thaw out. If the AC power is then restored before you return home, the contents can re-freeze and you may never know that your food is spoiled. This has given rise over the years to a number of freezer-alarm circuits and methods to detect thaw and re-freeze.

In this post, I’ll show you how to build a freezer alarm using a power operational amplifier (op amp). As an added benefit, it has zero power consumption – so you’ll have no worries about losing power or dead batteries. Figure 1 depicts the complete schematic of the freezer alarm.

Figure 1

Complete schematic of the freezer alarm; the OPA2541 actually has two of these circuits

Complete schematic of the freezer alarm; the OPA2541 actually has two of these circuits

TI has an excellent line of linear power op amps capable of delivering currents up to 10A, or that work on voltages up to 100V, with a wide range of features. For this application, what we need in particular is the metal case TO-3 hermetic OPA2541.

The first crucial step is selecting the housing. For this example, a cleaned, empty can of pet food made an ideal housing.

1. Fill the housing approximately two-thirds full with di-hydrogen monoxide (Figure 2).

Figure 2

To construct the circuit board, fill the housing approximately two-thirds full of dihydrogen monoxide

To construct the circuit board, fill the housing approximately two-thirds full of dihydrogen monoxide

2. Place the housing in the freezer until the dihydrogen monoxide has fully solidified.

3. Set the power op amp on top of the solidified dihydrogen monoxide (Figure 3).

Figure 3

Simply set the power op amp on top of the hardened dihydrogen monoxide to make the freezer alarm

Simply set the power op amp on top of the hardened dihydrogen monoxide to make the freezer alarm

4. Place the alarm in the freezer.

Set the alarm in a convenient place that’s visible every time you open the freezer. As long as you can see the power op amp, the food is safe to eat (Figure 4). If the power op amp is not on top and has become frozen into the dihydrogen monoxide (Figure 5), you have had a power loss and re-freeze and should discard the food in your freezer.

Figure 4

Typical placement of freezer alarm in freezer, and what you will see as long as everything is OK

Typical placement of freezer alarm in freezer, and what you will see as long as everything is OK

Figure 5

If you see this, your freezer has lost and regained power, thawing and then re-freezing its contents; you should discard the food inside. Even when you see that amplifier partly submerged, it would be a good practice to consider discarding the food.
Power can be lost for a host of reasons ranging from technical problems to natural disasters. Here we have shown the 'power' of TI op amps. When you are truly looking for a high power linear solution, check out the line of TI power op amps.

If you see this, your freezer has lost and regained power, thawing and then re-freezing its contents; you should discard the food inside. Even when you see that amplifier partly submerged, it would be a good practice to consider discarding the food. Power can be lost for a host of reasons ranging from technical problems to natural disasters. Here we have shown the “power” of TI op amps. When you are truly looking for a high power linear solution, check out the line of TI power op amps.

Does this solve your freezer problem? Post your feedback in the comments below.

Happy April Fools’ Day!

For more from Jerry Steele and his colleagues at TI:

See op amps in action in reference designs in the TI Designs library.

This post originally appeared on the Precision Hub here

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6 comments on “Make a Freezer Alarm Using a Power Op Amp with Zero Power Consumption

  1. Jean-Luc.Suchail
    April 1, 2015

    Is the type of OP amp absolutely mandatory ? Could the design be base on 2N3055, basically much cheaper ?

  2. Jerry Steele
    April 1, 2015

    The 2N3055 would work, but I think it will impair the excellent precision and linearity of the

    power op amp approach. This would be most evident in the partial defrost/refreeze condition and result in some ambiguity regarding whether the food is truly edible or not. And as we all know, if there is any doubt whatsover, do not eat it.

  3. Jean-Luc.Suchail
    April 1, 2015

    I have already found a chinese counterfact model based on a 6AL5 electronic vacuum tube,… but strangely it does not work !

  4. Tucson_Mike
    April 1, 2015

    Brilliant to apply this dual op amp (as opposed to a single channel) -> that provides reduncy for this critial health application. Since you can't put a price on avoiding food poisoning, selecting a device that will reliably break the relative high surface tension of dihydrogen monoxide in its liquid state is also a nice design touch. Micropackaged options would probably fall short (or not at all actually in the application). 

  5. RickyRed
    May 1, 2015

    After constructing the Freezer alarm circuit presented here, I considered making a sensitivity improvement.  I have two suggestions to make:

       1) The sensitivity will be improved by using one of the smaller sized cat food cans.  In this case, smaller is better.  The reduced thermal mass will allow the required dihydrogen monoxide to make an earlier phase transition, allowing increased sensitivity of the circuit.

       2a) Additionally, although significantly more cost, a heat sink could be conspicuously applied to the bottom surface of the can.  In this case, the increased surface area likely allows the heat, caused by freezer failure, to affect the phase transition of dihydrogen monoxide, triggering the alarm.

       2b) Although I did not prototype this part of the circuit, I believe that leaving the lid of the can partially attached will act as a heat sink.  If effective, this would increase sensitivity while having zero impact on the cost per unit.  (Care must be taken when handling the sharp edges of either cat food can or attached lid to prevent removal of biological material from the appendages of the experimenter.)

  6. Jerry Steele
    May 1, 2015

    The comments about increased sensitivity and the advantage of the dual

    op amp have not been lost on this designer. Furthermore, I am looking

    to further miniaturize the app and interface it to the Internet of Things.

    I think it needs enhancements so that we can use terms like gigabytes

    and megapixels or some other exciting modern jargon to have a marketable

    product that people will buy for no other reason than to join the club of

    those who have one.

    However these sort of design efforts take a long time despite my use

    of advanced tools such as Etch-a-Sketch.

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