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Freezer Alarm

Discussion in 'Electronic Basics' started by steve, May 19, 2013.

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  1. steve

    steve Guest

    Im want to build this circuit.

    http://www.discovercircuits.com/PDF-FILES/FREEZERALARM1.pdf

    I am a novice at this.

    The diagram states that the alarm will go off when the freezer gets to be 0C.

    Actually I think this is too late. Any freezer that goes kaput will hold the temp for perhaps 24 hours before it gets to 0c. By that time the food is now pretty much defrosted. And must really be used up.

    How can this circuit be changed to say go off at maybe -3 C. or there abouts.

    Thanks all.

    P.S.

    I cant seem to get any listed parts locally. I want to buy them from SAYAL electronics because they are near me and I can pick them up. But I cant seem to find any of the parts on their web site. I have tried to do comparisons but they dont seem to have any of those parts either. (and maybe IM doingsomething wrong) I'M sure they have what I need but I just dont have the skill to compare parts, if any Canuck (or other kind person could mach partsthat would be great)www.sayal.com

    I dont like buy things across the border or on line.
     
  2. Jamie

    Jamie Guest







    + 5 Volt
    |
    ++-------+
    + |
    + |
    .-. |
    | |<-+----+
    500K | |
    '-' Replace the 720K R with this
    |
    |
    .-.
    | |
    | |500K
    '-' To (-) input.
    +-----------------+
    |
    |
    |
    |
    .-.
    | |
    | | 1 MEG
    '-'
    |
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
    Jamie
     
  3. steve

    steve Guest

    Great thanks.
     
  4. That will work but you'll have to futz around to set the pot to the
    right place.
    So the original schematic might have been easier for you to understand
    if he used a 361 kohm resistor in the upper arm and the same 499 kohm
    resistor in the bottom. Then you just need to look up that
    thermistor, find it's resistance at -3 C and plug that resistance into
    the place where the 361 kohm is now... Does that make any sense?

    George H.
     
  5. amdx

    amdx Guest

    Any reason he couldn't use a 741 or a 339 or 1458?
    The LMC 7215 is a surface mount part not easy for him to get.
    The LMC7215 is a low power part, 1ua so if his product is battery
    powered the LMC7215 may be important.
    Mikek

    that may be important.
     
  6. normal freezer temp is -20C. I've not datalogged what the actual temp
    swing is in a typical freezer though. The defrost cycle would probably
    mess with that depending on where you measure the temp.
     
  7. Jamie

    Jamie Guest

    you must remember that he is operating in the 5 volt region, so the
    output of a 741/1458 is going to be in the 4+ volt range and the low
    output maybe .7 volts, it leaves a narrow window to work with but if you
    were to use a MFET as the switch with a Vgs(th) of 2.5V, I guess that
    would be fine.

    If you don't mind a little sloppy operation I guess it would be fine.


    Jamie
     
  8. True, but large restaurant type upright freezers can have defrost cycles.

    There's a gigantic Traulsen freezer at work with a really rediculous 3
    digit LED and 4 button "intelligent" controller. It's easy enough to set a
    temperature, but what the scrolling cryptic messages mean is beyond even
    the service people.

    They all say **** it and just do normal troubleshooting like on any normal
    refrigeration device.

    F- on the usability and intuitiveness of an "intela-traul" controller.
     
  9. steve

    steve Guest

    Thanks for the suggestions.
    I'm going to see if I get the parts then will probably get back to the group with questions.
    On the defrost thing. Most Deep freezes don't have defrosters I don't think.. Those that do only heat the coils behind the plastic wall compartment to remove frost and ice from the coils. This "should" not cool the food, if itdid then there would be a problem. This means that the sensor on these type of freezers should be in a small box or something in the freezer, not taped or stuck to the plastic wall of the freezer, as the wall of the freezer can get a little warm with the defrost cycle. Although there is probably places in the compartment that don't have cooling coils.
    Thanks.
    I'm sure I will be back again.
     
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