My Freezer Alarm or Freezing Alarm.

CREATED: "Oct 20, 2015"
UPDATED: "Dec 10, 2015"

"A Very Simple and Inexpensive" Circuit to Detect Warming in a Freezer or Fridge.

"Or a Very Simple and Inexpensive" Circuit to Detect getting "TOO COLD" Outside.

Twice Now I have Lost food because of a Upright Freezer Door Not being Closed Propery. I Fixed that problem with a Velcro Strip.

But Last week my Freezer Quit working for probably 3 day or so, and I Lost ALL THE FOOD in it.

SO TIME TO FIX THIS Possible PROBLEM on my Deep type Freezer.

I Was Not Impressed with Circuits I found on the Internet, So I Designed This Circuit.

VERY SIMPLE and EFFECTIVE. "OR with a Slight Change of Parts" A Circuit to Detect Freezing Outside.

I Recommend using the Piezo shown on the Schematic. It Works Very Good.

I would also recommend using a 9 or 12 Volt Adapter, Rather than a battery.

The Thermistor and it's Wire Connections should be Sealed in Epoxy, to prevent Moisture from affecting it Resistance.

You May want to Epoxy the Thermistor to a Small Aluminum Block, to give it MORE MASS. Or put it in a Tube Like I Did. This will help prevent Sudden Temperature Changes on the Thermistor when opening the door.

Do NOT put the Circuit Board, Piezo or Battery in the Freezer.


1) Leave the Battery or Power Adapter Disconnected.

2) Put the Thermistor Down about Half-Way in the Freezer or to the Back part of the Fridge. This is so it is Not Affected too much When opening the Door or Lid. Leave it there for about 5 Minutes to Stabilze in Temperature.

3) Now Connect up the Battery and adjust the Temperature Pot so the Sound just shuts off.

4) Open the door or Lid for a minute to make sure it is set OK and the Alarm does NOT Sound in Normal use. Or it may require a bit different setting to allow for Normal use of "Putting In things" or "Getting things out".


Not All Thermistors are the Same. I am using a 10K NTC Thermistor, Based on 25 Degrees Celsus, Philips Part Number "640-63103". This is an OLDER Part Number, but it is still Available.

Or Just get a Thermistor with a Value of 10K or Higher (A 25K or 50K one might be better as it will Consume LESS CURRENT.)

But Even with the Same Value, Other Thermistors can have Different Resistance Values at Other Temperatures.

With My Freezer at about -22 Degrees Celsus, (Compressor Shuts Off) My Thermistor Measures about 127K Ohms.

With My Freezer at about -16 Degrees Celsus, (Compressor Turns On Again) My Thermistor Measures about 95K Ohms.

So I will need to set my Alarm to Trigger at about -12 to -15 Degrees to Allow for Possible Temperature Variations, Especially when Cleaning it or Keeping the Freezer Open while Re-Aranging Stuff.

"Back to My Projects Page."

 The Schematics for Both these Circuits.

 The PCB for the Freezer Alarm.

 The PCB for the Freezing Alarm.

 The PCB Overlay of the Freezer Alarm.        The FREEZING Alarm is Simular, So I didn't do up a seperate Overlay.

 The Entire Circuit Overlay.
       This Thermistor is NOT Protected from Moisture Yet.

 My Finished Thermistor Probe.
       This Thermistor is NOW Protected and Holds the Cold Fairly Good.

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