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Freezing Temperature Alarm2
November 5, 2014

 

The circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).  A 9v battery powers the circuit.  The average 9ua current is so low that the battery should last for many years.
An inexpensive glass diode is used as the temperature sensor.  A small gage unshielded cable can be used to link the temperature sensor to the circuit. The cable length can be up to 20 feet for a small gage wire and longer if you use a heavy gage wire.  The diode and its leads should be covered in some epoxy to form an insulator, with will keep water from coming in contact with the part. The circuit can be housed in a small plastic box.  Some boxes have nice snap out compartments for a 9v battery.

A very low power +5v regulator is used to provide a solid +5v for most of the circuit.  A single low power voltage comparator (LM7215) from National Semiconductor is used to compare the voltages across a bridge network, consisting of the 1N4148 diode and a 1M resistor on one side and a 200K pot, 1.1M resistor and one 82K resistor on the other side.  When the temperature is below freezing, the voltage at the non-inverting (+) input exceeds the inverting input, causing the output of the comparator to swing from zero volts to +5v.  The 5v signal at the gate of the FET then turns on the transistor, which turns on the beeper.

Click on Drawing Below to view PDF version of Schematic


More  Alarm Circuits   Freezer / Frost Alarms
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Temperature Control Circuits      Themocoupler    Thermistor

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