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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, June 01, 2021 03:06 PM
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Alarm Circuits
Freezer / Frost Alarms |
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Freezing Temperature Alarm
designed
by David Johnson, P.E. Dec 13, 2004
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This electronic hobby 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 hobby circuit. The average 9ua current
is so low that the battery should last for many years. |
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An accurate glass bead thermistor is used as the
temperature sensor. Other sensors may have been possible, but by using a
thermistor, the overall power consumption can be kept low. A small gage unshielded
cable can be used to link the temperature sensor to the hobby 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 electronic circuit can be housed in a small plastic box. Some boxes have nice
snap out compartments for a 9v battery. |
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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 thermistor and a 499K resistor on one
side and a 720K and a 1M 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.
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Click on Circuit Below to view PDF of Schematic |
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Alarm Circuits
Freezer / Frost Alarms
Hobby Circuits List
eMail David A.
Johnson, P.E. about this circuit |
