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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:44:04 PM
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Freezing Temperature Alarm
designed by David Johnson, P.E. |
| 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. |
| 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. |
| 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 Schematic below to view PDF
version of this Circuit |
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