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Circuits designed by David Johnson,
P.E.
Last Updated on:
Monday, December 25, 2017 02:08 PM
Master Category List - Dave's Circuits
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| 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. |
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Click on Drawing Below to view PDF version of Schematic |
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