Electronic Circuits and electronic circuits, electronic schematics plus an extensive resource for hobbyists, inventors and engineers

DiscoverCircuits.com, has 45,000+ electronic circuits, cross-referenced into 500+ categories.
We have searched the web to help you find quick solutions & design ideas.

Got Designs?
Please eMail
if you want me to link to and/or post your original design
NOTE:  We make every effort to link to original material posted by the designer. 
Please contact us if our link is not to your site!  Thanks.

DiscoverCircuits.com -- Hobby Corner
Last Updated on: Tuesday, June 01, 2021 03:06 PM

Hobby Circuits List

The contents & graphics of Discovercircuits.com are copyright protected.
LINK to Dave's circuit, but DO NOT COPY any files to your WEB SITE server

More  Alarm Circuits   Freezer / Frost Alarms

Freezing Temperature Alarm
designed by David Johnson, P.E. Dec 13, 2004

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.

Click on Circuit Below to view PDF of Schematic

Freezing Temp Alarm, David Johnson, Dec 13, 2004

More  Alarm Circuits   Freezer / Frost Alarms

Hobby Circuits List
eMail David A. Johnson, P.E. about this circuit

HOME Schematics Index Hobby Corner Dave's Circuits Contact Info
Imagineering Ezine Dave Johnson & Associates Faraday Touch Switches

 About Us   |  Advertise on DiscoverCircuits.com   |   Report Broken Links  |    Link to DiscoverCircuits.com  |    Privacy Policy

Copyright  January, 1998 - June, 2021     David A. Johnson  All Rights reserved. 
 Linking is ALLOWED but COPYING any content or graphics to your web site is EXPRESSLY PROHIBITED.