In another project, it was shown how a package of silicon diodes could be developed into a solid state thermostat. Here is an analog version, which can be interfaced with a voltage-to-frequency converter for use with a frequency counter, or can be directly read by a 10 to 20 thousand-ohms-per volt multimeter. The circuit utilizes a pair of 4009 inverter sections, biased into the linear region to amplify the temperature effects upon the diode probe. In this application, the adjustment potentiometer, Rl, is set to give a mid-scale reading at room temperature on a typical multimeter set on the 6-Volt DC scale. If a separate 0-1 DC miliampere meter is available, it could be calibrated directly in degrees F or C, with a suitable resistance in series with the amplifier output.
Copyright by Bill Bytheway, K7TTY February 2012