Electronic Thermometers

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Introduction
Sensing Temperature Electonically
Ready-Made Sensors
Homebrewed Thermometers
Display Options
Building a Probe
Diode Demo Circuit
Direct Reading Thermometer
Next Step -- Refrigerator Control
Some Notes on Using Digital Panel Meters
Converting a 9V DPM to a 0-2V Ground-Referenced DPM
Theory of Operation of Thermometer Circuit
Circuit Modifications for Other Temperature Ranges (Added 8/9/99)


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Note: Throughout this document I make reference to Radio Shack and other companies who supply suitable components for these projects. Referencing them here does not necessarily constitute an endorsement of them or their products. Since Radio Shack has numerous local retail outlets, they are often a convenient source of electronic parts and can eliminate the need for mail-ordering. This is the main reason for referring to them here.} 
Temperature monitoring and control is a fundamental necessity of brewing. All-grain brewers must monitor not only mash temperature but that of the strike and sparge water as well. Extract brewers need to know when to remove specialty grains from the brew kettle to avoid leaching unpleasant flavors from the grains at high water temperatures. 
There are many kinds of thermometers available to the homebrewer. Each has advantages and disadvantages in their use. Mercury or alcohol thermometers can be very accurate but are prone to breakage if mishandled and usually must be read from the side, which make them impractical for checking temperatures deep inside a vessel. Dial thermometers offer good performance and accuracy and perhaps are the homebrewer's top choice, but often the dial scales are small and hard to read. Larger dial thermometers can be expensive and harder to find. 
Electronic thermometers offer a direct numerical readout and use a remote sensing probe, which allows one to easily read temperatures in difficult-to-reach areas. They can be home-built at low cost and rival or exceed the best dial thermometers in accuracy. Perhaps their best feature is that they provide a "front end" for electronic temperature controllers and other devices. 
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Sensing Temperature Electronically 
There are many ways to sense temperature using electronic components. Thermistors are resistors whose resistance changes with temperature. They are relatively inexpensive and are useful in certain control applications, but they don't have a "linear" response to temperature, so accurate conversion of a thermistor's resistance to a direct temperature readout is difficult for the hobbyist circuit builder. Thermocouples are little electrical "generators" whose voltage changes predictably with temperature, although again, they require special processing of their output to determine temperature. 
Perhaps the best electronic sensor for our applications is a piece of silicon available in the form of a diode. A diode is an inexpensive electronic component designed to allow current to flow through it in one direction, but which blocks current attempting to flow in the opposite direction. Picture a diode as an electronic "check valve" allowing electrons (instead of fluid) to flow in one direction only. As current flows in the diode, a voltage builds across it. For a given diode, this voltage depends on two things: the amount of current flowing through it, and the diode's temperature. If we can establish a fixed and known current, we can assume that any changes in the diode voltage are due only to changes in temperature. 
The change in the diode's voltage in response to temperature changes is very linear, so we've solved a big problem there. There are a couple of other problems though that will require addressing before we can use a diode in a direct-reading thermometer. First, most of the voltage cross the diode must be "subtracted" to leave only the part which changes proportionally to the temperature units we are interested in (normally degrees F or C). This "offset" voltage is roughly 0.5 to 0.7 volts. Next, the voltage change in response to temperature change is very small; about 1.22 thousandths of a volt (1.22 mV) per degree Fahrenheit (or 2.2 mV/deg C). Finally, the voltage drops as temperature rises, and rises as temperature drops. So in order to use a diode in a direct-reading thermometer, we must (1


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