For my smoker temperature monitor, I decided to use the same temperature probes that my storebought unit uses. Even if the project is a bust, I can use the probes either as replacements or as additional probes that I would manually switch between while using my single-input storebought monitor.
The brand name on my storebought monitor is Charmglow, a company which only makes grills, not temperature monitors. Thanks to the Internet, I learned that Charmglow re-brands monitors made in China and distributed by Maverick Industries. The replacement probes will cost $12 each.
My best guess is the probe sensor is a bi-metal thermocouple. The thermocouple junction creates a small (micro-volts) voltage that varies with temperature. Quantifying this voltage involves some complicated and messy electronic circuitry. Fortunately, someone has already done that for me and put it in a small 8-pin chip (integrated circuit). I can get the chip, MAX6675, for about $13 from Jameco. The MAX6675 will save me lots of time and effort because in addition to the interface for the thermocouple, the chip also contains circuitry to convert the small voltage value to a digital (ones and zeros) value that can be sent directly to the processor. I won't need a processor that does analog-to-digital conversion.