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  • Coffee Thermometer Circuit

  • Created: Sep 08, 2014

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A thermometer is a tool that is used to determine the temperature of solids, liquids, and gases. This tool has a liquid inside that is normally an alcohol solution or mercury in a reservoir. The volume of the liquid is linearly dependent on the temperature. This means that as the temperature of the object increases, the volume also increases. Consequently when the liquid is heated, it expands into a narrow tube. This tube has been calibrated to measure the temperature. The units of temperature are Celsius, Fahrenheit, and Kelvin. It is very important to determine which scale the thermometer is calibrated for.

The circuit uses a MCP6L91RT-E/OT. The MCP6L91/1R/2/4 family of operational amplifiers has a 10MHz gain bandwidth product and a low 850µA per amplifier quiescent current. These op-amps operate on supply voltages between 2.4V to 6.0V, with rail-to-rail input and output swing. They are available in the extended temperature range.

The coffee thermometer is composed of a voltage regulator, a temperature to voltage converter, a comparator and two LEDs. Roughly described, the circuit works this way: if the coffee temperature is not hot enough, the IC output is logical 0 and Q1 does not conduct current. Transistor Q2 at this time conducts and the red LED lights up. If the temperature is hot enough, the green LED lights up. IC2 measures the temperature and converts it into a voltage value. Because of this, IC2 must be dipped into the coffee. IC2 can also be installed inside an empty pen, which can serve as a probe. The output of IC2 increases in proportion to the temperature by 10 mV per degree. Therefore, if you feel that by 80 degrees Celsius the taste of the coffee is at its best, you must set the reference temperature at the minus input of the IC3 to 800 mV through P1. When the voltage level at the plus input of IC3 reaches 800 mV, the comparator output swings to logic 1 and Q1 conducts current. At this time Q2 turns OFF and the red LED is OFF. Otherwise, the green LED lights up showing that the coffee tastes good. There are two PCB layouts for this coffee thermometer circuit. Choose any of the two.



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