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Temperature Sensor
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The temperature sensor is a thermistor, a temperature-sensitive resistor. In order to analyze these changes in resistance, the thermistor must be included in a voltage divider to provide a varying voltage (0-5V) for the HC-12 ADC to process. Two opposite voltages are applied, with the thermistor at the positive end, and a fixed resistor at the negative end. The output is taken in the middle. The resistor value chosen matches the lowest temperature to be measured. In this case, the value is chosen to be 21.8k. Initially, values of +5V and –5V were used as rails, however this resulted in a range between 0V and 3.6V. In order to increase the accuracy of the reading, this range needs to be extended. This is accomplished by using rail voltages of –7V and +7V. |
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Initially we looked for a commercial waterproof probe, however these were all extremely expensive, often ranging from $50 - $100. So instead, we made a waterproof housing for a standard thermistor. This housing consists of a fuse casing sealed with hot glue at the end. While this does leave some air insulating the thermistor, this sensor was still found to be more responsive than some commercially available probes.
The sensor was calibrated by measuring resistances at varying temperatures. Then, those resistances were applied to the voltage divider using a potentiometer and the converted value on the HC12 recorded. |
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