PICAXE-18X & Philips EE: Power-saving Temperature Monitor

This application shows the use of the PICAXE-18X as a simple datalogging system, with a focus on very low power consumption by active control of the power supply of the DS18B20 temperature sensor.


Introduction

My son Joost and I jointly developed this simple datalogging system, and used it to test the quality of our fridge, as I had the impression that food (especially meat) could not be preserved long enough. Normally, food should be kept at an optimum temperature of about 4 degrees Celsius, so our goal was to see whether this temperature could be maintained by our fridge on average.


Hardware & Software Methods

The hardware setup is very simple, using input 0 to read the temperature values from the DS18B20 temperature sensor. Some leds were present at outputs 5-7 to signal whether the system is going to measure or to send previously measured data stored in the picaxe's EEPROM. In the initial phase of execution a switch at input 7 is read to determine whether data has to be sent (default case, input is high; switch left open) or measurements have to be made (switch pressed, input is low). After measurements have been made or data has been sent the computer enters low-power mode. Battery power of 4.5V is applied, with a 100uF capacitor fixed to the battery input pins. The Philips EE system has been used for prototyping.

Special precautions have been taken to reduce current consumption:

After power-down of the system it took several minutes for the 100uF capacitor to discharge to such a level that the system would come up again when the batteries were reconnected, indicating very low power consumption.

Code:


Results

The graph below shows the temperature measurements inside our fridge starting at around 23:00 hrs in the evening, with measurements ending the next morning at about 8:00 hrs. The fridge was set at maximum cooling. The door had to be opened to place the sensor in the middle of the fridge, and around 7:00 hrs the fridge was opened twice for breakfast.preparation. The graph was made using Microsoft Excel.

We can draw some interesting conclusions from this graph:

Since our fridge is quite old these results do not come as a complete surprise and do indicate that food cannot be conserved very long. The simple and somewhat ineffective cooling strategy was somewhat of a surprise though.


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