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Battery life
The Outdoor Unit is a low-power design. To get an idea of likely battery life we need to work out the average current used by the circuit. This isn't straightforward unless you happen to have a power meter. The method I used involved introducing a small 22 Ohm resistor into the power lead from the battery, and then measuring the voltage trace across this resistor over a minute period.
The photo shows the trace from my oscilloscope while the Controller PICAXE powers up, gets sensor readings and transmits to the Indoor Unit. During the rest of the minute there are regular 3-second small ramps in voltage.
Because the voltage is across a known resistor, this trace can be expressed as a graph of current against time. The area under the graph is the integral of the current over the minute interval - ie the total charge. This charge value, divided by 60 seconds is the average current.
I plotted a rough graph on graph paper, worked out the total area and got a result of average current of 0.68mA. Considering AA battery cells have a capacity of around 2500mAh, then each battery will be flat in approximately 3600 hours, or 5 months. The Max756 can't operate below about 0.7 volts so the batteries might, as a guide, last 3 to 4 months.
However in reality there are other factors involved such as operating temperature, type of battery etc.
At this current, Solar cells would be a realistic enhancement to this project, even for use in the UK.
The photo shows the trace from my oscilloscope while the Controller PICAXE powers up, gets sensor readings and transmits to the Indoor Unit. During the rest of the minute there are regular 3-second small ramps in voltage.
Because the voltage is across a known resistor, this trace can be expressed as a graph of current against time. The area under the graph is the integral of the current over the minute interval - ie the total charge. This charge value, divided by 60 seconds is the average current.
I plotted a rough graph on graph paper, worked out the total area and got a result of average current of 0.68mA. Considering AA battery cells have a capacity of around 2500mAh, then each battery will be flat in approximately 3600 hours, or 5 months. The Max756 can't operate below about 0.7 volts so the batteries might, as a guide, last 3 to 4 months.
However in reality there are other factors involved such as operating temperature, type of battery etc.
At this current, Solar cells would be a realistic enhancement to this project, even for use in the UK.
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