This is the second part of the meta-tutorial, where I talk about designing a cheap plant watering sensor. If you did not already read the first part, please do it now. It contains a lot information about constraints and decisions made which lead to this point.
The first part ended with step 11, building a working prototype with the selected key components. So let us start with the next steps in this journey.
Step 12: Analyse and Measure the Prototype
Never forget why you actually built a prototype. It is your tool to verify all assumptions you made in the design phase. To do this you need the right measuring instruments.
The Power Usage
I start measuring the current of the circuit. This will show if my assumptions about the battery life will be true. For this test I use a multimeter which has a good resolution measuring in the µA range. The multimeter I use is the Testo 760-3 which is not a very well known brand. Multimeters are usually really poor at measuring low currents on low voltages, so let us see if this will work.
I also use a Fluke 114, but this one has no current measurement. It is sometimes very handy to have two multimeters, one to measure the voltage and a second one to measure the current.
For the first test I program the MCU to do all the tests in a loop and connect the power directly to the second part of the circuit. Now the power is always on and I can measure the current used by the MCU while doing the measurement.
Just running, doing the measurement of the oscillator, the second part of the circuit uses at maximum ~1.2mA. This measurement phase should be as short as possible. Later we will analyse the timing. Continue Reading