As I wrote in the first part of my article, how to design a cheap plant watering sensor, I had troubles to get exact measurements of the current, using my Testo multimeter. I searched for a solution and found the µCurrent Gold device, from David L. Jones known for the EEVblog. Later a little bit more about the device, first the measurements.
I could do the measurements using the oscilloscope and not only measure the static current consumption, but the current consumption over time. The graph shown in the diagram, is the reading of if the sensor does one single measurement and flashes the LED.
The range of the device for the µA region is limited to 1250µA with the 3V coin cell I use, so the current consumption of the LED is way out of range. This is no problem, because is very easy to measure this much larger current using the multimeter. I did not measure it yet properly, but it will be 20mA or less, therefore I just assume the worst – 20mA for this part.
Now I can do a rough estimation of the real power consumption. Without the LED flash, it is just 56µAs (this is ~15nAh). With a single flash, it is much higher with 956µAs (~0.26µAh). Using these values, I can easily calculate the battery life for normal operation.
This simple device just converts the measured current into a voltage, which can easily measured using a multimeter or as I did, using a oscilloscope.
The clever design of this device reduces the burden voltage into the µV range (10-20µV). Read all details about the device here:
In a few days I will publish the 3rd part of my article, with details about the PCB prototype and the start of the final design.
If you have any questions, corrections or miss some information, please do not hesitate to leave a comment below.