What did we do
1. Reviewed and clarified the customers’ requirements. In addition to the 3+ year battery life a second key aspect of the project was to fit the electronics within a 4-slot wide DIN rail enclosure.
2. Sourced a suitable enclosure
3. Reviewed a physical sample with the client, and
4. Used the enclosure’s 3D CAD model as the basis for the physical printed circuit board constraints.
It was clear at the outset that there was insufficient room on a single printed circuit board for the electronics and the optional battery. As the battery was an optional power source it made sense for the battery to be affixed to a daughterboard. This allows it to be provided only where the installation requires battery power, this gives the potential for a cost saving where the units have a permanent power option.
Electronics risk prototype testing
The unit provides connectivity to two sets of wired temperature and humidity sensors which used I2C connectivity. The sensors needed to be up to five metres away from the main unit. While this worked directly over low cost cabling in a lab environment the signal integrity was poor due to cable capacitance. In a real-world environment with additional electrical noise there was a possibility of issues occurring in the field.
I2C extenders were prototyped to test actively driving the long cables. This significantly improved the signal integrity and therefore the real-world reliability. Later in the full design, options were included on the PCB to allow for the extenders to be removed. This is a later cost reduction option, should the installation cable lengths be reduced.
The mains current sensing was also prototyped and, following a risk assessment, was also tested to confirm correct operation.
Once the higher risk areas of the design had been prototyped, tested and proven the design was captured in a full schematic. It was internally peer reviewed and then passed to the client for their own review.
Printed circuit board
The printed circuit board layout was fairly straightforward, albeit quite tight to fit everything in the available space. Again, the finalised design was internally peer reviewed, then passed to the client for their own review.
Prototype manufacture and firmware coding
A handful of prototypes were manufactured. During that time the firmware for the unit was written and debugged by us using a development board.
When the physical prototypes were available the hardware was tested. A number of manufacturing errors were identified and, of course, were fixed. Finally, the code was ported across to the product hardware. The hardware was bought up and debugged to ensure full functionality was achieved for:
The remote temperature and humidity sensors,
Current measurement, and the interface with the radio module.
The printed circuit board was tested for mechanical fit in the selected enclosure and simply clicked into place.
Less power, longer battery life
Once the functionality was achieved, attention turned to power consumption optimisation. The base hardware power consumption was measured, and then work undertaken to reduce it. The target was a battery life of three years.
Next came power consumption optimisation with the firmware running. Using our high precision DC power analysis equipment, the current draw was quantified over a 2-hour period which contained 120 application measurement cycles. The measurement detail obtained was then used to optimise the microprocessor sleep times, sensor settling times and measurement times to generate a new measurement cycle time.
This power consumption activity resulted in an average current of 42uA while still providing the customer with the sample rate they required. The result of this activity was a battery life of 7 years, exceeding the original target by 233%, a fantastic result indeed!