Since most of the foundation software for this system is functional, the next posts will be more on the hardware aspects of the system starting from the power supply. As previously mentioned, the board design is more of a development platform than anything else, and so has a lot of provisions for alternative circuits and ease of modifications. The board was also divided into two main parts, power supply and peripheral.
The supply block can be of any four power sources: (1) Battery, (2) Solar (3) 12-VDC vehicle battery and (4) AC-mains power. TI's TPS767D301 is used to provide regulated 5Vdc and 3.3Vdc. Aside from having two output channels, this particular LDO regulator was chosen because of the high output current capability (up to 1A per regulator) yet an ultra low quiescent current. The adjustable regulator can easily be set via external resistor divider. See programming the TPS767D301 adjustable LDO regulator section page 17 of the datasheet.
The outdoor sensor module utilises harvested solar energy to charge the CR123A rechargeable battery while providing power to the sensor module. Battery is managed by TI's BQ25504, an ultra-low power boost converter with built-in battery management unit. This is the ideal power management IC solution for the outdoor sensor module. The circuit used for this project was adopted from the Solar Application Circuit as described on page 18 of the datasheet. This section also has an example on how to calculate the R values for a specific application. One would find the spreadsheet calculator to be very useful in the design process.
A couple lessons I've learned during the design process for this power source.
(1) if you are going to do/copy the schematic from the sample application, assume the OC and OK resistor values to be greater than 10MΩ, because
(2) the resolution of standard values above 10MΩ is low and would require a couple resistors to achieve the desired value. i.e. 15.6MΩ would realistically be 10MΩ and 5.6MΩ in series and this should be noted in the design of the schematic and laying out of components in the PCB design.
I was not mindful initially when I designed this section of the board, and it can be obviously seen on the R35, I only had a place holder for a single 0402 resistor. At least it was not that bad of a mod as there was room in the left side to accommodate two 0603s connected in series.
Below is the circuit in action connected to a solar panel MC-SP0.8-NF-GCS - MULTICOMP - SOLAR PANEL, 0.8W, 4V, NO FRAME | element14 New Zealand
Next one would be the supply block derived from AC mains. This supply is the second simplest power circuit (simplest one is the straight connection of battery to the dual LDO). Since this supply uses AC mains, and could potentially be dangerous, extreme care is fundamental during testing. The core component for this power block is the compact single output AC-DC converter from Vigortronix. VTX-214-005-105 - VIGORTRONIX - AC-DC CONV, FIXED, 1 O/P, 5W, 5V | element14 New Zealand. The compact size and 5W output are the key features for this module.
Initially, I thought of implementing a transformer-less power supply because I do not want to use a bulky and costly transformer. However, that can only provide low DC current and quite dangerous as there is no isolation from the high side. This compact AC-DC converter ticked the supply requirements for the smart switch and plug modules.
Lastly, the emission sensor module will be mounted somewhere in the vehicle and will be powered from the vehicle battery. For this specific module, TI's TPS62153A buck converter has been selected. This power management silicon has the following key features which were considered key aspects in the component selection:
- can provide up to 1A of continuous current at 5V output
- low quiescent current
- over temperature protection
- short circuit protection
- rated for automotive applications
Hope you enjoyed this post as much as I enjoyed populating and testing the boards, 'til next update.