The saga of the dual axis solar panel controller continues. As an R&D project I couldn’t put a lot of time into this design this week. Here is what I did get accomplished.
If you read the last post (see here) on this design I was into the firmware and software interface of this design. During that process I realized that the analog outputs of the accelerometer (ADXL335 Analog Devices) had too small of a range. The X axis output was +/-350mV centered around 1.65V. The Y and Z axes were +/- 600mV centered at 1.8V. Of additional note is the fact that the center point and range will vary from part to part. Roughly 75% of my analog-to-digital converter’s (ADC) resolution would be lost. So I added some differential amplifiers and other neat stuff to the schematic to get me back some bits.
Schematic of the analog signal changes (click to see full-size).
I still intend to use my revision 1 prototype to test the remaining portions of my circuit, like motor control. In fact, I should be able to finish most of my design using this prototype. However, I felt more like doing PCB layout than writing firmware this week, so I went ahead and made the PCB changes for the revision 2. PCB design is more Zen for me than writing firmware/software/probing things with oscilloscopes.
These were the changes I wanted to make:
1. Add op-amp differential amplifiers for each axis of the ADXL accelerometer as well as additional noise filter components. These circuits are under the red shaded area.
2. Change the debug / programming header to something a little more solid (0.05” pitch to 0.1” pitch).
3. Change from a horizontal to a vertical USB connector.
4. Use terminal blocks that can handle large gage wiring.
5. Move the user interface / programming buttons further apart.
You can see that I had to extend the second revision of the PCB a bit. I added the parts associated with the ADC to my board design. The PCB dimensions went from 2.8” x 3.2” to 2.8” x 3.5”, so not a very big change in size. In all, the changes took me about 4 hours to complete. Here’s the copper and silk screen layer associated with the new circuitry.