Background and Design
After figuring out the issues with my TeensyTrack V2, I went on to design my V3 tracker. V3 is nearly identical in structure to V2 with just a few changes.
Changes:
– Added a resistor-divider enable signal to the boost regulator to enable only when solar voltage is high enough.
– Added RF out DC decoupling capacitors and and pull-down resistors to bleed static electricity that could build on the antenna.
– Changed dipole feed to be from two si5351 outputs 180 degrees out of phase for higher output power rather than one output and ground feeding the dipole.
– Replaced BMP390 temp sensor with LM75B temp sensor.
– Added series resistors to solar voltage measurement MCU analog input to prevent backpowering through ESD diodes.
– Added status LEDs and UART debugging header.
– Added series solar inputs on each side for free-hanging solar panel install and additional mounting holes.
The enable signal control was made easy because of the built-in hysteresis of the boost regulator.
PCB Design
PCB is a standard 4-layer impedance-controlled PCB with a signal-gnd-gnd-signal stackup. RF traces are 50 ohm microstrips. Even though everything here is low-speed I try to keep GND passthrough vias next to all signal vias.
Assembly and Fixes
After a quick assembly and some initial tests I did come across one major design issue. The schematic symbol I downloaded for the boost regulator was incorrect and the enable and output voltage select pins were reversed. Let this be a lesson to always double check your symbols and footprints against the component datasheet. In one of the photos below you can see where I had to move around the resistors to get the voltage divider resistors and voltage set resistors in the right spot. It ended up working with this fix.
Testing
After getting the software working, I soldered on 4 solar cells and a 20m dipole and strung it up in a tree in the middle of the night in order to catch the sunrise to make sure everything powered up correctly as the solar voltage rose. I found that the boost regulator still leaked voltage through as it the input voltage went up, and I needed to set the MCU BOD voltage to 2.6V to get it to be stable.
As the sun went up the tracker powered up and started transmitting. It was heard all over the Eastern United States by the WSPR network as can be seen in the last image!
Check out my Picoballoon page for launches using this tracker!