PC Board Art (A Work In Progress!)
Far harder than I expected was the layout of the PC board(s) and the planning of how the components, fasteners, and connectors will all come together. I have bought perhaps a dozen different cases and rejected almost all of them for one reason or another. The first three boards show the evolution of the project from concept to final board . Boards 2 and higher measure 80mm X 52 mm. The latter 2 boards are conceptual and have not been etched.
The blue traces are the bottom side... the red traces are on top of the board. Yellow silkscreen outlines show the component placement.
|Board 1: This board includes a pressure transducer which has since been eliminated. it can be seen as an outline to the upper left. My skills at layout and creating logical, clean traces is a joke. While this board would probably work, it would be prone to interference, and the sheer number of components makes failure of an individual component more likely. Simpler is often better!|
|Board 2: MUCH better! I haven't wired up the PIC uController, but 90% of the components are in place. Molex C-grid connectors will make attachment of external devices a cinch. The three identical clusters are the I-Pak MOSFETS and support components. The cluster above the 28-pin DIP is the AD595, "Thermocouple meter on a chip", which replaces a much more complex thermocouple interface on the first board.|
An evolution of board 2, above. This is the final board
for what we call "PCB Mark 1". An order was
placed with Express PCB for a large number, which turned
out to be excellent and functionally perfect, no jumpers
or corrections to the layout required.
The upper row holds the input section, with jacks for RC, tach, and LCD out. Below these is the uController, still a PIC16F876. 3 MOSFETS can be seen to the right of the PIC's 28 pin DIP layout; these control Pump, Glow, and Starter circuits, with corresponding output jacks on the right edge of the board. Power-in components occupy the lower right portion. The EGT section is to the lower left, to keep its sensitive circuitry as isolated as possible from the noise of the outputs. Note the blue ground plane around this critical area.
Obviously a densely populated board. All of the components are through-hole, making it feasible to produce this ECU as a kit.
Through hundreds of hours of bench testing and live running, no failures of any components have been noted!
After a lot of thought on safety and hardware. I designed
this dual-processor ECU. The only common components are
power supply. The uControllers run independently of each
other. uProcessor #2 is a PIC16F628, whose functions
include gathering external data (R/C and RPM) and sending
this data serially to the primary controller, a
PIC16F876. Its secondary function is to monitor the
"health" of the primary processor... in the
event of a primary processor failure, the PIC16F628 can
secure the pump by triggering a fuse device. The MOSFETS
have been placed vertically to save room, as they run
The plain 7805 5V regulator has been replaced with a superb Texas Instruments 5V LDO (Low Dropout) Regulator, which assists in brownout prevention during high-amperage motor starts. Another output has been added; a fuel solenoid port. Both the fuel solenoid and gas solenoid are controlled with a high-side dual FET switch, eliminating a bunch of components.
Compare the size of the uProcessor layout with board 3. The only way to cram these components onto this board is to switch to a number of surface mount devices, which certainly save space but make kit potential a bit tricky. To help further my ability to use SMD components, I acquired a Hakko 850 hot-air rework station.
This board is still conceptual and has not been produced.
Pushing size and functionality to the limit! The massive
object center-left is a Linx SC-PA tranciever for a wireless
link to a data module on the ground! Lots of testing of
this concept to go (range? interference? legality?) but
the potential is very cool. Being able to
monitor critical parameters inflight would be a boon. An
audible alarm on the data display could alert a pilot to
a turbine/ECU problem in real time.
RadioMetrix in the U.K. has a better solution in its TX3/RX3 combination, allowing legal use in Europe at 868 MHz, and the U.S. market at 914 MHz. In typical U.S. fashion, the U.S. version would require FCC approval of the final design, a process which would cost ~$2,500. Ouch. But the European version at 868 MHz is pre-approved without further testing. This whole wireless concept is something that I would like to pursue.