Engine Gauge PCB Design, Updated!

My original gauge controller boards were done in Eagle, long before Autodesk got their claws into them and murdered them. Fortunately, KiCAD does a pretty good job of importing Eagle projects.

That got me started – I ended up basically redrawing the schematic from scratch, but that was probably the better choice anyway.

Revisiting The Engine Gauges…

Back in 2015, I began working on replicas of the Fuel Flow, FTIT, and RPM gauges. The gauge design at the time was based around an air-core motor manufactured by SimCo (specifically, https://www.simcoltd.com/products/coil-bobbin-assemblies)

Air-core motors are frequently used in automotive instrument clusters for things like the speedometer, fuel gauge, etc. The units I had were part of a group-buy that I participated in that was run by a number of F-16 cockpit builders. Air-core motors are similar to synchro resolvers in that they’re driven by a sine/cosine signal. Unfortunately for me, I was unable to work out how to drive the motors silently. While they moved fine, there was a high-pitched tone emitted by the motor, which realistically wasn’t something that could be easily ignored. At the time I lacked the attention span to tackle the problem and figure out what the root cause was.

The gauge design sat idle until around June of 2016. About that time I was able to collect enough attention span to chase down a simpler solution to the problem. Instead of working out why the air-core motors whined like I’d stolen their candy, I just swapped over to Switec stepper motors!

It turns out in the years between the group-buy of air-core motors and when I started actually working with them, a Swiss company named Switec had developed a range of stepper motors specifically designed for use in automotive instrument clusters and other pointer-style gauges. The motors turned out to be a perfect solution to my noise problem. They had the range and resolution I needed, the gear hysteresis was no where near as bad as an R/C airplane servo, and they were inexpensive.

Another bonus turned out to be their low operating current. The coils in a Switec motor only consume 20mA each, making them safe to directly drive off the output pins of an Arduino Nano, and no driver chip was required.

Controlling the Power Supplies

This flight simulator involves a number of different power supplies that are used to supply the panel and instrument lighting (5VDC), various indicator lamps (24VDC), the control loading system (24VDC), and the CANBus I/O interfaces and instrument controllers (12VDC). For safety and operational reasons, there’s a need to be able to control what power supply comes online and when. For example, if there isn’t at least one engine running, you don’t need any panel or instrument lighting, and the control loader shouldn’t be powered up unless the simulator software itself is running.

Powering the Control Loader

One of the final tasks I have to do with the VPForce FFB system I’m using for the F-15’s control loader is to permanently connect the power supply it uses.