Welcome

Mike's Flight Deck is an introduction to home cockpit building, the hobby that takes off where flight simulation game software ends. When staring at a 17" monitor on a crowded desk, and pushing, pulling and twisting a wimpy joystick just doesn't do any more, it's time to build a simulated cockpit or flight deck. This site offers information on how to do just that.

Mike's Flight Deck is also home to Mike's Flight Deck Books, a very, very small company that sells the books I've written about building flight simulators. So far I've written two books. Book 1 is about making steam-gauge style instruments for your simulator. If you're interested in knowing more about it or ordering a copy, click on the left book cover picture. Book 2  covers just about all aspects of building a flight simulator at home. Clicking on the right image will lead to its information and ordering page.

         ("Book 1")                    ("Book 2")

What's New: 

Projection Depth of Field (6 February 2010)

I've been investigating depth of field as applied to projection systems. There's a lot written about DoF when taking pictures, but little specifically about it when projecting an image. As it turns out, it's not difficult to derive DoF formulas from basics. It's really a question of geometry and how sharp-eyed you are.

DoF isn't really about focus. It's about an acceptable range of almost-in-focus. DoF in photography is based on the idea that a disk 0.01" (0.25mm) in diameter viewed at a distance of one foot (30 cm) is just barely distinguishable from a dimensionless point of equal brightness. There is some argument about this. Some use a slightly larger disk. Others say an image de-focused this much would be objectionably fuzzy. The arguments are really a matter of degree rather than of basic concept.

The fundamental concept is that there is a zone in front a lens within which objects will appear to be acceptably sharp. For photography, this plays out as detail or softness of objects in a print. For projection it is the sharpness of the image relative to the distance between the screen and lens for a given focus setting on the lens.

Disregarding any geometric distortion, projection DoF is an issue when you want to project onto something other than a flat screen orthogonal to the projection screen axis. For example, perhaps your projector doesn't have lens shift and you want to use an non-orthogonal projection angle. Or maybe you want to project onto a curved screen and want to know the minimum radius of curvature before focus becomes an issue.

So, here are the basics. When a projection system is perfectly focused, a dimensionless point on the screen is illuminated by a conical bundle of light rays that extend from every point within the lens exit aperture and converge on the point on the screen. If the screen is moved closer to the screen, the screen intercepts a circular area of the cone. The closer the screen is, the larger the area of the circular intercept, and the more de-focused the point appears.

Now for the geometry: remember similar triangles, long, long ago in grade school? No? Well, anyway... The proportion of the spot size to the front depth of focus boundary is the same as the proportion of the lens aperture to the screen distance.

There are a few more details to consider. 

Lens aperture in the drawing is the diameter of the lens exit aperture, not the f number. Lens aperture diameter is the lens focal length divided by the f number.

Spot size is the acceptable de-focusing of a theoretical point. It depends on how far away the viewer is from the screen. The 0.01" number is for a one foot viewing distance. If viewed from five feet, it can be five times as large. On the other hand, this is the spot size that is just visible to a viewer of average visual acuity. Perhaps you have better than average eye sight, or simply don't want the de-focusing to be noticeable at all. In those cases, your choice of acceptable spot size might be one half or one third the size with a commensurate reduction in DoF. 

Depth of Field is an objective calculation that increases with projection distance and viewing distance, but decreases with increasing lens exit diameter. Depth of Field is also a somewhat subjective concept that depends on the acuity of a human viewer.

Interfacing a Real A/C Exhaust Gas Temperature Gauge - part 2  (16 January 2010)

The firmware for the EGT adapter is complete and tested on a proto board. The PIC is the chip on the left of the proto board. The read and white clip leads hooked to the output of the PWM output filter. The other ends of the leads connect to a digital voltmeter. The components to the right of the white clip lead form the voltage inverter which is part of the tachometer adapter I tested earlier.

This test set up uses a signal generator to make pulses like the RC servo control pulses. The voltmeter readings verified that the voltage from the PWM filter varies from 0 to 40 millivolts as needed by the EGT gauge. I've sent a programmed PIC to Rob for testing with an OC servo card and a real EGT gauge.  Soon we'll get a real world test.

Interfacing a Real A/C Exhaust Gas Temperature Gauge (10 January 2010)

This is a spinoff from the tachometer interfacing project below. What we want to do here is interface a different type gauge using the same OpenCockpits RC servo control card. The EGT (exhaust gas temperature) gauge is an electronic instrument designed to respond to the low voltage output of a thermocouple. Rob has made a few measurements and determined that a 40 millivolt input will cause a full scale deflection. 

Rather than start we're just going to modify the tachometer adapter. We'll replace the PIC firmware that generates the variable frequency square wave output used to drive the tach with a fixed frequency, variable duty cycle output. We'll also replace the tachometer-driving output transistor with a resistor-capacitor network which will scale the output voltage from the 5 volt max sent by the PIC to 40 millivolts, the max needed by the EGT. The caps in the network will filter the EGT control signal so the gauge sees a smooth voltage rather than the pulsating PIC output.

Here's where we are now:

And we have an early printed circuit design as well:

This board actually combines a single EGT adapter with a pair of tach adapters.

This is why I like microcontrollers, neat projects for not very much cash. (PIC16F648As are selling for $2.25 a piece from Jameco.)

Interfacing a Real A/C Tachometer - part 3 (8 January 2010)

The firmware is operating properly with sets signals. Rob will check its functionality with a real tachometer.

Here's the complete schematic. The whole thing is powered by the RC servo interface card. The card supplies +5 volts to power the servo. We use the 5 volts for the PIC and for a low power inverter.

And here's an early pass at circuit board artwork. This places three PICs on a single board along with a shared inverter for the output transistors.

Interfacing a Real A/C Tachometer - part 2 (3 January 2010)

As we left this project, my collaborator, Rob, was going to use the test oscillator diagramed in the December 9th note to try driving a real aircraft tachometer, and to get some measurements we could use to determine the tach's input resistance. Turns out the oscillator would not make the tach function. Rob also tried connecting the audio signal that drives his computer speakers, and that did the trick. He used a small audio signal generator app to produce a sine wave signal, and the tach woke up. 

This told use that the tach worked and that we needed a drive signal that went negative in addition to going positive. And while the first version of the test oscillator did not do that and could not activate the tach display, it did provide us with test voltage measurements that allowed us to calculate the tach's input resistance. We needed to know this so we could determine how much power was needed to drive it.

All that lead us to a revised version of the test oscillator, shown below. Why stay with this when we knew we could drive the tach with an audio sine wave? Because the test oscillator signal is very easy to generate. 

Version 2 of the test oscillator proved successful. We now know how to drive the tach with an easily made signal.

This brings us to the next step, developing a low cost interface.

As it turns out, Rob already has an OpenCockpits RC servo interface card. So not only does he have a chunk of nice hardware, he has the supporting software behind it. Software to bridge the gap between the flight simulation application and the interface hardware is a significant part of the project. So, our thought is that if we could somehow convert the RC servo style control pulse to the variable frequency square ware needed by the tach, we'd have a simple, low-cost solution.

This adapter is the approach we're working on now. It's based on a PIC16F648A micro controller. As you can see in the next figure, it doesn't take much hardware to make it go. Even after adding a small inverter to generate the -5 volt supply, we'll still be looking at only about $10 in parts. The bulk of the functionality comes from the firmware running on the PIC.

In general terms, the PIC uses an internal timer to measure the width of the RC servo control signal connected to pin 6. It does some scaling, checking, and binary hand-waving to calculate the parameter needed to generate a suitable tach control signal which it outputs though pin 17. The transistor makes sure the signal swings both below and above the 0 volt line.

At this point, I've got the firmware written, but not fully tested. Hopefully, I'll get to it over the next few days.

Go to here for previous rants.

Something, somewhere in this site was tweaked, changed, added, deleted or otherwise updated on 02/06/10.

It's possible that I'm not as smart as I think I am. (Occasionally, I have moments when I know this to be true. Fortunately the feeling passes quickly.) Although I have tried to make this information as accurate as I can, it is not only possible, but also quite likely, that errors lurk within. I cannot and do not warrant these pages to be error free and correct. Further I accept no liability for the use of this information (or misinformation). If, after reading this, you are still interested, please be aware that the contents of this site are protected by copyright (copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 by John M. Powell). Nonetheless, you may copy this material subject to these three conditions: (1) the copyright notice is copied and presented along with the material, (2) the copy is used for non-commercial purposes, and (3) the source of the material is properly credited. And of course, you may link to this page.