But why go to all the trouble of building simulated instruments? Why not use real aircraft instruments? After all, if we're going to have to pay megabucks for a simulated instrument (a slight exaggeration), or become a clock maker (you actually don't, it just helps), why not just use the real thing?
Well, why not? It's certainly possible to get real aircraft instruments. Used instruments turn up fairly often on auction websites like Ebay. There are a number of companies that cater to the flight community that offer new and used flight instruments. Trade-A-Plane is a print and on-line "green sheet" where you can buy a complete airplane if you want. Surprisingly, some new instruments aren't all that expensive compared to some of the commercially available simulated ones (though admittedly, both are too expensive for me). Basic flight instruments for the experimental aviation crowd might be worth looking at. Check out Aircraft Spruce and Specialty, and Wicks Aircraft.
Before plunking down a wad of cash for even a used instrument you need to know if it's even possible to interface to, and control, a real aircraft instrument. The definitive answer is yes, no, maybe and it depends. This answer pretty well covers it all, but, as I'm feeling somewhat gregarious, I think I'll elaborate a bit anyway.
Magnetic Field Sensing Instruments
Consider the magnetic compass. Both the whiskey compass and a vertical card compass work basically like the little toy you perhaps had as a child. A small magnet is carefully balanced and allowed to align itself with the horizontal component of the earth's magnetic field. To control it you can wind a set of simple coils at right angles to surround the compass. Passing carefully controlled current through these coils will produce a magnetic field that adds to the earth's field. A little vector arithmetic to figure out how much current going where for each simulated heading, and that compass will do just what you want it to. You can even get fancy and toss a few error terms into your calculations and model north-turning error as well. Bottom line is yes, you can interface to a real aircraft magnetic compass.
Consider instruments that directly incorporate a gyro, such as the artificial horizon, the rate of turn indicator, and the directional gyroscope. These instruments are interesting because, unlike the other instruments, the indicators or pointers in these gyro-based instruments are designed not to move. The gyro in each is configured to maintain a fixed orientation. The indicator or pointer is mounted on the gyro-stabilized platform inside the instrument. It is the instrument housing (and the aircraft bolted to it) that moves around the indicator or pointer. This presents a rather large hurtle to effectively using this class of instrument in a flight simulator. Because, unless you can alter the laws of physics, the bottom line is no you can't use real gyro instruments in your sim.
Air Pressure sensing Instruments
Consider airspeed indicators, (non-radar) altimeters and rate of climb instruments. These instruments work by measuring air pressure and the rate of change of air pressure. If you can accurately generate slowly varying pressures, you can indeed interface to and control these instruments. Remember though, that we are talking about controlling air pressure accurately to within small fractions of a PSI. Maybe you have the skills to do this and maybe you don't. If the pressure control is not precise enough, the instrument reading will not agree with the simulation's internal state and the overall system will be disappointing. So, the bottom line on using pressure sensing type instruments in a sim is maybe.
Servo-ed and Other Instruments
Consider the various remote reading instruments like servo-ed compasses and attitude direction indicators, or engine gauges like RPM, oil pressure, and exhaust gas temperature. These work on a variety of principles. The ease or possibility of using these instruments depends upon which type you end up with. Bottom line then is it depends.
The Bottom, Bottom Line
Aircraft instruments work on a variety of principles. In a number of cases it is possible to supply simulated inputs to these instruments and make them work effectively within a flight simulator. These simulated inputs take the form of varying pressures, varying magnetic fields, electrical pulses of varying frequency and multi-phase electrical signals with varying phase. Some of these inputs will be simple but many will be quite complex. For this complexity you get to look at real aircraft instruments in your sim. Some instruments cannot be effectively incorporated within a simulator without modifications that effectively turn them into simulated instruments. If worn out or defective instruments are available at low cost, they may be of greatest value as a starting point for building simulated units. So, the bottom, bottom line is really, "it depends", because it comes down to how skillful you are in adapting your system to the inputs required by whatever real flight instruments you find.
Let's say you've got your eye on some surplus military instruments and you're about to bump your Ebay bid up from the "loose change" to "serious cash" level. You're pretty sure it's a remote indicator of some sort. Here's a very rough idea of what you're about to get into.
Many of these remote reporting instruments make use of devices called synchros. ("Selsyns" and "autosyns" are similar though older. Today's synchros are much more accurate.) Synchros are used to synchronize the rotational position of shafts that are electrically, rather than mechanically connected. For example, a float in a fuel tank may rotate a synchro shaft near a fuel cell. This rotation is repeated in the cockpit by another synchro within the fuel quantity gauge. In a somewhat more complicated application, the relative movement of a gyro stabilized platform is repeated within the cockpit as a slaved gyro compass or attitude indicator.
As a representative instrument, here are a couple of shots of the insides of a surplus, remote indicating Lear ADI. (Photos are courtesy of Roger Wielgus.)
Small synchros do not produce a great deal of torque. Getting a relatively large indicating device, like the sphere in this ADI, to respond quickly, was often handled by pairing a particular kind of synchro, called a control transformer, with a motor. The control transformer sensed the actual position of the indicator relative to the desired position and produced a signal proportional to the difference. This difference signal was amplified, and used to power the motor. In the case of dual axis instruments, Like this ADI, this arrangement is implemented twice, once for pitch and again for roll.
With regard to this particular instrument, the amplifier was remotely located. Other instruments have an internal amplifier.
Not all instruments incorporating synchros make use of this servo arrangement. Instruments with only a light weight pointer like an engine RPM gauge or an ADF head, do not have high torque requirements, and so can make do with an unaided synchro. This improves the possibility of their use by a hobbyist, but does not guarantee success.
Utilizing these instruments in an unmodified form is possible, but by no means is it a trivial task. These instruments work on 400 Hz power, make use of a technology few are familiar with and documentation is generally in short supply. Nonetheless, these instruments functioned in aircraft and a person skilled in electronics can reverse engineer their operation and make use of them in a simulator.
So,... if you really are considering bumping that Ebay bid into the "serious cash" region, do your homework first. Learn all you can about synchro systems and have a game plan in place for how you're going to interface to the instrument.
For More Info
To investigate synchros in more detail, take a look at "Synchro and Resolver Engineering Handbook" from Moog, and "All About Synchros, Resolvers and Data Acquisition" by Michael W. Johnson.
You will also find an overview of synchros as used in aircraft instruments in Aviation Electronics by Keith W. Bose, and Modern Aviation Electronics by Albert Helfrick. (At least it's in the first edition, probably later editions too, though I don't know that for a fact.)
A good reference for learning how basic flight instruments function is Fundamental of Aircraft Flight and Engine Instruments by Jack Andersen. The book was published by Hayden Book Company in 1969, and is no longer in print. However, with a bit of perseverance, it can be found through used book dealers. Another possibility is Aircraft Instruments and Integrated Systems by E. H. J. Pallett. This one is much newer (1992), and though I haven't personally read it, its table of contents looks pretty good. It's available through Avionics.com.