Center Pedestal

Switches & Indicators

Korry - the Holy Grail of Switch-Hunting Hobbyists

You will hear a lot about is something called a "Korry" switch. Here's the deal. Some years ago there was an engineer named Boris Korry who worked for Boeing. He left and set up shop in 1937 making illuminated push button switches. These were rather special switches for the time as they had been designed to allow the lamp to be replaced from the front of the panel, apparently a first back then. He sold a lot of his switches to Boeing for use in the Boeing 307 Stratocruiser, and the company he founded is still in business today as a subsidiary of Easterline Technologies. Today, Korry's product line has expanded beyond switches to include panels, transluminated knobs (they picked up the Aerospace Knob Corp. line), displays and lighting products. They focus on the commercial aviation and aerospace markets with high reliability, expensive products. You probably won't be buying Korry products new (unless you consider ~US$130 per switch a bargain), but you can check out the technical specs on the Korry web site. You might, however, run across Korry products on the surplus or salvage market.

Korry is certainly not the only supplier of switches and indicators to the commercial aviation industry. There's Janco, Otto Controls, Eaton , Jay-El and Mason Electric,  just to mention a few. But for some reason Korry seem to hold great appeal, perhaps because they have such a noble history.

If you can't find the grail, here are some "paper cups"

In any case, high reliability aerospace and aviation switches, regardless of manufacturer, command a sizeable premium. If you're not pleased with the prospect of spending four or five thousand dollars on lighted push button switches and indicators/annunciators, you probably will want to consider some alternatives. One possibly viable approach is to buy products that aren't targeted toward the aerospace markets. Honeywell's AML series and Miyama "Mountain switches" (sold through Mouser) are two of a great many alternatives.

Twisting instead of pushing...

You have undoubtedly noted that some switches on the flight deck are turned rather than pushed. And you have perhaps heard people referring to "rotary encoders". So, (1) what's a rotary encoder anyway, and (2) why don't you just use a regular rotary switch?

1. Whatza Rotary Encoder?

Well, there are absolute rotary encoders and there are incremental encoders. An absolute encoder has a number of binary outputs that indicate the switch shaft's absolute rotational position referenced to some spot on the switch's body. For example, a three bit absolute rotary encoder will divide the rotational position into eight sectors. Absolute encoders are available in a variety of style with a variety of resolutions.

Rotary encoders

An incremental rotary encoder, unlike its absolute brethren, won't tell you where the shaft is positioned. It will only tell which direction the shaft is being turned and how fast. Incremental encoders have two outputs called phases. Each outputs a square wave. Turning the shaft one direction causes one phase to lead the other by 90 degrees. Reversing the direction will cause the other phase to lead. The frequency of the output is proportional to the rotational speed of the shaft.

Rotary encoders

Rotary encoders may be optical, or they may be mechanical. Optical encoders use a disk with holes or slots to gate light moving between paired LEDs and phototransistors. Because there is no mechanical contact with the disk, extremely reliable optical encoders can be made with lifetimes measured in the millions of rotations. A mechanical encoder contains a pair of mechanically actuated electrical switches that very definitely have contact internally. Its operational lifetime is relatively shorter, measured in the tens of thousands of rotations.

2. Why not use a regular rotary switch?

With the definitions out of the way, let's get back to an earlier question, 'why use a rotary encoder instead of a regular switch?' The answer goes back to the way the fight sim software works. The software is not built for a bunch of crazies like us to wrap a flight deck around it. It was built to operate on as many basic home PCs as possible, and that means using a keyboard for input. Most frequently, the software takes keystrokes to make relative changes to something. (You know, push "u" once to make it go up a notch, push "u" a lot to make it go up a bunch, that sort of thing.)

When people started looking for relatively easy ways to interface replica flight controls to recreational simulator software, the most straightforward solution was to emulate the character stream from a keyboard. Say a person using a sim wants to change something that would have a rotary switch in a real plane, for example, the navcom frequency. Said user turns a knob in the sim that is fastened to a rotary encoder. Said rotary encoder sends pulses to a bit of electronics which is configured to send the appropriate character or stream of characters to the software to make the desired change. Having changed its internal variable representing the frequency, said software then updates the navcom display to the new frequency.

This may not seem like the simplest way to solve this problem, but please note that it does not require modifying the simulator software in the least. In fact, you can park everything outside the PC and just plug into the keyboard port. So, while it perhaps is not exactly trivial to do, it may in fact turn out to be one of the more simple approaches to interfacing rotary switches. In fact, this is in general the approach used by the Hagstrom keyboard encoder.

I want to acknowledge that there are some situations where the use of incremental encoders is a bit lame. In particular, an incremental encoder is not so hot when you have a rotary switch with fixed detents, and a knob that points to markings on the panel. The software has no way to determine where the knob is positioned upon power up. It will only get notification of changes from the switch's current position, not an absolute setting of the knob. Incremental encoders work great in situations where the software is controlling the display of whatever variable the encoder is supposed to change, like the earlier example of a radio's frequency.

As for the second question regarding optical versus mechanical encoders, I see no reason not to use the less expensive mechanical variety. Optical encoders offer enhanced reliability often at significantly enhanced price. Optical encoders are great in situations that require the encoder shaft to turn continuously, for instance when the job at hand is to keep track of a lead screw's rotational position on a CNC machine. There is no reason to pay for that sort of robust reliability when there are so many other worthy things to buy, like triple chocolate cake and…

An Alternative to a Rotary Encoder

There is a special switch that makes a sequence of closures on one set of contacts when turned clockwise, but not counter-clockwise. It makes a sequence of closures on another set of contacts when turned counter-clockwise.

These switches can be wired to a keyboard encoder or a hacked keyboard.

Versions of this switch are made by Knitter-Switch, by Alps Electric Co, Ltd., and by Alpha/Xicon.  Knitter-Switch calls the item a "rotary pulse switch" (model MRP1-20). Alps calls theirs a "pulse/rotation sensor"  (model number SRBM1L).  Elfa carries Alps products. Alpha/Xicon calls their switch a "pulse switch". Mouser carries it. Look for part number 105-SR10030-PS.

Toggle Switch Guards

A switch guard is a spring loaded cover that prevents the switch from being inadvertently tripped. It's used on switches you'd  rather not have flipped at an inappropriate time. For example, the fuel pump shutoff on the Bell 206B. As to why you might want some, if you're simulating a cockpit that actually uses them, having them in your pit as well is a nice touch.

But, they can be hard to find.

A common type used is the MS25224 (dash 1 if you want red). It's made by companies like Eaton Aerospace and Cutler-Hammer. Some non-mil spec switch guards are made by Philmore-Datak.

The cheapest sources for MS25224s seems to be aircraft parts suppliers like Wicks Aircraft and Aircraft Spruce and Specialty. On-line auctions, surplus stores and boating supply stores(!) push the prices up. The Philmore-Datak part may or may not turn up at small electronics parts stores. Check the Philmore-Datak website for retailers. If you're in the UK, check out Maplin. They have a "mil-style missile switch cover" that looks identical but is relatively less expensive. Search for part number N42AT.  Jaycar Electronics (Australia) carries one as well (#ST0578).