LIGrlMMING If you intend to fly at night, illumination of the instrument panel is highly recommended! Further, some

Rheostat dimming has some attractive advantages. First, it's simple - a single component does the job. Second, it's fairly low cost.

By BOB NUCKOLLS EAA 205021 The AeroElectric Connection 6936 Bainbridge Rd. Wichita, KS 67226-1008

means for controlling lighting intensity have been standard features of instrument lighting systems from the very first. Most general aviation aircraft use a simple rheostat. The

Rheostat dimming also has lots of disadvantages: It's not "linear," that is, lighting levels change slowly over most of the control travel only to change rapidly over a small remainder of travel. Airframe manufacturers in the '60s and 70s designed custom

rheostat is a variable resistor; connected in series with a string of lights control adjusts the resistance until to be dimmed. Turning the panel the desired intensity is achieved.

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Figure 1. Mounting Plate Fabrication Details 50 JUNE 1993

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(2 PLACES)

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Figure 2. Assembly Details dimming rheostats to correct this problem. The system was still simple but costs went up a lot! Rheostats are simple controls, but they are not regulators. Small changes in system bus voltage allow noticeable changes in lighting intensity. Flashing beacons often caused small aberrations in panel lighting with every flash . . . very irritating. Rheostats for large arrays of post-lights dissipate lots of heat. Further, the required control device can approach 3" in diameter! For homebuilders, finding the proper

rheostat for ideal control can be frustrating if not impossible. The choice is further complicated when later changes to a lighting system force a search for a new dimming rheostat. The ideal dimming control should be small and dissipate no heat on the panel. It should be a true regulator; small changes in bus voltage should not influence light levels. It should generate no radio noise. Lighting levels with respect to knob rotation should be relatively linear and smooth. The system should not go to

zero-volts on lamps when set for minimum intensity; panel lamps should be kept "warm" with about 4 volts on them (in a 14 volt system) to minimize vibration fatigue and start up surges to filaments. The device should be capable of working 2 to 16 light bulbs without revision. Finally, if our wish list were complete, the system should weigh 1 ounce, occupy 1 cubic inch of space and cost $5! I would like to illustrate a simple, cost effective system which nearly meets all points on our wish list. InSPORT AVIATION 51

INST

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SOLID STATE DIMMER MODULE

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INSTRUMENT/MAP LIGHTING LOAD 4-12 VDC AND UP TO 1.0 AMPS

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[NOTE 3]

Figure 3. Schematic and Wiring stalled prices compare favorably with rheostats (a 1 inch, 12 watt rheostat now sells for over $20!). The following will describe a solid-state, illumination controller which anyone can build. A complete set of parts will cost about $20 per dimmer. The panel control for this system is 1/2" in diameter. It mounts in a 1/4" hole and supports a 1/2" diameter knob on a 1/8" shaft. Panel control leadwires may be any desired length; the heat dissipating portion of the dimmer is mounted off the panel. If only small loads (2-6 lamps) are anticipated, the auxiliary heat sink (Item 12) may be deleted. The active device is a true variable voltage regulator which will prevent normal aberrations in bus voltage from pulsing the panel lights. Installed weight will be close to 0.3 Ibs. per dimmer. To reassure the faint of heart, there is nothing "electronic" about building these things. If you can put bolts through Erector Set parts to duplicate 52 JUNE 1993

a suggested construction, then you can build one of these dimmers. I encourage anyone with a need to give this project a try. The hardest part is doing a good job on the metal work. After that, 20 minutes time with a soldering iron will finish it up. Parts are easy to g e t . . . call Digi-Key for a catalog, however, they will take your order on first call by reading to them right from this bill of materials. UPS will drop all the goodies at your door in a few days. Radio Shack stores are everywhere . . . here's what you will need.

Bill of Materials Item 1 - Uninsulated terminals. 24 assorted sizes to a package from Radio Shack. Cat. #64-3030. These may be crimped or soldered; solder is recommended. Item 2 - 2.5" x 3.25" x .062" alum alloy fabricated per Figure 1. Item 3 - 3-point terminal strip from

Digi-Key. Cat. #CBB103-ND. Item 4 - 4-40 x .63" machine screw and two .25" hex nuts. Used 2 places.

(Note 3)

Item 5 - 4-40 x .25" machine screw and .25" hex nut. Item 6 - 2 2 microfarad, 16 volt, aluminum electrolytic capacitor. Digi-Key Cat. #P1311. Used 2 places. Item 7 - 6 8 ohm, 1/2 watt, carbon film resistor. Digi-Key Cat. #68H. (Pkg. of 5 for about 28 cents) Item 8-110 ohm, 1/2 watt, carbon film resistor. Digi-Key Cat. #11 OH. (Pkg. of 5 for about 28 cents) Item 9 - TO-3 Transistor Socket. Digi-Key Cat. #4601K-ND. (Note 2) Item 10-0.1 microfarad, 50 volt, metal film capacitor. Digi-Key Cat. #P4525.

Item 11 - #6 x .50" sheet metal screw. Used 2 places. Item 12 - Heatsink for TO-3 case transistors. Digi-Key Cat. #HS110ND. (Note 2) Item 13 - Mica insulating washer.

Digi-Key Cat. #4662K-ND. (Pkg. of 10 for about 53 cents) (Note 1) Item 14 - Voltage regulator. DigiKey Cat. #LM317K. Item 15 - 500 ohm potentiometer, 1/2" diam., .125" shaft. Digi-Key Cat. #RV6N501-ND. Item 16 - Knob, 1/2" diam., 1/8" shaft. Digi-Key Cat. #8554K-ND. Item 17 -1/8" heat shrink tubing, semi-rigid. Digi-Key Cat. #SR332-2ND. Item 18 - Twisted pair of 22 awg wire.

Construction Notes

threads of installed fasteners to add some vibration proofing. DO NOT glue the terminal strip screws . . . these need to remain "dry" to allow maintenance without twisting screw heads off! Note 5 - Make mounting plate from .062" thick soft aluminum alloy to allow sharp bend at mounting flange. Lay out and drill on holes flat pattern; make bend last. ALTERNATELY Make heatsink plate from -T6 aluminum; it machines better. Then fabricate or purchase some angle brackets to fasten to lower edge mounting in airplane. Hole codes: A = #30 drill (.128") or 1/8"

Note 1 - Coat both sides of mica B = #2 drill (.221") or 15/64" washer (13) with thin coating of siliC = #11 drill (.189") or 3/16" con heat sink compound. Radio Note 6 - Carefully deburr all holes Shack Cat. #276-1372 or equal. Note 2 - Heat sink (12) is optional. (Code B) in immediate contract with If lighting circuit loads do not exceed mica washer (1 3). Tiny burrs will 0.5 amps, heatsink may be deleted. puncture mica and negate insulating When heatsink is NOT used, molded qualities. hole bushings for socket (9) will have Note 7 - After soldering wire (18) to lugs of potentiometer (15), cover to be sanded off so that they do not extend past surface of mounting joints with heat shrink tubing (17). Apply RTV sealant (bathtub caulk plate. Note 3 - Extended screw and sec- smells like vinegar) around lugs for ond nut provides electrical ground additional mechanical support against breakage from vibration. stud connection. Note 4 - Apply some super-glue to NOTE - Failure here is FAIL SAFE -

dimmer goes to full bright if wire breaks. Note 8 - Drill out end holes of heatsink with #2 drill (.221") and deburr. Note 9 - Vendors: Digi-Key, Inc., Thief River Falls, MN, 1-800/3444539. Radio-Shack - Check your local "white" pages. +

Bibliography Written coverage plus on-line consulting services on this and other electrical systems topics are available from the AeroElectric Connection, 6936 Bainbridge Rd., Wichita, KS 67226-1008, phone 316/685-8617. CompuServe I.D. 72770,552. $42 buys all materials in print (1 3 chapters, 5 appendices, 200+ pages) plus one year's subscription to newsletters and the next issue of chapters on specific topics.

Acknowledgements Thanks to Richard Kirkland, EAA 21770, for proofing this article and making some excellent suggestions for improvements.

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