Bob's Shop Notes:
Smart (Current Sensing) Annunciation . . .

A number of builders have asked about adding annunciator lights to indicate when various systems are in the ON state. The first thought that comes to mind is to simply connect a lamp to the output side of the controlling switch . . . switch ON, light ON.

Suppose you could build a device that not only shows that the switch is ON, but that the device controlled by that switch is really drawing current? This could be an important component of the annunciator indication . . . not only is power being applied to the switched device, that device is intact and drawing current. By sensing both voltage and current, the annunciator lamp can be made to stay dark if, for example, a navigation light bulb is burned out, a landing light is shot, or a pitot heater is burned out.

Many of you may be aware of modern automotive designs that use sophisticated, integrated circuits to sense accessory current and operate the annunciator lamp. These are neat chips, easy to obtain and use . . . provided you're reasonably gifted with the use of a soldering iron and perhaps etched circuit board layouts. The technique I'm going to describe uses a simple magnetic sensing device called a "reed switch"

Click here for larger image A visit with camera in hand to a production line of certified piston aircraft allowed me to record and share a fabrication technique for soft penetration. This technique has a long history of laboratory testing for effectiveness, production line convenience, and field maintainability. In this case, all of the wiring comes through a single penetration fitting . . . but there's no reason why multiple, smaller fittings wouldn't work too . . .

Here we see how a stainless weldment bolted to the firewall with steel hardware provides the structural component of a transition for wires and other relatively "soft" materials that need to run from cockpit to engine compartment. Note generous flange area outside the tube to flange interface that is sealed with fire-stop when the flange is bolted into place.

Click here for larger image The fittings for this airplane are made from 0.050" stainless. Thickness and attaching geometry are a function of how much support the fitting needs to provide for the bundle of transitioning wires and tubes. The material shown here is pretty hefty stuff and may have been selected as handier to weld than thinner material.

Builders can certainly experiment with thinner material and alternative joining techniques. Periodic inspections will show whether or not there are issues of mechanical robustness . . . not strong enough, they'll simply come apart. Given that fires are VERY rare, the failure of an experimental fitting doesn't represent a great threat as long as you do reasonably complete inspections during normal P/M activities . . . like every oil change. The worst thing that happens is that you have to build a more robust transition fitting and replace the broken one.

Click here for larger image Looking up the business end of the finished transition. What's not visible in this view is the packing placed around wires so that the second hose clamp doesn't have to put a super-crush on the fire sleeve . . . . more on this later.

Click here for larger image The flight-ready firewall penetration. A second hose clamp brings the fire sleeve down for a snug fit on the wire bundle. If necessary, a filler around the wire bundle makes for a better seal with less crush under the second hose clamp . . .

Click here for larger image On another airplane, we find a similar technique except that the stainless steel firewall fitting is straight, no 90-degree bend. Otherwise, installation and functionality is same as shown above.

Click here for larger image This view illustrates an interesting packing material used to build up the wire bundle size. A piece of fire sleeve was cut down the side and made into a kind of wrapping tape. Note that all exposed edges of the fire sleeve are "doped" with the recommended sealer to preclude entry of moisture and to keep the edges from fraying . . .

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