Courtesy…those little white lights that stay on for a while after you shut the ignition switch off in your automobile? Somehow that common courtesy never translated across into airplanes. And yet, getting out of an airplane in the dark is no more fun than getting out of your automobile in the dark.
Once again, this is a reader suggestion that we come up with a simple solution for the problem of darkness outside the doors on your pretty homebuilt airplane. It is really a quite simple problem if you understand the whys and wherefores of the simple 555 timer chip.
Honestly, this is one of those little “extras” that make homebuilding an aircraft worthwhile. Not even the heavy iron amongst us (Cirrus, twin Beech, Cessna…none of them) have this little gem. The delightful thing is that it is so easy to build and construct.
Again, a trip back in time. Back in the days when dinosaurs were roaming the earth (around 1970 or so), we needed a timer chip that would be a “do all, do everything” part. Hans Camenzind at Signetics Corporation designed this wonderful little part that takes the place of 23 transistors, 15 resistors, and two diodes. To date, there have been about 30 billion of these little jewels produced in the last 45 years.
You won’t find courtesy lights in most certified aircraft, but you can add them to your Experimental by building this circuit.
You can operate the 555 in about five different modes, but the two that are the most common are the “monostable” and the “astable.”
The astable, the one we are not going to use comes in handy when we need a fairly inexpensive and reliable clock to keep time. Think of a music metronome going “tick-tock-tick-tock…” and by means of a single control (potentiometer), we can make it go faster or slower; so long as it has power, it just sits there and toggles back and forth forever. (Astable [a-stable] means “away from stable,” or continuously in motion.)
The monostable mode is the one we are going to use. Monostable means that it has only one stable state, and if forced into a “tick,” it will soon revert to a “tock” and stay there until forced to move again by some other signal—say, like a signal that the master switch has been turned off and we are ready to depart the airplane.
So, if the master is turned off, and the chip is forced to “tick”, then we can use a transistor or two to make that tick turn on the courtesy lights. Then, sometime later, we can turn off those same lights when the chip returns to “tock.” (“Ve haf vays of making you tock.”)
Most of us in the engineering design field find ways of “diddling” (ahem, heuristically engineering) a circuit after we do the paper design to make it just a little bit better—this one, not so much. From my first hen-scratching on the tablet a month ago, through the pretty design on the computer schematic drafter today, I changed not one part or value. It is one of those rare birds that just fell into place all at once. Maybe, after doing this for nearly 60 years, I’m learning how to do it right.
There are two factoids that make this whole design work:
• FAR Part 23 (the rules for normal/utility/aerobatic light aircraft, which don’t directly govern homebuilt design, but is good practice anyway) says that you can have up to 5 amps of draw directly from the battery without going through the master switch or master relay (23.1361.b.3). This is commonly called a “keep-alive” circuit because it keeps the clock running, the frequency memory for the radios, and other things that need to be kept going at all times.
• When you turn off the master switch, it disconnects the battery from the entire electrical system with the exception of the 5 amps mentioned above from keep-alive.
However, all circuits connected to the keep-alive breaker should not draw much current or you will wind up with a dead battery every time you go out to fly your airplane. This one that I show you draws about 600 micro amperes (600 μA or 0.6 mA) when the light shuts off. To put this in perspective, your 35 amp-hour battery will power this circuit for just about 60-thousand hours, or a little less than seven years.
On the other hand, the components chosen (especially Q101) will handle lamp loads of up to 1 amp without any special handling; you can easily go to 2 amps of lamps with even a minimal amount of heat sinking.
Another design note worth considering…the original 555 bipolar transistor design has ten times the resting (quiescent) current of the CMOS part used in this circuit. The price of the bipolar versus the CMOS part is 40¢ versus 80¢ and not worth worrying about. Similarly, the LM358 is a “popcorn part” and is widely available at about 50¢ each.
One thing not to be worried about is Q101. ANY NPN bipolar power transistor can be used here without any changes in the circuit.
There may be some question about why I included the small LM358 op-amp in the circuit and didn’t simply use the negative trigger pulse from the power-on bus to start the 555 timing. The answer is that a lot of small motors (the turn and bank, the electric horizon) and capacitors (every radio on line) will not create an instantaneous sharp pulse at Trg on the 555 timer. By using the op-amp in the comparator mode, this slow descent from on to off becomes a sharp pulse and is then used to trigger the 555 on.
The amount of time that the courtesy light stays on is determined by the time constant of R101 and C101. If you multiply them together to get the time constant, you find that they come up with a constant of 100 seconds, which is midway between one and two minutes. You want a shorter or longer time? Ei-ther decrease or increase (respectively) the value of C101. You could also vary the value of R101, but getting stable resistor values above one megohm is somewhat difficult.
S1 was almost an afterthought, as well as switching diodes D101/102. The question was asked, “How do you turn the lights on manually?” The answer is that Q101 will turn on when either the 555 is providing the driving current through D102 or the switch provides the current through D101.
Ledengin makes a really neat line of 5-watt white LEDS for less than $10 a pop, and you have your choice of three colors of white: warm, neutral, and cool (or if you prefer color temperature °K, 3100, 4100, and 5500 respectively). They run on exactly 1 amp of current (each), so if you want courtesy on both wings, heat sink Q101 and get two of the little rascals (www.mouser.com is a distributor). They only require about 3 volts at an amp, so if you want to do a series string, you will be under an amp for both.
If you really want to get spiffy, for about $15 or $30 you can have 10- and 20-watt lamps respectively at 0.7 amps.
Now, the clever amongst you are noticing that I only used half of U101…there is another complete amplifier on this chip and you know me…if there is another unused device, I can’t leave well enough alone. Expect to see a modification to this circuit as an addendum to a future article as soon as I can figure out a useful circuit for the remaining op-amp. Suggestions?
As if last month’s ground plane aircraft COM band antenna wasn’t enough, one of you wrote in and asked if I could do a simple and cheap NAV band antenna the same way. Well, yes…and no. Next time, I promise. Stay tuned.
Jim Weir is the chief avioniker at RST Engineering. He answers avionics questions in the Maintenance Bay forum at www.pilotsofamerica.com. His wife, Cyndi Weir, was his high school sweetheart 50 years ago and now she keeps Jim from making stupid blunders in spelling and grammar. Check out www.rst-engr.com/kitplanes for previous articles and supplements.