In at least one aircraft the stall warning tone was quite loud, so that a gunner towards the rear could easily hear it. The sounding device was listed as a "Klaxon" and there is no way anybody in a small plane could not hear it. Not even with the doors off and the engines at max power! Fortunately, unlike in a 737Max, all that the stall detect system did was give a warning. Actually, that would be a MUCH BETTER system to put in those planes.

 

"If the stall or gear not down alarm goes off, I don’t care if it gets broadcast"
Even if it blocks the frequency for other pilots? Although broadcasting "Stall Stall Stall" or "Landing Gear Landing Gear Landing Gear" on the approach frequency might act as a pseudo CVR to tell everyone why you crashed.

Ken

It would only be broadcast if I were transmitting anyway - so no harm done to others, and I’m sure ATC would rather I lowered the landing gear and avoid crashing on their nice tidy runway that hear what I had to say
True tho’ - it would rather give the game away!!

 

Once again, a very inexpensive isolation audio transformer along with a level setting resistor and possibly a series capacitor will provide a safe way to connect the warning device to the intercom system. providing power so that the internal battery does not run down and fail is another concern, but once the audio connection is isolated the power does not need to be isolated, and so a simple linear regulator would work.
But what is the voltage of the planes electrical system? That matters a bit. AND there is the concern of having the warning device switched on while flying. It only helps when it is powered on.

 

So if I go down the isolating transformer route, what difference does the impedance of the transformer make? (assuming i’m choosing one with a 1:1 ratio). I see ranges from hundreds of ohms through kilo-ohms.
Thanks in advance!

 

Do we have the output impedance of your mp3 device and the headset?

 

Do we have the output impedance of your mp3 device and the headset?

That would be handy, but the data sheet for the mp3 device doesn’t list any output impedance. I think my headset is rated 200-2200 ohms.

 

If you have an oscilloscope you can measure it. Open and loaded. The formula is

Z = (A1*RL/A2) - RL
Where:
Z = Output impedance
A1 = Open circuit voltage amplitude
A2 = Loaded voltage amplitude
RL = Load resistance

Just send a sine wave at 1khz through it

 

I

If you have an oscilloscope you can measure it. Open and loaded. The formula is

Z = (A1*RL/A2) - RL
Where:
Z = Output impedance
A1 = Open circuit voltage amplitude
A2 = Loaded voltage amplitude
RL = Load resistance

Just send a sine wave at 1khz through it

I’ll give that a go when I get a chance - so then I would just need a transformer which approximately matched its impedance? I think the intercom is designed to accept a range of input impedances as it has to deal with a range of headsets.

 

Matching impedance will affect levels (as in too high and distortion or too low and low levels) and affect your frequency response. So putting it simply yes.

 

Haven't seen it mentioned, and perhaps things have changed in 20 years, but back then avionics microphones were carbon mikes. I believe they were rated at 600 ohms.

 

So if I go down the isolating transformer route, what difference does the impedance of the transformer make? (assuming i’m choosing one with a 1:1 ratio). I see ranges from hundreds of ohms through kilo-ohms.
Thanks in advance!

You would need a voltage step down, from the earphone drive level towards a microphone signal level and since the efficiency of the power transfer would not matter in this type of application a transformer with a 500 ohm primary connected to the MP3 player headphone output and a lower impedance connected to the microphone input would work. Of course, this is presuming that the microphone inputs are set up to handle voltages from 10 millivolts up to 100 millivolts, since I have no clues about those particular microphones. A step-down ratio of 10:1 should work. But start out with the player volume turned fairly low.
The transformer can be physically very small, since size relates to power handling ability.

 

Thanks everyone for your help - it gives me a lot to think about and a lot more to learn about! I sincerely hope I never need this project to work in earnest, but I’ll definitely keep working at it!

 

Thanks everyone for your help - it gives me a lot to think about and a lot more to learn about! I sincerely hope I never need this project to work in earnest, but I’ll definitely keep working at it!

The stall detect could be backed up by a pitot tube feeding a pressure switch to warn of low airspeed, and combine that with a switch triggered by the climb angle, and the signal could trigger a Klaxon, and no electronics required. For the landing/wheels thing, a switch on the landing gear plus one to trigger when the flaps were put in the landing position, presuming that happens on your plane, and very simple logic to warn if flaps were down and wheels were not. There you have some independent ways of triggering a warning. At least I think that they should work. I have not designed any aircraft safety systems so there might be some missed details.

 

The stall detect could be backed up by a pitot tube feeding a pressure switch to warn of low airspeed, and combine that with a switch triggered by the climb angle, and the signal could trigger a Klaxon, and no electronics required. For the landing/wheels thing, a switch on the landing gear plus one to trigger when the flaps were put in the landing position, presuming that happens on your plane, and very simple logic to warn if flaps were down and wheels were not. There you have some independent ways of triggering a warning. At least I think that they should work. I have not designed any aircraft safety systems so there might be some missed details.

That’s exactly what the aircraft already has, the problem is that the warning tones sound over a speaker in the cockpit, not over the intercom system. Given that manufacturers work really hard to ensure that headsets block out as much external noise as possible (both passively and by noise cancelling) it’s not surprising the existing alarms are easy to miss.
Modification of any of the electrical systems is a major no-no in aviation (changing a light bulb is about the most advanced thing you’re allowed to do without being a qualified technician) so my project can’t just take electrical inputs from the alarm system already in place - it has to listen out for the existing alarm for its trigger. Rules about connecting extra audio sources to the intercom are less clear. Most modern headsets allow music or phone signals to be included, so I think I’m ok with an audio alarm, as long as it doesn’t fry the intercom!

 

That’s exactly what the aircraft already has, the problem is that the warning tones sound over a speaker in the cockpit, not over the intercom system. Given that manufacturers work really hard to ensure that headsets block out as much external noise as possible (both passively and by noise cancelling) it’s not surprising the existing alarms are easy to miss.
Modification of any of the electrical systems is a major no-no in aviation (changing a light bulb is about the most advanced thing you’re allowed to do without being a qualified technician) so my project can’t just take electrical inputs from the alarm system already in place - it has to listen out for the existing alarm for its trigger. Rules about connecting extra audio sources to the intercom are less clear. Most modern headsets allow music or phone signals to be included, so I think I’m ok with an audio alarm, as long as it doesn’t fry the intercom!

OK, now it is clear. The only private aircraft that I have been offered a ride in was not airworthy and so I declined the offer.
The isolation transformer will work, but the volume control will need adjustments, probably. And I am wondering still about what can be done for powering the MP3 device. Some connection to the power system will be needed.

 

I believe the DFPlayer has onboard volume adjustment. Does the plane have a plug for charging phones? You could pull line to a regulator.

 

OK, now it is clear. The only private aircraft that I have been offered a ride in was not airworthy and so I declined the offer.
The isolation transformer will work, but the volume control will need adjustments, probably. And I am wondering still about what can be done for powering the MP3 device. Some connection to the power system will be needed.

My current solution (no pun intended) is to plug the device in to a usb charger which sits in the cigarette lighter socket - there’s also a backup battery in case it doesn’t get plugged in, in which case it reports its battery status every half hour over the intercom, and also when it’s powering down because the battery is too low.