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Best master switch saver?

BruceMe

Well Known Member
I'm handy with electronics and I'm sick of killing my battery because I forget to turn off the master... I know, this is a "millenial problem" like FADEC and glass panels. But it's getting old and a $5 circuit can fix it.

Requirements:

  • Must not accidentally turn off on final after a long flight (simple timer)
  • Must have very-very low current when off (<1pA)
  • Be cheap and reliable
  • Avoid failing-off... ie if it dies, stay on

Design

Use a micro-controller connected to a sensitive vibration detector that resets a one-hour timer. After one hour of near-total-non-movement or 2 seconds of holding the power-switch; then collapse the power supply and de-energy the master solenoid. An LED would indicate "On" when solid, blink when in some state of turning-off.

It would be installed in place of the master switch on an aircraft and could be built using a single LED momentary switch.






Components;


I'll post an updates as I'm building and when it's done.
 
I'm imagining a simple piezo buzzer mounted out of sight behind the panel, and powered by the master bus through the normally-closed side of a standard oil pressure switch.

Same principle as the "Low Oil Pressure" annunciator light in common use. A light can be easy to miss. An annoyingly loud buzzing noise, not so much.
 
Or you could just put a pressure switch under your seat (like cars have for seatbelts), connect it to the master power and a piezo horn. As soon as to get out and the master is still on the horn blows... I'm not sure I would trust the only source for aircraft power on a handful of sensitive electronic parts. IMHO.
 
strobes

I always leave my strobes on - can't miss those when I forget the master.
 
Thanks

Buzzer is simpler and I've had one like that, I've had this idea for a long time. And it's never after a flight that I leave the master on... it's after working on it.
 
+1 strobes on all the time

Never thought I would leave my master switch on. After 20+ years of flying, it finally happened. So now I fall into the group of people that leave the strobes on all the time. When I get out, strobes will be a gentle reminder that I left the master on. Not sure about the master switch on a timer plan. Sounds like a potential point of failure.

+1 strobes
 
car keys

If working on the plane, tape your car keys to the master switch, or hang them on the switch if it's a toggle switch. Can't leave without your car keys!
 
Your circuit operates on the positive side of the master relay. Most are wired with the master switch shorting the negative side to ground:

http://www.aircraftspruce.com/catalog/pdf/11-03161draw.pdf

They are wired that way to prevent the possibility of the unfused master power switch leads shorting to ground. With the ground side being switched, the worst a short will cause is for the master relay will turn on unexpectedly. If you use your circuit, you should at least redesign it to work on the negative side of the relay.

For ground testing, I'm just using a 13.8V supply connected directly to the battery through an Anderson Powerpole connector. It powers everything and acts as a float charger if (when) I forget and leave the master switch on. Alternately, you could plug the power supply into a power outlet (power inlet in this case), and never turn the power switch on.
 
Or buy yourself an iFly740 , it powers up with the master switch and powers down when the master is turned off . Never had a problem since its been on a ram mount there in front of the pilot. And the secondary benefit is that its an excellent VFR gps and includes wx and traffic with a bit extra.
 
The solution I have seen before (other than leaving the strobes on, good idea there) is a simple low-voltage alarm. If the master is on and the bus voltage falls below, say, 13V or thereabouts, you get a light and beeper. It will be off any time the engine is running, and on when the master is ON but the engine is not running.

Of course you don't want to listen to it whine and complain when you first turn the master on. A simple timer will take care of that -- longer than your usual engine start, shorter than it will take you to close up and leave the hangar. It's not completely foolproof, but you'd have to work to accidentally leave the master on.

Hacking one up with a couple of chips (555 and maybe an op-amp or a couple of transistors) would be an interesting evening's project, but writing the code, schematic and PCB layout to do it with a single $1 PIC took me all of an hour or so. I included a provision for a blinky LED, a piezo beeper (some of which are QUITE loud and annoying), and a mute switch that will mute the beeping but not shut off the blinking LED. What I had in mind was a momentary pushbutton with an LED, to provide mute and test functions.
 
I have the avionics cooling fan energized with the master...can't hear it in-flight, but would notice after shut-down if the master were left on. Strobes, as discussed is a great idea. I'd be careful about putting some tricky "system" into that circuit.
 
I'm imagining a simple piezo buzzer mounted out of sight behind the panel, and powered by the master bus through the normally-closed side of a standard oil pressure switch.

Same principle as the "Low Oil Pressure" annunciator light in common use. A light can be easy to miss. An annoyingly loud buzzing noise, not so much.

This is the best solution in my opinion. Super simple, and works well.

Similarly, in a club I was in, we had in our checklist to always leave the rotating beacon ON in our 172. If you left the master switch on, you'd immediately see and hear the beacon when you got out of the airplane. It worked perfectly and we had zero dead batteries on the club plane after adding that one item to the "securing aircraft" checklist.
 
Guys: I solved the problem with the oil pressure switch, available in the aviation isle of your favorite auto parts store, that turns all the red lights and symbols on when you turn the car ignition switch on before starting. I installed the switch on the block Vans supplies to mount oil, fuel and vacuum lines and pressure switches. At the oil pressure line and switch I mounted the no oil pressure switch that is normally closed at no pressure and normally open with 3 psi or more oil pressure. I used it as a ground by running a hot wire from the master switch to a radio shack "bing bong" noise maker then to the no oil pressure ground. Shut down the engine, wait for the oil pressure to drop below 3 psi and presto, the loudest most irritating noise you can imagine until the master switch is turned off. No more $50 battery jumps. Dan from Reno
 
Seems that it would get annoying if you're working on something that needs electrical power in the hangar. And if you put a disabling switch in that device, that'll be the one time you leave the master on.

Or am I missing something?

Dave
 
Seems that it would get annoying if you're working on something that needs electrical power in the hangar. And if you put a disabling switch in that device, that'll be the one time you leave the master on.

Or am I missing something?

Dave
You're right. Great point. I just added a few lines of code to beep every couple of minutes even when muted, just so you don't outsmart yourself while working in the hangar.
 
Check list prior to leaving hangar

Car Keys
Plane Keys
Cold one for the road
Lights off
Alarm Set
Door locked

Check....
 
Hobbs oil pressure sensor on mine lights a bright red "low oil pressure" LED on my panel when engine shut down. Simple, inexpensive, easy to wire.... hard to miss.
 
I'm with Mel. I still wire the alternator light because it works as a master warning light as well. :)

Vic
 
I bought a Genius (that's the name) smart charger. It has a recovery mode useful when the battery gets run down.

I recently ran my battery down to 4 volts because the charger didn't provide enough current when I left the master on while databases were loading, and I thought I could come back the next day. Step one in the recover was to use a dumb charger at low current rate to get the battery voltage up to 8 volts because the Genius would not recharge starting at such a low voltage. Next, I used the Genius charger in recovery mode. Once the battery was up to full voltage, I then ran the battery down to 8 volts again and then repeated the recovery. So instead of a fatally run down battery, mine is good as new again.

And no, I'm not that smart, but my buddy Mike is.
 
Just install an EFIS

Ever since I installed an 10" EFIS, I have not left the master switch on. Simple solution, you should install an EFIS ;)

The only time I have left the Master on was at Oshkosh. A volunteer was walking right up to the plane after shutdown. Instead of finishing my checklist and seeming "rude", I went for the mags to make sure they were off, then I talked to the friendly volunteer who welcomed me to oshkosh. Next thing you know, I was out of the airplane going about my business, but the master was still on. Lesson learned, follow the checklist and apologize later.
 
I still have an electric TC in the panel....... With all that grinding going on, I have never left the master on.
 
I have an "Idiot light" on my alternator. Hard to miss.

I have a similar setup in my RV-4. Canopy Open Light very bright and conspicuous on the upper left of the panel. On anytime the master is on and the canopy is open (microswitch to sense canopy lock rod).

Pretty hard to miss and serves an important dual purpose.

Steve
RV-4
Bearhawk 4PL
V35TC Bonanza
 
Hobbs oil pressure sensor on mine lights a bright red "low oil pressure" LED on my panel when engine shut down.
That's what I did, only I used a blinking red LED next to the master switch. The LED blinks with the master on and no oil pressure.
 
Your circuit operates on the positive side of the master relay...

I believe I've fixed this. It's now a pull-down configuration. I need to test the power sense voltage divider and at that OFF will actually trigger the cascading shutdown, but I think this is it (see the origonal post, it automatically pulls the latest version)
 
Or you could just put a pressure switch under your seat (like cars have for seatbelts), connect it to the master power and a piezo horn. As soon as to get out and the master is still on the horn blows... I'm not sure I would trust the only source for aircraft power on a handful of sensitive electronic parts. IMHO.

Or, as soon as you hit some moderate turbulence that lifts you up off the seat a touch...doesn't sound like a good idea.

I do it the old-fashioned way. A CHECKLIST. Yes, you have to actually use it.
 
I believe I've fixed this. It's now a pull-down configuration. I need to test the power sense voltage divider and at that OFF will actually trigger the cascading shutdown, but I think this is it (see the origonal post, it automatically pulls the latest version)

Your new circuit switches the low side of the solenoid now, but you're still routing an unfused positive wire from the battery to your master pushbutton switch, which is the main thing a standard master switch circuit is trying to avoid.

Would you put a fuse on the power line going to your circuit? If so, it looks like a blown fuse will cause the master solenoid to shut off. A standard master switch circuit is designed to safely avoid putting fuses in the circuit, and also fail in the on state if the one wire going to the switch shorts to anything on the airframe.

And it's never after a flight that I leave the master on... it's after working on it.

Then maybe it would be safer to leave the normal master switch alone, and optimize your circuit to be used only for maintenance purposes.
 
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