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Trim Selector Switch & Indicator

GalinHdz

GalinHdz

Well Known Member
Back in 2012 when I designed my all glass instrument panel, I thought I had covered every possible situation you could encounter while flying. I was quickly proven wrong when I did my first flight with my wife. As we flew along she rested her hand on the co-pilot joystick, and since both joysticks have trim control, she accidentally changed the way the airplane was trimmed. Since I fly actual IMC, this was not part of an acceptable design to me. So I re-wired the trim circuit and installed a trim selector switch. This way the trim can only be controlled by the pilot flying while the other trim is inactive. This setup worked to perfection, until last month.

I was flying with a pilot friend who wanted to do some practice approaches. He flew the airplane for about 1hr then it was my turn to do some approaches. But no matter how hard I tried I was fighting the airplane and could not keep it where I wanted. After about 1/2hr I realized the trim switch was still set to co-pilot so no matter how much I moved the trim, nothing was happening. As soon as I turned the switch to pilot, I could properly trim the airplane and everything fell in to place. I realized that unless I looked down at the selector switch I couldn’t tell who had control of the trim. The last thing I want to do while flying actual IMC is to be looking down to check the trim selector switch position. Again this was not part of an acceptable design to me. So I went back to the drawing board to come up with an acceptable solution. The process is too long to post so here is a link to a .pdf file that details my "solution".

Trim Control Selector Switch & Indicator

YMMV

:cool:
 
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From your notes I think you mis-understood how leds work. If you still have questions I can explain further.

Just curious why you didn't just connect the pilot and co-pilot switches in parallel? Then whoever is activating the trim would automatically have control.
 
rapid_ascent said:
From your notes I think you mis-understood how leds work. If you still have questions I can explain further.

Just curious why you didn't just connect the pilot and co-pilot switches in parallel? Then whoever is activating the trim would automatically have control.
Click to expand...

If you parallel the switches, then if one pilot commands nose down while the other pilot commands nose up, you?ll blow the fuse or CB.
 
safety

Interesting idea and implementation. Just curious if some auditory or tactile feedback would also be useful in the case where you are trying to trim but didn't have control. Something like one of these in the stick:

https://www.robotshop.com/uk/miniature-vibrating-motors.html

or perhaps a sound file that plays a message like "Trim Control on other stick" using something like this:

http://www.electronics123.com/shop/...-module-for-toys-gift-boxes-9791?category=322

Personally, I'd avoid red lights unless they are indicating something bad.
 
rapid_ascent said:
From your notes I think you mis-understood how leds work. If you still have questions I can explain further.
Click to expand...
Being an EE I have a good understanding of how semiconductors work. I used "voltage" in my write up since it is how the majority of the non EE people think. But thanks for the offer, I can always learn more.

rapid_ascent said:
Just curious why you didn't just connect the pilot and co-pilot switches in parallel? Then whoever is activating the trim would automatically have control.
Click to expand...
That is exactly how I had the circuit originally configured but having both persons with trim control is what I wanted to prevent.

:cool:
 
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rv8ch said:
Interesting idea and implementation. Just curious if some auditory or tactile feedback would also be useful in the case where you are trying to trim but didn't have control. Something like one of these in the stick
Click to expand...
The "tactile" feedback is that the airplane doesn't trim no matter how much you try so the stick gets very hard to move. But, at least in my case, that didn't prove adequate.

rv8ch said:
Personally, I'd avoid red lights unless they are indicating something bad.
Click to expand...
I agree. Not being able to trim the airplane while on an instrument approach in actual IMC qualifies as "something bad" to me. YMMV

:cool:
 
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Use a switch to select who is in charge

I used Infinity grips for pilot and co-pilot. I put a DPDT toggle switch on the panel that provides the common ground path for the stick trim hat and flap switch. The switch positions places control with the pilot or co-pilot, never both.

Reasons:
- Did not want to have a co-pilot (especially a non-pilot) messing with trim or flaps unless I specifically allowed it.
- Wanted to have a way to make the co-pilot seat kid safe.
- Wanted a backup if I broke off the pilot trim or flap switches (in this case the pilot reaches over to the co-pilot stick).

Simple and effective.
Carl
 
I have a switch similar to Carl's where I can disable the co-pilot stick functions by opening up the ground to the co-pilot stick. This stops the passenger from hiting something by mistake. I'm not too worried about the 2 pilot case.
 
I have a switch similar to Carl's where I can disable the co-pilot stick functions by opening up the ground to the co-pilot stick. This stops the passenger from hiting something by mistake. I'm not too worried about the 2 pilot case.
 
seems a bit of overkill when a couple of 2 cent resistors would have worked fine. but to each his own. great looking panel by the way.

bob burns
 
Trim

Setting up your plane the way you want it is one of the best parts of EAB world. My solution is to have trim switches on the pilots stick only and redundant back ups on the panel where either seat can use.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Carl Froehlich said:
I used Infinity grips for pilot and co-pilot. I put a DPDT toggle switch on the panel that provides the common ground path for the stick trim hat and flap switch. The switch positions places control with the pilot or co-pilot, never both.

Reasons:
- Did not want to have a co-pilot (especially a non-pilot) messing with trim or flaps unless I specifically allowed it.
- Wanted to have a way to make the co-pilot seat kid safe.
- Wanted a backup if I broke off the pilot trim or flap switches (in this case the pilot reaches over to the co-pilot stick).

Simple and effective.
Carl
Click to expand...
If you notice that is exactly what the panel switch does and for the same reasons. It connects the ground to the pilot flying while disconnecting the ground for the pilot not flying. The only thing I added now was a switch position indicator. Like you say, simple and effective.
n82rb said:
seems a bit of overkill when a couple of 2 cent resistors would have worked fine. but to each his own. great looking panel by the way.

bob burns
Click to expand...
I am curious. How would you do it with a couple of resistors?

:cool:
 
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Carl Froehlich said:
I used Infinity grips for pilot and co-pilot. I put a DPDT toggle switch on the panel that provides the common ground path for the stick trim hat and flap switch. The switch positions places control with the pilot or co-pilot, never both.

Reasons:
- Did not want to have a co-pilot (especially a non-pilot) messing with trim or flaps unless I specifically allowed it.
- Wanted to have a way to make the co-pilot seat kid safe.
- Wanted a backup if I broke off the pilot trim or flap switches (in this case the pilot reaches over to the co-pilot stick).

Simple and effective.
Carl
Click to expand...

I did the exact same thing with my infinity grips. P or CP
 
BobTurner said:
I think this was in reference to powering the LEDs off 12 volts.
Click to expand...
That is the only thing I could imagine but I could be wrong.

The reason I used this DC - DC converter is that, unlike regular LED's, Bi Color LED's are very sensitive to voltage variations. Trying to provide 2.2v with no more than about a 0.2v difference is almost impossible with resistors and a typical aviation alternator. So this device is not only works as a voltage converter but a very accurate voltage regulator. And it cost less than $8.00.

:cool:
 
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I would wire it with the pilot always active and the co-pilot switched. While you could have the first scenario again, you could never have the second. This seems like a better solution with reduced consequences for forgetting to place the switch in the proper position. The pilot will always be able to deal with an untrimmed condition and only the co-pilot could experience a problem.

I am sure you know, but relays should be used so that only one switch can be active and controlling at any time (wired so Pilot switch can interupt the co-pilot curcuit. This eliminates that risk of a short.

Larry
 
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rapid_ascent said:
I have a switch similar to Carl's where I can disable the co-pilot stick functions by opening up the ground to the co-pilot stick. This stops the passenger from hiting something by mistake. I'm not too worried about the 2 pilot case.
Click to expand...

I did what you did Ray.

Installed (in my case 2) switch(s) that enable or disable the GIB stick buttons.

I installed 2 so I can enable the small stuff - pitch and roll trim with one switch for the novice pax and another switch for those who fly - Starter, Fuel pump, A/P Disc and Flaps.

I wasn’t “thinking” whilst building and in the beginning wired the rear MIC trigger button to those switches...:confused: Well....once I started getting ready for first flight I had a nice ground loop I induced :mad:

I removed that wiring and just figured that MIC button was safe to stay hot. I mean, if someone is playing MAV back there and going to guns....I’ll hear them clicking away and then stop them!
 
Galin,

I think the reference to a couple of resistors has to do with how LEDs are driven. LEDs are driven normally by current and are current limited. The LED voltage that you are referring to is at a specific current. So... You can run LEDs off of 12V you just need to add a resistor in series. That resistor have a voltage drop across it, 12V - Vled. The resistor is selected by setting the desired current. R = (12V - Vled) / I(led drive current).
 
rapid_ascent said:
Galin,

I think the reference to a couple of resistors has to do with how LEDs are driven. LEDs are driven normally by current and are current limited. The LED voltage that you are referring to is at a specific current. So... You can run LEDs off of 12V you just need to add a resistor in series. That resistor have a voltage drop across it, 12V - Vled. The resistor is selected by setting the desired current. R = (12V - Vled) / I(led drive current).
Click to expand...
In the typical airplane circuit you can't expect to control voltage within the tolerances of these kinds of LED's with just a resistor. Since the airplane electrical circuitry can't remain within tolerances, then you have to design for this. In all cases, if you exceed the maximum breakdown voltage your current goes into runaway and you fry the semiconductor. Bi Color LED's are much more sensitive to voltage variations than typical LED's so resistors are not a good choice for this particular application.

You can see how current is affected by voltage variations to this LED on this Bi Color LED Spec Sheet. You need about 2.0v to get them both to work but once you get beyond about 2.3v, your current goes into runaway and your LED goes "POOF". Interesting enough in this particular case the RED LED goes "POOF" way before the GREEN one does.

Remember a resistor will let current go through based on the voltage it sees. Less current will flow through a resistor at 12.0v than will pass trough the same resistor at 14.4v. In this particular application, the voltage difference(s) in typical airplane circuits are outside this LED's tolerance of about 0.3v. Think of the converter/regulator as a very accurate and fast variable resistor and you will get what I mean.

We can do heavy discussion about this but it is irrelevant to this situation. Just keep the voltage within tolerance and everything else falls into place. That is why I opted for the $8.00 converter/regulator in this particular application not a resistor.

And yes "POOF" is a highly technical term. :D
 
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GalinHdz said:
If you notice that is exactly what the panel switch does and for the same reasons. It connects the ground to the pilot flying while disconnecting the ground for the pilot not flying. The only thing I added now was a switch position indicator. Like you say, simple and effective.

I am curious. How would you do it with a couple of resistors?

:cool:
Click to expand...

actually since you are using bi-colored leds i would spend and extra buck for a 7800 series regulator to get a stable voltage supply and then go with a current limiting resistor.

bob
 
Just to be "Cheeky", ;), my solution would be to have a manual trim knob in between the seats that either pilot could reach, but only one pilot at a time could operate..........

Sorry, couldn't resist! :p
 
The resistor solution works fine if you set the resistor value to limit the current appropriately at full regulated voltage. The LED will be somewhat dimmer at a lower voltage. As an aside, the LED "dimness" could also serve as a warning that the buss voltage is dropping for a poor mans charging system monitor.

To be a little more elegant a linier voltage regulator such as this one could be used at $2 each and small enough to be part of the wire harness,
https://www.digikey.com/product-det...LM1085IT-5.0-NOPB/LM1085IT-5.0-NOPB-ND/363564
to first regulate the voltage to 5V and then a fixed resistor to set the forward voltage. Or just choose a linier regulator with a 2V output. They are out there. I'm just too lazy to look up the part number. This way voltage changes, won't change the brightness.

One further refinement is to add a pot into the circuit to dim the indicators for night flying. Pots are not linier as far as light output, but do a nice job dimming LEDs. If the right range is chosen, full bright/dim will be the extremes of the pot. (Or just add a switch to insert a fixed resistor for bright/dim control)
 
Easy fix

For those of you who are not electrical engineers and have a G3X touch system; Garmin makes the GDL 27. It takes care of prioritizing the pilot over the co-pilot switch inputs. For guys like me who have NO electrical savvy, this is of course the way to go, although pricey. It does a bunch of other stuff too and makes a newbie builder very happy.
 
leok said:
To be a little more elegant a linier voltage regulator such as this one could be used at $2 each and small enough to be part of the wire harness,
https://www.digikey.com/product-det...LM1085IT-5.0-NOPB/LM1085IT-5.0-NOPB-ND/363564
to first regulate the voltage to 5V and then a fixed resistor to set the forward voltage. Or just choose a linier regulator with a 2V output. They are out there. I'm just too lazy to look up the part number. This way voltage changes, won't change the brightness.

One further refinement is to add a pot into the circuit to dim the indicators for night flying. Pots are not linier as far as light output, but do a nice job dimming LEDs. If the right range is chosen, full bright/dim will be the extremes of the pot. (Or just add a switch to insert a fixed resistor for bright/dim control)
Click to expand...
That is basically what I did but I went with a small adjustable linear regulator so I could set a specific voltage based on the LED I selected. I actually set the voltage at 2.17v. At 2.17v both colors show up perfectly and they are dimmed enough that I don't have to worry about night flying or burning them out. Other than that, your and my approach to the situation are the same.

:cool:
 
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GalinHdz said:
That is basically what I did but I went with a small adjustable linear regulator so I could set a specific voltage based on the LED I selected. I actually set the voltage at 2.17v. At 2.17v both colors show up perfectly and they are dimmed enough that I don't have to worry about night flying or burning them out. Other than that, your and my approach to the situation are the same.

:cool:
Click to expand...

No problem, didn't mean to imply your solution was any different.

My response was for those who don't know much about linear regulators. They are available in adjustable form where a few added components fix the output voltage. This requires a small circuit board to connect everything. That appears to be what you have with an adjustable resistor to set output voltage.

They also come in fixed output where the "extra components" are in the IC. For these an additional resistor can be used in series to make the final adjustments.

For low current applications like the LED indicator lights no heat sink is needed. The package is very small. The electronics are very simple. If dimming is a goal, an added pot will do the job nicely.

I have two 5V linear regulators in my system. One drives the Arduino which controls the heater servos. The other provides current to a small MP3 player. The MP3 player draws more current and so required a small heat sink to support the output.

In any case, nice application of current regulation and LED indicators in your aircraft. :D
 
Oh no, I didn't take it as negative at all. There are more than one way to skin a cat and we sure skinned this one.

;)
 
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