Tx Mixer mod

[wingnutt]

i came across this idea in another thread (http://www.rcgroups.com/forums/showthread.php?t=392553)
and would like to get a little more input about the theory behind it before actually comitting to tearing my Tx apart.

the mod creates a v-tail or elevon mixer at the Tx by connecting the two potentiometer wiper posts together (through a 5k ohm resistor) of the channels you want to mix. also, if you use a 5k pot instead of the resistor you will have an 'adjustable' mixer.

this makes sense, but i have a few questions.

1. what is the 5k ohm resistor for? is it a buffer to keep from directly coupling the two pots?

2. if you use a pot instead of the resistor to make it adjustable, wouldn't you be directly connecting (shorting) the two wipers together at one end of the pot rotation?

3. when using the pot, what kind of adjustment are you getting? is it a ratio of say rudder travel to to elevator travel for a v-tail mixer?

any help on this would be greatly appreciated.

thx,
wingnutt

[Miami Mike]

I agree with #2, and that gives you the answer to #1. If the two wipers are connected with no resistance between them, and you move the stick such that the two potentiometers are at opposite ends of travel, you'll have a short circuit. I noticed that you pointed this out yourself in this post (http://www.rcgroups.com/forums/showthread.php?p=4064465&postcount=10), and so far "Bare" hasn't answered. I think you're right. I suspect that "Bare" is telling you to do something that he's never tried himself.

[wingnutt]

thx MM, i appreciate the input. after some extra thought on the subject the resistor not only acts as a buffer, but also limits the travel on the servos to half travel for each channel equaling full travel only when the gymbal is moved to a 'corner'. with this in mind i will have to double check the values of the pots already in the gymbals before blindly going with the 5k ohm value.

wingnutt.

[HO-229]

The 5K ohm pot/resistor is a joke.....

I would not perform this mod to your TX I tried it on an old TX & had no luck making it work.
There is info on the forum to correctly mod your TX to do mixing

Best of luck

[Miami Mike]

The 5K ohm pot/resistor is a joke.....Right. I went to great pains to prove that on the Flying Wings forum, but wingnutt seem to be in denial so I gave up.

http://www.rcgroups.com/forums/showthread.php?p=4068750&postcount=16

[Acetronics]

Hi,

See here for serious stuff ... :

http://www.lazygator.n4jsb.net/files/ace/acerctx/11g505.pdf

Alain

[Miami Mike]

Thanks! Just a few minutes before I read your post and looked up that PDF document, I had already come to the conclusion that you'd need some sort of active components, such as op-amps, in order to achieve elevon mixing.

The reason is that you need to impliment same-direction servo rotation from one potentiometer voltage and opposite-direction servo rotation from the other potentiometer voltage, and there's simply no way to do that with a resistor network, no matter how complicated you make it.

[Acetronics]

Hi, Mike

35 years of modelling learnt me a modeller has always something to trim up ... even his plane flies perfect !!!
So let's build trimmable systems !!!... humour !!!

Another good reason is to take extreme care of the existing systems ... and reach predictable results.

Try this resistor mod. on a NE 5044 coder ... and you could easily loose 1 channel from times to times !!! that's written between the lines in the datasheet ...

Alain

[arneansper]

I have added the mixer to TX. Some details are here: http://home.cyber.ee/arne/mixer/mixer.html. Trimming it (especially for flying wing where elevator is extra sensitive) is very tedious. And you must shield it well.

Arne

[HO-229]

Miami Mike,
WOW..... You gave allot of your time in those threads.
I am working to mod my old attack4 TX to have mixing and I came across the correct mod instructions after giving the 5k a try.

[Acetronics]

Hi, Arne

I've noticed the Norton's amps i.e. 3900s drawings are much more difficult to keep away from disturbations ... prefer the 324s schemes.
it's much much easier to place "miracle capacitors" !!!

Alain

[AndyOne]

I tried it on an old TX & had no luck making it work.
Best of luck

In most old transmitter circuits from the days where all descrete components were used, the stick pots were directly used in part of the timing circuit and not as in modern transmitters where they just controlled a DC voltage which is interpreted as a pulse width by the processor. The older style of circuit can't be used with an add on mixer because the pulses appear at the pot so there is no actual DC level to mix.

Andy.

[Acetronics]

In most old transmitter circuits from the days where all descrete components were used, the stick pots were directly used in part of the timing circuit and not as in modern transmitters where they just controlled a DC voltage which is interpreted as a pulse width by the processor. The older style of circuit can't be used with an add on mixer because the pulses appear at the pot so there is no actual DC level to mix.

Andy.

Hi, Andy

The only way here is to have the mixer aboard the plane, between receiver and servos.
as no more analogic mixers ( Sanwa ... or similar build around CA 3086 arrays ) can be bought, PIC's do that really nice ...

I also still have an old scheme with a LM3900 ... for vintage fliers !!!

Alain

[wingnutt]

MiamiMike,

Not so much denial as blind faith. I just figured that if it was published in RCM without any major retraction, that there had to be some value to it and was determined to give it a shot. Tooooo much benefiet of the doubt so to say. this is not to say that I believe EVERYTHING i read.

after some consideration during my flight, you are right. it will take more than a simple resistor to make a mixer.

BTW, Oshkosh was awesome. if you havent been - GO!!!

wingnutt.

[HO-229]

Then inexpensive GWS & Attack4 TX use DC pot voltage signals to the encoder

I know for a fact the 5k pot will not work on these transmitters and/or any other TX of this type for that matter, so the best of luck to anyone who made the 5k pot work.

Acetronics has right... use the 324 Quad OpAmp to do your mixing & forget about the 5k pot.
The mod is not that difficult to perform only requires a few components
Less than $10 for all the components from DigiKey & that is using good quality pots

[AndyOne]

best of luck to anyone who made the 5k pot work.

As Miami Mike said in post #7 adding a resistor between the wipers of the 2 pots simply doesn't work to provide the kind of mixing you need for elevons or V-tail as you need some inversion in the system.

I actually tried it to see what would happen and I can report that both servos move in the same direction and even if you reverse one of the functions on the TX they always both move in the oposite direction QED.

Andy.