HOWTO: DIY Throttle Hold Switch

[dvdouden]

Here's a little howto for adding a throttle hold switch to the Esky transmitter. Although I came up with this idea myself, others have done this before me.

Here's what you need:
-an on-on switch
-a 5K Ohm trim pot (potentiometer)
-some shrink tube
-some wires (I recycled some servo wire)
-soldering iron + solder
-drill
-screwdriver
-wire stripping device
-CA (superglue)

Now before I get to what you need to do, let me first explain which results we want and how we're going to get them.

What we're looking for is a switch that will put the throttle at such a level that the main motor will stop. This will provide an alternative to closing the throttle much faster than by stick and will make sure we don't develop any nasty habits that will come back to us when we progress to CP helicopters with an idle up mode.

To do this, we need to make the transmitter "think" we've moved the throttle stick all the way down. The throttle stick have a potentiometer (pot) underneath them, like the pots on the 4-in-1 for setting up the gyro, but a whole lot bigger. The pot acts as a variable resistor, the amount of resistance (ohms) is determined by the position of the pot (read: throttle stick). This resistance determine how many volts go through the circuit, the amount of volts is measured and determines the value of the throttle channel. My explanation may not be entirely accurate, but I don't know that much about electronics, so please forgive any mistakes, corrections are welcome
What we want to do is add a resistor similar to the resistance the pot has at zero throttle, and then use a switch to switch between the pot and our newly added resistor. We could measure the resistance of the pot at zero throttle, then find ourselves some similar resistor(s) and use those, but it's much easier to get ourselves a second pot with similar values. The advantage of that is that we can play with the settings a bit, always a good thing
There's not much room in the transmitter, so we're going to use a special type of pot for this: a trim pot. These so called trim potentiometers allow for very precise setup. A normal pot will turn just 270 degrees (of which just a small portion is actually used by the transmitter), but a trim pot will turn (depending on the model) about 12 complete revolutions. Also, it won't get out of its position when there's some shocks or vibrations.

Anyway, onto the technical bits. What we want to do is the following:

Image originally from here, adjusted by me to match wire colors and setup.

The blue wire is the one we're interested in, it's the input for the pot, adding a switch to that allows us to switch between the two pots. The other two wires (black and red) are output, we can simply tie the output wires of both pots to each other.

Time to do some hacking!

Step 1:
Determine where you want your switch, I placed it on the right shoulder of the TX, before the trainer switch, because that's where the throttle hold on my Futaba 7CH is located. If you look closely at your TX, you can see that there's already some circles in the plastic, and if you open the TX, you can also see that there are already two holes in the front where switches and trim knobs are located on the King 3 TX for example.

Step 2:
Remove the batteries, open the TX. Measure the diameter of your switch and drill a hole in the location where you want it. Be careful that you don't kill anything (the TX or your hands).

Step 3:
Cut the wires going from the PCB to the throttle stick pot. It's the red/blue/black trio going to the pot in the middle. You can recognize the throttle stick by moving it while looking at the pots Cut the wires somewhere in the middle so you'll have plenty of wire to work with on both ends. Separate the wires from each other and strip the isolation as much as you think you'll need.

Step 4:
Get a new piece of (preferably) red wire, this will go from the red wire you just cut to the trim pot, so make sure it's long enough, best to have a few inches too much than too little Strip the end of that as well. Put some shrink tube (as much as you think you'll need) on one of the red wires, solder all three red wires back together, put the shrink tube around the soldered part and shrink it. Repeat this process for the black wire.

Step 5:
Get a new piece of (preferably) blue wire (I used white), this will go from the blue wire coming from the PCB to the switch. Strip it, put some shrink tube in place and solder it to the blue wire coming from the PCB. Shrink the tube around the solder joint. Solder the other end to the middle leg of the switch, usually indicated by a 2 (add tube to that too if you think it's necessary).

Step 6:
Get a new piece of wire that will go from the switch to the throttle stick pot. Solder it to either the left or right leg of the switch (indicated with a 1 or a 3). Doesn't really matter which one, you can determine how you want to install your switch later on. Solder the other piece to the blue wire coming from the throttle stick pot. Don't forget the tubing. Solder a second piece of wire to the last leg of the switch.

Step 7:
If you did everything correctly, you should have three wires left. One new red wire, one new black wire, and one new blue wire (again, I used a really white-ish shade of blue). These will all go to the trim pot. The blue wire will go to the center leg (indicated with a 2) of the trim pot, the others go on the remaining legs (doesn't really matter how). Solder it all, tube if necessary, and you're done soldering!
Step 8:
Now comes the fun part: setup.
First prepare your Bee. Disconnect the tail rotor and pull the rotor head off.

Put the trim pot around center. Install the batteries in the TX, make sure the throttle stick is at zero and the throttle hold switch is in "normal" mode (see above diagram for orientation), and turn the TX on. Turn the Bee on, check the throttle. Now once you'll flick the throttle switch the Bee will spin up to around hovering speed, adjust the trim pot until the main motor stops, or even a bit below that. Turn the Bee off, set the throttle stick to 50% and turn the Bee back on, it should initialize because you're still in throttle hold. Put the switch back to normal and the Bee should spin up.
If all works as described: congratulations, you've just added a throttle hold switch to your cheap Esky TX!

Step 8:
Install the switch, CA the trim pot, put everything back together (don't forget to reconnect the tail motor) and fly, fly, fly!

Here's some pictures to clear things up:
overall view:


The trimpot CA'd to the TX


Wires from the PCB to throttle stick pot


Installed switch:


End result:

[JettaManDan]

sweet mod! crazy.....

[dayshaddow]

now why the heck couldnt esky read this and add it to all there tx's
sweet professional job there mate

[Viet-Lama]

Originally Posted by dayshaddow

now why the heck couldnt esky read this and add it to all there tx's
sweet professional job there mate

+1 GREAT JOB!

[dvdouden]

Thanks for the replies guys The quality isn't bad, but not superb either, some sloppy soldering here and there, but then again, I'm a programmer, not an electrics guy

The whole mod costs about $4 and 90 minutes to do. Parts should be available at any electronics store (Conrad, Radio Shack, etc).

I don't know what's the usual orientation of a throttle hold switch, currently it's in hold mode when the switch is flicked towards me. But it's easy to change. Just remove it and put it back rotated half a turn

I'll see if I can spare some time to make a small YouTube video of this for those that don't like reading

[Birdkiller]

Great MOd DD!!

[actionscript]

Looks really cool, and can't wait to try it....but ummmmm....what does it do?

[dvdouden]

Originally Posted by actionscript

Looks really cool, and can't wait to try it....but ummmmm....what does it do?

LOL The throttle hold switch will turn your main motor off with just a flick of the switch. It's a feature most of the more expensive transmitters have. First of all it's an emergency switch. Say you're about to crash your Bee and you know it. Best thing to do is get off the throttle completely to minimize damage, so you pull your throttle stick all the way down. Now imagine you've just progressed from Bee to T- Rex (clone) and you're in idle up mode. Zero on the throttle stick now suddenly means 100% throttle and full negative pitch, so your T- Rex is now slamming into the ground at full speed and instead of minimizing damage you did the exact opposite, and you'll be hitting yourself wondering why you've learned that nasty habit .

This is why a TX should have a throttle hold switch. Throttle hold on cp birds cuts the throttle while still allowing control over pitch, so you get to do so called auto rotations. On an fixed pitch bird like our Bees, it should just put throttle at zero, which is exactly what this mod does.

Secondly, it's a safety switch. No accidental bumping into the throttle stick as you bend over to inspect the bird and such (been there) or while you're walking with it (done that) as the throttle stick is completely ignored by this mod while in throttle hold.

Hope that explains it

[actionscript]

I get it, the other day my TX strap got caught on the throttle while I was trying to adjust the gain and the bird took off full speed into the roof of the garage.

Amazingly no damage whatsoever, but it would be great to have an off button.

And I don't crash anymore....so not sure I would ever need it in flight.

[Birdkiller]

Originally Posted by actionscript

I get it, the other day my TX strap got caught on the throttle while I was trying to adjust the gain and the bird took off full speed into the roof of the garage.

Amazingly no damage whatsoever, but it would be great to have an off button.

And I don't crash anymore....so not sure I would ever need it in flight.

[Viet-Lama]

Originally Posted by dvdouden

I don't know what's the usual orientation of a throttle hold switch, currently it's in hold mode when the switch is flicked towards me. But it's easy to change. Just remove it and put it back rotated half a turn

I relocated the "throttle hold" switch on my DX6i and placed it exactly where yours in the photo.
When I fly I rest my pointer finger in front of it and its super easy and natural to push forward to kill the throttle before a crash.

[dvdouden]

Originally Posted by Viet-Lama

I relocated the "throttle hold" switch on my DX6i and placed it exactly where yours in the photo.
When I fly I rest my pointer finger in front of it and its super easy and natural to push forward to kill the throttle before a crash.

If that's the normal way to operate switch then I'm going to have to reverse mine before I get used to it Already got to try the switch at the field, lost half of the tail rotor in the process The worst part is that I'm stuck using the stock tail until my replacement tail rotors (plural) arrive, bye bye tail response

Still have some switches and pots left in my parts storage, now what shall we do with those?

[Birdkiller]

Originally Posted by dvdouden

If that's the normal way to operate switch then I'm going to have to reverse mine before I get used to it Already got to try the switch at the field, lost half of the tail rotor in the process The worst part is that I'm stuck using the stock tail until my replacement tail rotors (plural) arrive, bye bye tail response

Still have some switches and pots left in my parts storage, now what shall we do with those?

No thats not the " normal" way, but very convenient.

So how did you loose half your TR?? were you trying out your new ' switch ' during flight!!??

[dvdouden]

Originally Posted by Birdkiller

No thats not the " normal" way, but very convenient.

So how did you loose half your TR?? were you trying out your new ' switch ' during flight!!??

Was in the air for about a minute, doing hard right rudder, the Bee was in its twentieth or so spin when the tail motor decided to stop completely, ended up hitting the ground while doing three spins per second, tail was the first thing to bite the dirt. Post crash inspection shows nothing strange, tail is working fine except it's slightly out of balance (missing half the rotor).
Pulled the tail boom out and installed my other boom with a stock tail (which I fortunately carry with me in the flight case) and got to fly the rest of the pack without problems.

[Birdkiller]

Originally Posted by dvdouden

Was in the air for about a minute, doing hard right rudder, the Bee was in its twentieth or so spin when the tail motor decided to stop completely, ended up hitting the ground while doing three spins per second, tail was the first thing to bite the dirt. Post crash inspection shows nothing strange, tail is working fine except it's slightly out of balance (missing half the rotor).
Pulled the tail boom out and installed my other boom with a stock tail (which I fortunately carry with me in the flight case) and got to fly the rest of the pack without problems.

Awesome !!! Gotta love the bees!!

[radu7]

Great idea and instructions! Would this mod work on a CP (HB CP3 for example) if you were in idle-up mode? It seems like it would immediately give your heli full throttle at full negative pitch if you flipped it while idle-up was on. If that is the case could you wire the idle-up switch such that it is only active when the throttle hold switch is off? I realize this is the HBFP forum and that idle-up isn't a factor, but thought I'd ask.

[Kmc5019]

Sorry to revive a very old thread but i had a question about this custom addition...

I did this mod to my Honey Bee King 2 CP heli. The mod works great, except that the throttle signal is mixed with collective so when the throttle hold engaged not only removed motor rpm, but also sets the blades to zero pitch. Is there a way to figure out how to parse the collective and rpm signals such that pitch is preserved when RPM is cut with the throttle hold?

[JettaManDan]

Originally Posted by Kmc5019

Sorry to revive a very old thread but i had a question about this custom addition...

I did this mod to my Honey Bee King 2 CP heli. The mod works great, except that the throttle signal is mixed with collective so when the throttle hold engaged not only removed motor rpm, but also sets the blades to zero pitch. Is there a way to figure out how to parse the collective and rpm signals such that pitch is preserved when RPM is cut with the throttle hold?

no not really....even with computer radio's you get the same thing...head goes into negative pitch when the hold is on....

[Luke Warmwater]

Originally Posted by JettaManDan

no not really....even with computer radio's you get the same thing...head goes into negative pitch when the hold is on....

.....unless of course you program a different pitch curve..... By default, on my DX7s, the pitch curve in TH is 0 25 50 75 100....but I can program it to whatever I want!

[HELIHANKSTER]

Originally Posted by Kmc5019

Sorry to revive a very old thread but i had a question about this custom addition...

I did this mod to my Honey Bee King 2 CP heli. The mod works great, except that the throttle signal is mixed with collective so when the throttle hold engaged not only removed motor rpm, but also sets the blades to zero pitch. Is there a way to figure out how to parse the collective and rpm signals such that pitch is preserved when RPM is cut with the throttle hold?

Mmmm…since it’s a King 2, the transmitter your have is probably the 0406A model. There is a switch in the back of the battery compartment that turns off ccpm mixing all together, but that’s not going to help you in this situation. I could be wrong, but being a non-programmable radio, I don’t think you will be able to separate the throttle and swash movement with this radio.

Originally Posted by Luke Warmwater

.....unless of course you program a different pitch curve..... By default, on my DX7s, the pitch curve in TH is 0 25 50 75 100....but I can program it to whatever I want!

….+1 Most programmable radio out there allow you to change/adjust pitch values in all flight modes including throttle Hold. If you think about it, it would be hard to pull off a planned auto if the pitch went to straight negative and stay there when you hit throttle hold.