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Jan 20, 2007, 02:18 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Mini-HowTo

Modifying an HS-81 servo for 180 degree rotation


For several applications, including FPV flying, it is advantageous to have a servo that can rotate through 180 degrees. Typical servos only rotate about 90 degrees though. One can use linkage systems to double the "throw" of the servo. But in cases where this is not possible (direct drive for example), the servo either needs to be modified, or a "doubler" device needs to be installed. I've used one of these doublers, and while it worked good, I couldn't get 180 degrees (due to a mechanical limit of the servo). I also wanted to reduce the clutter inside my plane. So removing the doubler not only cleared up some space and extra wire, but also reduced the total weight.

So here is how you modify an HS81 servo for 180 degree operation. First to know, is that the servo IS capable of mechanically moving through 180 degrees, but there are two modifications necessary to make this possible. The first involves removing a mechanical end stop, or limiting tab located inside the upper housing. This is there to protect the electronics inside the servo, should it try to move more than the normal amount. However, this usually limits the travel to somewhere around 150 degrees. Second involves soldering small resistors to the potentiometer inside the case.

Start by removing any servo horn attached, and then the 4 screws located at the bottom of the case. Once these are removed, you can pull the servo apart.
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Jan 20, 2007, 02:21 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Take the upper cover, and turn it over. You should be able to see a small tab that the output shaft will press against when turned too far. This is the mechanical stop. Using a sharp X-acto blade, remove the stop, being careful not to cut into anything else. In particular, if the output hole is marred, this will interfere with the output shaft and affect the performance of the servo.
Jan 20, 2007, 02:22 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Once that's done, the next modification requires a little bit of soldering. Inside the case, remove the screw holding the potentiometer (or pot) in place. Push the pot out of the case, careful not to damage any of the wires. You will see three connections to the pot... red, yellow, and green wires. We want to install two 2K Ohm resistors, one on each outer leg of the pot, in series with the wires. What this does is trick the servo into thinking it needs to rotate more, effectively doubling the travel. Keep in mind, your mileage may vary. 2K Ohm resistors work for me. They may not in all cases though, so some experimentation may be required. I used 1/8 watt 1% resistors. These have low drift, so there hopefully will not be much change from varying temperature swings. 1/4 watt can be used, but 1/8 watt will be easier. There's not much room in there to work with, so the smaller the better. What I do is remove the two outer wires, bend the tab over so it's flat, and then solder the resistor and wire on. This keeps everything tucked as far in as possible. Remember, do one side at a time, so you don't forget which wire goes where!
Jan 20, 2007, 02:25 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Re-install the pot and screw into the case being careful again not to damage anything. Before we put the servo back together, it is recommended that you mark the output shaft relative to center position of the pot. The pot has more than 180 degree of travel, but not by a whole lot. It's VERY IMPORTANT that you not over-extend the range of the pot. In testing, I did this by accident and wiped out a nylon gear. The output gear locks into a tab on the output shaft on the pot. And this is very easy to damage if you go too far. You can also damage the wiper inside the pot. In the below pictures, I reinstalled the output gear on the pot to show you how much travel you typically have. It does not give you too much leeway on either side of 180. Especially since the servo horn does not sit square with the servo when the pot is centered. So to make sure you know exactly where the position of the pot is, mark the top of the output shaft.
Jan 20, 2007, 02:29 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
After you're done, put the servo back together (don't forget to lube the gears), and hook it up on the bench for final adjustments. The travel limits in the transmitter need to be set. A computer radio is highly recommended here. Start with normal travel limits, and SLOWLY rotate the servo 90 degrees each way. If it looks like you're going to go past that, STOP and reduce the travel. Continue slowly until you have achieved exactly 90 degrees each way. If you feel ANY resistance to rotation, STOP and determine the cause. Remember, when the pot is centered in it's travel, the servo horn is actually offset to one side due to the way the output shaft spline is molded. This means that you will have less clearance in one direction than the other. This is very important to remember.

Back to the end point adjustment... In my particular testing with a Futaba 9C, I ended up with end points of 99/95% and a sub-trim (for proper centering) of -25. Again, your mileage may vary, and it can also depend on which spline offset you go with for the control horn.
Jan 20, 2007, 02:31 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
One thing to look out for. Do NOT use the metal gear version for 180 degree operation. I know it would be nice to have the comfort of metal gears, but the reason for not using the metal gear HS81 is clear in the pictures below. The gear teeth do not continue all the ways around the gear. When the servo is rotated 90 degrees to either side, the output gear is dangerously close to binding or loosing contact with the other gear. The nylon gear however, has teeth all the ways around, so there is no problem with this one.
Last edited by twinturbostang; Jan 20, 2007 at 03:02 AM.
Jan 20, 2007, 02:39 AM
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Mangus's Avatar
Excellent, thanks! If I may ask, where did you get the 2K resistors? Radio Snatch only has 1k and 2.2k (2 x 1k would work, but there isn't much space).
Jan 20, 2007, 02:44 AM
Fly FPV, sleep; repeat
twinturbostang's Avatar
Walla! You're done. Here's a quick video showing it in action.

BTW, AnthonyRC has a guide to modifying an HS-85 servo here. If you are in need of something with more torque and a ball bearing, I suggest using that one. However, the HS-85 is a little bit heavier, a little larger, and costs twice as much. So choose what's important to you and then purchase the appropriate servo for the job.
Last edited by twinturbostang; Jan 20, 2007 at 03:00 AM.
Jan 20, 2007, 02:55 AM
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twinturbostang's Avatar
Quote:
Originally Posted by Mangus
Excellent, thanks! If I may ask, where did you get the 2K resistors? Radio Snatch only has 1k and 2.2k (2 x 1k would work, but there isn't much space).
I got mine from an electrical engineer at work for the friendly discount of free. But many of the online stores should carry a large assortment of resistors. Digikey is one possible place. The 2.2K resistors might work for you, so you might try them before purchasing any new ones. However, proceed carefully and make sure you don't over extend the pot. I was warned against doing that, and I accidentally did it anyways! lol As mentioned above though, I didn't break the pot, but instead tore up the output gear. It's only nylon, so when forced, the hole that the pot shaft goes into strips easily.
Jan 20, 2007, 03:08 PM
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Mangus's Avatar
Quote:
Originally Posted by twinturbostang
I got mine from an electrical engineer at work for the friendly discount of free. But many of the online stores should carry a large assortment of resistors. Digikey is one possible place. The 2.2K resistors might work for you, so you might try them before purchasing any new ones. However, proceed carefully and make sure you don't over extend the pot. I was warned against doing that, and I accidentally did it anyways! lol As mentioned above though, I didn't break the pot, but instead tore up the output gear. It's only nylon, so when forced, the hole that the pot shaft goes into strips easily.
Digikey most certainly has them (or Mouser, or Jameco, etc), but the problem there is the S/H for $1 worth of electronics.

I think there's an electronics parts supplier around here somewhere. Either way, I'll be trying the 2.2k resistors first.

Thanks again!
Jan 22, 2007, 08:18 AM
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al_b's Avatar
Can somebody please confirm the colored bands on the resistor? I have a bag of resistors, but just can't make out what the colors on the 2K are in the photo so I can pick out the right one.

Thanks.

Al.
Jan 22, 2007, 09:01 AM
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Mangus's Avatar
Quote:
Originally Posted by al_b
Can somebody please confirm the colored bands on the resistor? I have a bag of resistors, but just can't make out what the colors on the 2K are in the photo so I can pick out the right one.

Thanks.

Al.
http://www.diyguitarist.com/Images/R...ColorCodes.jpg
Jan 22, 2007, 09:41 AM
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al_b's Avatar
Thanks, but that doesn't really help me. What are the colors on the resistor in the photograph?

Al.
Jan 22, 2007, 10:03 AM
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Mangus's Avatar
Quote:
Originally Posted by al_b
Thanks, but that doesn't really help me. What are the colors on the resistor in the photograph?

Al.
Sure it helps you. Look closer.

Each color on the resistor represents a digit of its value. In the image I attached, it shows you what each color means (from 0-9). The top resistor in the image: the first color is yellow. Looking at the chart, yellow = 4. The second color is purple. Purple = 7. The third color is the multiplier and is orange. Orange = x 1000. The second-to-last color stripe on a resistor (regardless of the number of stripes) is always the multiplier.

47 x 1000 = 47,000, or 47 Kohm.

The last color stripe is always the tolerance. Gold means +/- 5%, meaning the actual measured resistor value will fall in the range of 49,350 and 44,761 ohms.

-M
Jan 22, 2007, 10:06 AM
Registered User
Mangus's Avatar
I"ll go ahead and give you the answer as another example:

2k resistor = Red (2), black (0) red (x100), and (preferrably) Gold (tolerance).

Hopefully that chart makes more sense now.


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