convert/modify standard servo to retract

[3drc tim]

Hey guys,

Just checking in here before I start what should be a simple project. I have a new bird that needs three retract servos. I have three standard servos that put out over 51 oz./in. and they should work fine (one per retract only.)
These are Hitec BB 325 servos with "karbo" gears.

I have searched and searched and there seems to be but one article on making a 180 degree retract servo, and it is a Futaba 148. There are thousands of articles on how to get continuous rotation for camera stands and robots.

The Futaba article says use one 2.7K resistor and one 1.3K resistor on each outside leg of the pot. I am not sure that this is correct with the pot value in the Hitec but it may be. I have already opened up the servo and am familar with how to make the mod., if it only requires two resistors.

Whew!! My question is has anyone of you all done one of these modifications? Are the resistor values OK for the Hitec? Any general suggestions on this modification?

Thanks to all the brains on this site for helping out.

Tim

[3drc tim]

Well, this must be the most boring thread on the board!! I tried the 2.2Ks on each pot lead and the servo overtravels the pot by at least 20 degrees on each side throwing the gears all over the bench. So--I tried some 1.5Ks to reduce the throw back to the 180 degree range and it still over travels the pot which offers maybe 200 degrees of movement. Any help or thoughts??

Thanks, Tim

[ElectroLawndart]

3drc,

Try putting a 5K linear taper pot on each side of the servo pot with some alligator clips. Tune the pots until you have the desired amount of travel. Disconnect the pots without moving the knobs. Measure with your ohm meter and you have your resistor values.

Does the servo gears have a limit tab on them? I remember having to cut a tab on the gears when modifying them for constant rotation but I'm not sure if this applies to 180 degrees of rotation.

Hope this helps.
Dart

[3drc tim]

Thanks for the reply Dart. I thought no one out there cared or knew about this topic. The main drive gear did have a tab but I cut it out. The tab of course prevents over travel.

I understand using the trim pots to check this out; good idea. I was hoping someone out there has done this before me. I spoke to Hitec late this afternoon and they suggested 2.2Ks to start and then suggested going down to 1.5Ks. They both travel over 180 degrees and this of course pops the gears because it over travels the pot.

I am still wondering why it is suggested on one guys web page that you use a 1.3K on one leg of the pot and a 2.7K on the other leg of a Futaba 148 servo. Why are they different values. Is it the total of the pot plus the two legs or why would they be different values? Thoughts?

Thanks for any help.

Tim

[Daniel7866]

Thanks for the reply Dart. I thought no one out there cared or knew about this topic. The main drive gear did have a tab but I cut it out. The tab of course prevents over travel.

I understand using the trim pots to check this out; good idea. I was hoping someone out there has done this before me. I spoke to Hitec late this afternoon and they suggested 2.2Ks to start and then suggested going down to 1.5Ks. They both travel over 180 degrees and this of course pops the gears because it over travels the pot.

I am still wondering why it is suggested on one guys web page that you use a 1.3K on one leg of the pot and a 2.7K on the other leg of a Futaba 148 servo. Why are they different values. Is it the total of the pot plus the two legs or why would they be different values? Thoughts?

Thanks for any help.

Tim

Hi,

The reason for different resistor values is from the fact that with THIS servo at neutral (1.5msec) the pot is not at centre. So to keep it centered with a larger travel you have to keep the same resistor ratio.

Daniel

[fhhuber506771]

Just plopping in resistors to replace the pot won't give you a fixed endpoint. The servo uses a COMPARATOR to sense if it needs to move the motor or not.

If the comparison is not within tollerance, the motor is activated in the direction that SHOULd make it match, and it keeps going until the value desired is achieved.

Plopping 2 resistances across the Pot makes it so the comparison NEVER matches except at one fixed setting of the transmitter.

So.. the method you are trying to use will never work.

You can find 2 pairs of resistors... and use a DPDT switch to swap which pair is replacing the pot. Then when you flip the retract switch, the servo will move until the switch flips, putting the correct resistances in the circuit.

The switch will have to be a FAST acting type... such as a pair of microswitches "ganged" to get the DPDT. It has to SNAP from one pair of resistors to the other with essentially no time having both sets out of the circuit.

[Rodney]

He is placing the resistors in series with each end of the pot, not in parallel with the pot. This does work, if you need more movement, increase the values of the resistors, if less movement decrease the values. Yes, each end point might require a different value of resistor depending on how the pot body is mounted in the case. If you can rotate the pot body, you can adjust it so that equal values are required on each end of the pot.

[3drc tim]

Guys,

Thanks for your feedback. I talked to the GURU at Hitec today and he is confused as to why the 2.2K resistors over travel. I told him that I went down to 1.5K resistors and he said just keep going down or get some trim pots to test what resistance would work. Rodney, you are correct that the components are mounted in series off each outside leg of the pot. The pot body in this 325 series Hitec is a square pot (dimensionally) and mounts inside the case so that it is at center (reason now known for same values on each leg.

I have tried setting the ATV values of the transmitter to 50% on the gear channel and that did not help either.

FHhuber; I am not taking the pot out of the circuit not am I putting resistance across the pot, the resistors are in series with the outside legs. This method has worked for others (from my research) but I just cannot seem to get the values right.

I am stumped. Maybe my next step is to get 1K resistors to reduce the over travel. Going down is correct, right?

Thanks for all your help guys, Tim

[Cirip]

Hi,

Here's another idea. Instead of using off the coff resistor values, what about measuring/reading the pot value, then adding series resistors that are 10...20% of the total pot value. For instance, if the pot is 5K, then a good starting range for the series resistors might be 0.5...1K. Test then readjust if required.

Good luck,
Cirip

[3drc tim]

Well, thanks guys for your help.

I finally figured it out. I subsituted resistors until I was in the correct area and moved in with smaller resistors until it was perfect. The I bought the correct sizes in 1/2 watt but still couldn't make it work. After awhile I noticed that I had the wires reversed on the outside of the pot.

After that it worked perfect; I set the ATVs for just under 150% and got 200 degrees of throw and can reduce the throw by reducing the travel advance.

Perfect. The final for the HS 325BB servo is a 2.2K on one leg and a 3.0K on the other outside leg. Bottom line, I needed three retract servos and now have 3 standard BB servos with 51 oz.in each per wheel and saved a couple of hundred bucks. I'm pleased.

Thanks again, Tim

[vintage1]

Now how to make them really slow to act..:D

like 5 seconds end to end..

[Daniel7866]

Glad to kow it's working.
Is your servo centered with stick at neutral?

Daniel

[Daniel7866]

Now how to make them really slow to act..:D

like 5 seconds end to end..
Pouring SAE200 oil in gearbox? :D :D

[3drc tim]

The servos are centered; however this is of little concern to me as I am only going to use them to go from one end of travel to the other plugged into the retract channel (no center.)

As far as slowing down the servo, in my research I did run into some "plug in line" circuits that are available in commercially and in project form. I may be wrong but I think 'Alans" page has a schematic.

Tim

[Daniel7866]

The servos are centered; however this is of little concern to me as I am only going to use them to go from one end of travel to the other plugged into the retract channel (no center.)

As far as slowing down the servo, in my research I did run into some "plug in line" circuits that are available in commercially and in project form. I may be wrong but I think 'Alans" page has a schematic.

Tim

Have a look at this one http://www.rc-cam.com/ .

Daniel

[3drc tim]

Thanks Daniel, this site has a lot of interesting projects including lighting, hi and low power switches, servo slowing, and much more.

Tim


Have a look at this one http://www.rc-cam.com/ .

Daniel