Mechanics and Construction > Mechanics and Construction

Beginner... would love a recommendation


Hi... I've found your site to be quite informative and thought I'd run this little problem by you guys.  What I'm trying to do isn't robotic, really, but does involve a high-torque servo spinning 360 degrees continuously.  I only need it to switch on, switch off and have variable speed with a top end of about 75 rpm.  The low end can be 30 rpm, let's say.

I've read your page on how to modify a servo for 360 degree rotation so I bought a JR DS8411SA and a 5 cell 4200 rechargable battery pack.

I guess what I'm looking for now is a simple controller that can send a signal to the motor to start it spinning, and allow me to adjust the speed with a pot, maybe.  I would be open to purchasing another motor that you deem more suitable for this application, but it would need to the same or higher torque spec as the JR I have.

What do you think?

Thanks for your time!


Ok what you need is something called a servo tester. This is just a squave wave emitting device that has a tweakable wave length with a pot.

You can either make one:

Or buy one:

Neither list is exhaustive, just what I can find quickly online for examples.

Also, if you want it remote controlled, for about ~$100 you can get a transmitter and reciever (plug and play - very simple) to control servo direction and speed wirelessly. This might help if your interested:

You probably dont need this, but if you want preprogrammed actions for multiple servos, there is the microcontroller:

Awesome... thank you.  I went over to the hobby shop yesterday and picked up one of those "custom electronics" servo testers.  Works great!  Although, it's really only effective from 10 to 2 o'clock.  I'm wondering if I can build one with a different pot that might give me more range in that effective resistance zone.

Also, about that microcontroller... Can you tell me if I would be able to program the servo to perform a set number (maybe 20) of 360 revolutions that start at 75 rpm and evenly ramp down to 60 rpm?

Thanks for your time, patience and expertise.


Some servos respond differently to the input square wave wavelength. If you look up spec sheets of the servo, you will see something like:
"Clockwise/Pulse Traveling 1500 to 1900usec"
If your servo tester only goes to say 1700usec, then your servo wont rotate the full direction. You basically want the servo tester range to be greater than the servo range.

So you could consider getting another servo that works in the range of your servo tester (check datasheets), or you can attempt to hand modify the servo tester.

To do this, find the capacitor inside the device, and change it to another. If you place a second capacitor in parallel with the current one, this will increase capacitance and hence slow the timer down (increasing wavelength). Reducing the capacitance will increase the timer speed. But I really couldnt tell you how well this will increase the range of the tester since I suck at R/C circuits . . .

Programming the microcontroller to ramp down/up a servo speed is easily possible. You just give it a list of speeds and tell it the time delay to wait before it goes to the next speed on the list.

However, after you modify a servo, it loses position feedback control. This basically means that the servo angle will slowly drift as it rotates with no way to correct for it. It might rotate 19 times, or maybe 21 . . . of course if your situation is very predictable you can tweak the code until its satisfactorily close.

As for 75rpm . . . that is a little fast for a servo, and too fast for some. Check the spec sheets just to make sure. You will see something that says '0.15sec/60 degrees at no load' . . . which means .9sec/360, or 66.7rpm.

60sec/(.15sec*6) = 66.7rpm

Digital servos are generally faster.


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