[Moses_C]

Hi again everyone! :wave:

Maybe some of you may remember me....the last time i posted a question on controlling the speed of my grass trimmer not too long ago, and i'm so glad u all manage to help me resolve that successfully! :banana:
Tq!

Ok, this time, i really need some help on building a simple PWM control for a small DC Motor, which i plan to attach it to my relatively new Spotting Scope in order to remotely turn the focusing knob...as to achieve sharp focus using my hand (manual control) would generate terrible vibration and sometimes almost unattainable but desired focus on the subject, even after many attempts (i.e. rocking back and forth)

I suppose any of you familiar with using a telescope or sth similar would easily understand how hard it is to focus properly esp. when using a very long focal length scope. That's why, there are commercially made remote focuser, also known as motor focuser, which is nothing more than a small dc motor or maybe a servo (i'm not sure) connected by a cable to a handheld device which house the PWM circuitry, control buttons and battery.

They are typically expensive as they are not a commonly used thing, and btw, i got my scope thru someone while he was in the US..so to buy that focuser and have it sent over to my country would be even more expensive bcoz of the shipment fees and taxes..and lots of procedural hassle with local authorities.

So, i hope once again the community here would be able to help me out....like you, Al the Man, Keebler and many more....who gave me very good ideas and suggestions to solve my previous prob. Tho its not related to CNC (sorry for that), but i know the knowledge you all have here is more than enough to resolve it easily.

I found a few PWM circuit based on the 555 timer, and some video i watched on youtube...the idea of using the timer is great, as its simple and low cost. (i prefer such solution actually) However, at very low speed the torque generated by the circuit caused the motor to literally jump around everywhere! I watched that happening in a video on youtube...and then i thought to myself this isnt going to work unless it is very smooth and slow on the focusing knob.

So, is there anyone who knows how to fine tune the 555 timer PWM circuit to make it very slow and smooth? As i mentioned b4, im still a beginner at most and would not be able to build a circuit from scratch. Also, is there any other circuit that you all know which can provide a forward and backward control (with variable speed control, if possible..as it'll be much more flexible for me). Think that's all for now..

Thank you all again for any form of help. :wave:

[underthetire]

How much movement do you need?

I would think this
HobbyKing R/C Hobby Store : Servos and parts>Servo Tester

with a good servo from them will get you pretty close.

[Al_The_Man]

I agree the R/C or Robot group sites may show a solution, also you could experiment with the method that was originally used in similar applications for positioning Radar antenna.
Two small stepper motors coupled together , one as the driving element, the other the receiver, with the anticipated load you may have, no other electronics should be needed.
As long as the stepper resolution was sufficiently small enough?
Another similar method is to use two resolvers for very low resolution.
Al.

[Moses_C]

How much movement do you need?

I would think this
HobbyKing R/C Hobby Store : Servos and parts>Servo Tester

with a good servo from them will get you pretty close.

Hi there,

Thanks much for the suggestions and link! It's a pretty cool site, really..i wasn't aware of such a shop prior to you telling me.

I just checked out the servo tester thru the link, but i don't even know how to use it..what a newbie i am... Well, i will definitely have to do some reading up first on this, before trying or buying this..

Nevertheless, tq very much for the help. :cheers:

[Moses_C]

I agree the R/C or Robot group sites may show a solution, also you could experiment with the method that was originally used in similar applications for positioning Radar antenna.
Two small stepper motors coupled together , one as the driving element, the other the receiver, with the anticipated load you may have, no other electronics should be needed.
As long as the stepper resolution was sufficiently small enough?
Another similar method is to use two resolvers for very low resolution.
Al.

Hi Al,

Tq for the help too!

Ok, i will find out more based on ur recommendations...try them out if i manage to understand the whole setup, and find the right parts locally.

Yeah, sorry to have to post it here..i will look for the robot sites as u suggest...if u know any good ones, pls do let me know, as i am not a member of such kind of sites yet. Maybe that's a much better place to discuss things about motors and stuff..

TQ! :cheers:

[underthetire]

Hi there,

Thanks much for the suggestions and link! It's a pretty cool site, really..i wasn't aware of such a shop prior to you telling me.

I just checked out the servo tester thru the link, but i don't even know how to use it..what a newbie i am... Well, i will definitely have to do some reading up first on this, before trying or buying this..

Nevertheless, tq very much for the help. :cheers:

Pretty easy actually. Plug servo in, plug 4.8-6VDC in the power plug, and set mode to manual. Knob then turns the servo. A little, or a lot, or just to a position.

[vger]

Moses,

Years ago I used parts from an old JVC video camera to remotely turn a 10 turn potentiometer. I used the zoom control button and circuit board with the zoom motor and gears to turn the pot. The button (zoom in/out) was some sort of variable resistance control and allowed for very slow, fine, smooth, control of the motor direction and speed. It operated from 12 volts. If you can find an old camera like that, you may be able to hack it apart.

[Moses_C]

Moses,

Years ago I used parts from an old JVC video camera to remotely turn a 10 turn potentiometer. I used the zoom control button and circuit board with the zoom motor and gears to turn the pot. The button (zoom in/out) was some sort of variable resistance control and allowed for very slow, fine, smooth, control of the motor direction and speed. It operated from 12 volts. If you can find an old camera like that, you may be able to hack it apart.

Hi Vger!

I did just what you said, hacking apart my old digital nikon compact camera and using its miniature motor plus some additional bits and parts to control the focuser knob. I'm just at best half way thru the project coz quite i'm busy since the first day i asked for help on this matter.

Sorry for the late reply. Thanks for sharing your own experience and success.

To the rest, thank you too for all the help.

God bless ya all!(group)