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Fiddling with an autofocusser


Bizibilder

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I'm making myself an electric focusser by following Abernus' description here: DIY Focuser and I'm not sure that I'm quite getting it right! My electrical knowledge is poor - but I'm learning!!

I have assembled the test circuit shown to find out whether the thing works (so there is no switch, for example) the whole thing being held with croc clips! The problem is that by using either 4.5v or 6v and either a 4K7 or 1k pot in any combination I can only get the servo (Futaba S148) to run when using around 10% of the available "turn" of the pot. The rest of the "turn" is effectivly "off". I am loathe to try 9v (or more!) as the servo is rated at 4.8-6v.

Any ideas from you electrical wizards out there???

PS sorry about the use of Bizibilder's own conventions on the diagram! I hope it is clear :glasses1:

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Thanks martin_h - I can't even copy from a picture properly! Anyhow, I've reassembled the circuit correctly and still only get around 10% of a full turn from "full speed" to "off" at both voltages and swapping both pots! Is there something else that I should be doing?

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Ah Ha! I now have better speed control with movement of around 15% of the pot! At least I can set it to "fast" and "slow" as needed. I'm just surprised I can't get "full scale" speed control - I suppose there is a low voltage limit where there simply isn't enough "umph" to get the servo turning?

Thanks martin_h for your time and trouble.

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I agree that a PWM circuit would give better controll....have you got a variable dew controller you could try it on? Connect the dew controller to 6volts and try that...it wont be perfect because the controller is designed for 12v input, but it would be worth a try at 6v.

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PWM is what they are designed for.

One easy way of using a RC servo is to convert it to continuous rotation. This requires the internal feedback pot to be mechanically disconnected and left in the middle of it's tavel. Any physical stops on the output gears removed.

Then use a simple servo tester (555 timer circuit can be used) to operate it.

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At the moment I have simply bypassed the servo circuit by soldering my servo input wires to the motor terminals. The gears have been modded to remove any physical barrier to continuous rotation. I am simply using the servo motor and gearbox wired pretty much directly to the batteries.

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The only problem with trying to control a motor with a variable resistor is that the voltage wil drop to a point where the motor will stall and that will be well above 0v and with a load on the motor that will be higher still so you wont get much in the way of controll...PWM supplies the full voltage to the motor all the time, it just sends it in pulses and varies the time between pulses to speed it up or slow it down

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The other problem with your potentiometer is that the value is too high.

I = U / R

I = 6V / 4700 Ohm

I = 1.3mA

So with that you could draw a max of 1.3mA of current. I doubt that this is enough for a motor.

Then, if you go lower with the resistor value, current increases but you have to be careful not to overload the potentiometer. Check the wattage it's designed for and don't go higher than that.

Say your motor needs 50mA at 6Volt. 6V * 50mA = 300mW. So to be on the safe side the potentiometer should be rated to 500mW or higher. As for the resistance value, assuming 50mA again:

R = U / I

R = 6V / 50mA

R = 120 Ohm

Though it would be far better to use a voltage regulator circuit of some sort (for a continuous rotation motor) or a PWN circuit for a servo.

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I've assembled the beastie and it works well now that I have the proper controller. I have attached a couple of pics to show the insides and outsides. I now have to decide how to attach it to the scope.

(You will notice that I've been careful not to show my atrocious soldering!!)

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  • 6 months later...

Finally got this thing attached to a scope! The small pulley and belt was purchased from:

motionco_Pulleys & Belts_Aluminium MXL Timing Pulleys_Buy Online

(Thanks to someone on SGL for recomending the supplier - btw exellent and quick service from them!). The bracketry was made by me.

The controller gives a rotation of the focusser from 1rev in about 12seconds to about 1 rev in 2 seconds. You can "blip" at slow speed to move the focusser knob somewhat less than 5° of a revolution at a guess - which should be fine enough. The little toothed belt is quite happy driving on the normal focusser knob and can be removed easily for "rough" focussing if required.

The only problem I can forsee is that by mounting the bracket on the tube ring I can't rotate the scope (as its a refractor that shouldn't matter) but it means that the finder bracket is now in an akward position and as I guide with the finder I may have to make a new arrangement for mounting this.

The controller now has two switches - one is the forward and back toggle and the second (on the end of the box next to where the wire comes out) is an on-off switch which I had to add as the power supply was running the batteries down even though the toggle was "off"! - it obviously still draws a small current somehow!

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Good job and very interesting :) And thanks for the link for small pulleys and belts - very useful - been looking for something like that :(

BTW what are you using for the servo/motor? The original link (which this is based on) no longer works. I was going to use a Maplin motor/gearbox (SN36P) but what you have is much smaller and neater. The motor/gearboxes are much more powerful than needed for this.

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Good job and very interesting :) And thanks for the link for small pulleys and belts - very useful - been looking for something like that :(

BTW what are you using for the servo/motor? The original link (which this is based on) no longer works. I was going to use a Maplin motor/gearbox (SN36P) but what you have is much smaller and neater. The motor/gearboxes are much more powerful than needed for this.

The servo looks like a Futaba s148. A very common basic servo in radio control models.

Link: Phoenix Model Products Futaba Analogue Servos

It's the one at the top of the page.

EDIT: Maplin one here - Futaba S148 Servo : Servos : Maplin

HTH

Lee

PS... (Another edit) How about using a servo tester? Again, fairly common in the radio control model world?

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The servo is a Futaba S148 (I wish I'd seen the site you quote Lee as I paid about £14.99 from Maplin for mine!!) but you can use others - all you need is the case, motor and gears, the electronics get scrapped. The pulley is a PL024AL 24 tooth aluminium pulley. I modified it by drilling it out to fit a small raised ring on the servo output disc and turning away the bit that normally holds the grub-screw. It is held together with Araldite. The belt BMXL090 90Tooth 1/4" wide 7.2" long.
I must admit that luckily I bought a couple of belts and pulleys to "try". I would suggest that this pulley is fine but I would go for a slightly longer belt - maybe 120-150 teeth - just to give you a bit more room for the installation.

NB The device now operates from a 9v battery not the pair of 1.5v as shown in the photo

PS Lee - what does a servo tester do? Excuse my ignorance but I'm an electronics numby!!

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Hi Bizibilder,

No problem - I'm into radio control models and use them to exercise / test suspect servos. They are sometimes automatic and just rotate the servo end to end and usually have a centre point check function (so that they can be checked for accuracy and that they go back to the same point each time)

There are a few about that are adjustable which I would have thought would have been ideal for this sort of set-up. They are probably cheaper to buy than build!

Phoenix Model Products Servo Gears & Accessories

Either of the top two will do. You'll need a 4 cell battery too (4.8v). You can either get yourself a typical radio control model pack or 4 qty 'D' or 'C' batteries.

Phoenix Model Products Receiver Batteries

(see item BR274 at the bottom)

The only down side that I can see with using a radio control model pack is charging it. The alternative is standard batteries and a battery holder such as this one...

C Size Battery Holders : Battery Boxes : Maplin

Hope that helps :)

EDIT: This device relies on the servo being complete with all of it's electronics in tact.

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