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DIY motorised focuser


JamesF
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I'm sorting out some bits to build my own motorised focuser, so I thought I'd open up the existing Skywatcher one that I have and see what it looked like. This is what I believe is the correct circuit diagram for it (I've not marked the resistor/capacitor values in yet):

post-10871-0-52839100-1375211364_thumb.p

Can anyone tell me if that looks sane? It looks to me as if the speed controller pot controls the output of the voltage regulator and the switches arrange for one pair of wires to the motor to be at that voltage and the other pair are pulled to ground. What happens inside the motor I'm not sure.

I'm not sure the design is going to work for what I had in mind, but I rather like the two-button design compared with the ones I've seen where a single switch is pushed one way to move the focuser in, say, and the other way to move it out.

James

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Shouldn't the regulator be supplying the motor +? Looks like the motor gets the 9v supply and the regulator pulls some down to ground like you suggest? Doesn't strike me as an efficient way to do things

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Well I'm afraid I can't offer much help with the schematic, however electric focusers are very handy bits of kit.

I recently built a battery-less ( servo to servo) focuser. It works very well.

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Shouldn't the regulator be supplying the motor +? Looks like the motor gets the 9v supply and the regulator pulls some down to ground like you suggest? Doesn't strike me as an efficient way to do things

Could be just the scan of my diagram. When the buttons are both "up", both pairs of wires to the motor are connected to ground. When either button is pressed down, one throw connets 9V to Vin on the voltage regulator, and the other throw connects its pair of motor wires to Vout from the voltage regulator. I could probably have drawn the diagram to make that more obvious.

James

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Well I'm afraid I can't offer much help with the schematic, however electric focusers are very handy bits of kit.

They are. I love having the one on my Mak, hence the intention to build another that I can fit to the PST.

James

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Hi James,

it looks fine to me ( although I'm used to seeing the LM317 with the Adj pin at the bottom and the Vin and Vout at the sides ) and should work. Just watch the resistor values. I trust that this is for use with a DC motor and nothing else?

hope this helps,

Alan

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it looks fine to me ( although I'm used to seeing the LM317 with the Adj pin at the bottom and the Vin and Vout at the sides ) and should work. Just watch the resistor values. I trust that this is for use with a DC motor and nothing else?

It is indeed. My intention for my own focuser is to mod an RC servo to allow it to rotate continuously and use that. I don't think it requires quite so much complexity.

James

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Why would one use a voltage regulator in this instance rather than just a voltage divider? I know a voltage divider would be wasteful, but it's not going to be conducting for very long periods of time and there's still going to be a voltage drop across the regulator too. And as the entire unit is battery-only there's presumably no need to protect from excessive voltage. So what advantage does the regulator give?

James

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The regulator gives a low resistance and a steady voltage resulting in a more constant speed particularly at low speeds - it makes a big difference. DC motors draw a higher current when starting making a simple resistive divider ineffective.

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The regulator gives a low resistance and a steady voltage resulting in a more constant speed particularly at low speeds - it makes a big difference. DC motors draw a higher current when starting making a simple resistive divider ineffective.

Ah, that makes sense.

I am thinking that it would be quite useful to be able to run everything off 12V rather than a mixture of 6V, 9V and 12V as I've had in the past. The servo requires up to 6V and (I believe) the LM317 can't provide Vout higher than 4V below Vin. I'm thinking therefore that I ought to be able to choose my resistors such that the highest possible output of the LM317 is 6V (when the pot provides negligible resistance), running down to a very small voltage at the pot's maximum resistance. (I guess I may not end up using an LM317 here. Depends what's available when I place the order for the bits I don't already have.)

James

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Actually, I guess it would be even better to make one of the resistors a preset pot, allowing me to set the maximum output voltage lower than 6V if that turned out to be moving the focuser faster than I really wanted.

James

Edited by JamesF
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It is indeed. My intention for my own focuser is to mod an RC servo to allow it to rotate continuously and use that. I don't think it requires quite so much complexity.

Just a quick one James, but RC Servos are run using PWM and not constant voltage/current. To vary the speed of one modified for continous rotation requires attaching some kind of controller that provides the PWM you require and will increase the complexity of the circuit.

There are plenty of internet links about servo motor operation that can give you the details you require.

hope this helps,

Alan

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If you remove the stop and the pot plus circuit board you have a tiny 5v DC motor with gearbox that you can use like any other DC motor. ie. control the speed with a variable voltage low resistance supply and swap motor connections to reverse the direction. With some of the tiny servos you can't actually remove the pot and have to butcher it to allow continuous rotation.

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That is exactly my intention, Gina :)

I've been pondering today on switching. I don't like a single centre-off toggle switch as some people use because I just know that if I have a toggle sticking up I'll catch it on something and end up throwing the controller across the obsy or patio :) I do like the dual buttons on my existing Skywatcher controller, but it's also struck me that I could use a momentary centre-off rocker switch. I just need to work out the wiring so I can reverse the polarity without ending up shorting Vout to ground. That will be this evening's job I think.

James

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Mostly because my gut feeling is that I'll probably have better control over the range of speed and accuracy if I do it this way. I agree that what you suggest is a perfectly workable approach though. If I were just wanting it for visual use I'd probably do just that. It would be ideal for a "hands-off" focusing system for an unpowered visual setup. In fact, I might well be tempted to do just that with my dob :)

James

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Have you seen this thread James? http://stargazerslou...ser bizibilder (Ignore the first bit and go on to the control board part). I use mine regularly (I now have identical modified servo's on each scope so all I have to do is swap the power box around) and find it a godsend I use a centre off toggle switch and have not had a problem with it.

Mine works on 10:1 dual speed focussers and I can get movement from "too fast" to a "nudge" of maybe 5° at the servo - which is geared down by the belt and pulleys. I've never needed finer control.

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I've been trying to find the thread describing my first fosuser circuit using an LM 317T and centre off toggle switch fitted with a lever to make it like a large rocker switch. Pressing one end moved the focus one way and the other end the other way. No joy so far.

I have found a photo of my earlier dual imaging rig with two 1100D DSLRs with the focus control box beside it. I may have the circuit diagram on my Linux box where I have a specialised electron circuit drawing app. I'll look later.

post-13131-0-41337200-1375297168_thumb.j

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If you remove the stop and the pot plus circuit board you have a tiny 5v DC motor with gearbox that you can use like any other DC motor. ie. control the speed with a variable voltage low resistance supply and swap motor connections to reverse the direction. With some of the tiny servos you can't actually remove the pot and have to butcher it to allow continuous rotation.

Ah, I see what you're doing. In the past I have just removed the stop from the gearing which then allows you to use a standard servo circuit to control backwards/forwards and speed. I didn't realise that you were going to remove all the control circuitry.

Alan

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I just went for simple. A stripped servo, with a belt drive to the focusser. The belt can be lifted off for manual operation. A 4.8v Ni-Cad pack with a DPDT Centre Off toggle switch controls the motor via a long cable which can be unplugged ( via a servo plug ) once focus is achieved. The pulley on the servo is smaller than the fine speed control on the focusser, further gearing the drive, so I have good fine tuning of the focus.

flatfield2.jpg

Edited by ArmyAirForce
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