Simple servo question

This has nothing to do with RC planes, but I figure you guy's/gal's would know about this stuff.
I am looking at using some RC servo's in an underwater housing I am
making for my video camera (to actuate the camera controls). I've been reading up on how servos work, and I have a simple question. I am not looking for the ability to get a specific degree of movement out of the servo; rather, I want the max amount of movement, all the time. Can I just apply 5V to the servo, no pulsing, to achieve this?
Or do I need to make a pulsed circuit, with a mostly "on" duty cycle.
thanx
Bryan
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Bryan Heit wrote:

why use a servo at all when a solenoid is all you need? as a servo will not respond at all to 5v and a pulsed circuit needs to be calibrated to the servo from memory it needs a 1ms pulse for one end and 2ms for the other and if you modify the servo to move with just 5v its just going to go round and round
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funfly3 wrote:

I have about a dozen mini servos sitting in a box. I'd have to buy a solenoid.
Bryan
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Bryan Heit wrote:

or buy the bits to control the servo ?
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On Fri, 08 Dec 2006 10:11:42 -0700, Bryan Heit

No, you need to apply about 5 volts to the power leads, and a 5 volt pulse to the control lead. It needs to be repeated about every 50 mS and have a variable period to control the servo arm position. 1.5 mS is center, full travel is about 1.1mS in one direction, and about 1.9 mS in the other. A 555 timer can provide such a pulse train, I can provide a simple schematic if you wish. For a water-tight switch, you should probably use a reed switch on the case inside, and a moveable magnet on the outside.
The exact voltages are not too critical, most servos work fairly well over a 4-6 volt range, maybe more with some, but do a trial with yours first.
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Bryan Heit wrote:

No..not to a standard servo.
You wither need to doctor the servo, or build a circuit to get the pulse input you want.
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A good while ago someone posted drawings on making a simple 16K Campac for Futaba FF8 and FF9. Does anyone still have the details or better still did anyone ever come up with a 64 K version? TIA Dave Armitage
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Dave,
Do a Yahoo or Google search for clonepac.

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wrote:

The original Clonepac (16K) drawing can be found here: http://home.comcast.net/~day_trippr/rcstuff/clonepac.pdf
/daytripper
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Thanx everyone for your replies. Looks like my master plan will not work; but maybe I can dissect the servos and use their electric engine somehow. Does anyone know what type of engine they use (stepper motor, etc).
Bryan
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| Thanx everyone for your replies. Looks like my master plan will not | work; but maybe I can dissect the servos and use their electric engine | somehow. Does anyone know what type of engine they use (stepper motor, | etc). | | Bryan
Dc motor with feedback potentiometer. With the old Expert series servos we used to reverse the servo output by reversing the polarity of the motor wires and the outside leads of the potentiometer. Useful for 2 servo elevators and flaps.
You may be able to get to the amplifier part of the servo and just inject a small dc voltage at the output.
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On Mon, 11 Dec 2006 08:09:39 -0700, Bryan Heit

They use a small DC brushed motor, usually rated at around 3 volts. I have removed the motor driver/amp board and feedback pot and used it just as an actuator. Generally they will provide reasonable torque off a single cell (1.5v) when used like this.
That may be a solution for your application. Remove all the guts except the motor, and remember to remove the stops (often a moulding on the case frame) then the servo will just rotate when power is applied. With the correct shaped cam on the output, maybe it will do what you want? You said ":I am not looking for the ability to get a specific degree of movement out of the servo; rather, I want the max amount of movement, all the time."
Barry Lennox
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Barry Lennox wrote:

Similar issues have been covered here before
http://www.rcgroups.com/forums/forumdisplay.php?f 9
I'd try a search and ask there.
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Barry Lennox wrote:

This sounds like the best bet. I'm basically using the servos to push buttons on a camera so removing the stops may not be necessary. Major thing will be if the motor produces enough force to press the buttons, but given that these things could move elevators and whatnot in flight, I doubt this'll be an issue, and if it is some sort of lever should be able to fix that.
Thanx everyone for your help
Bryan
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| This sounds like the best bet. I'm basically using the servos to push | buttons on a camera so removing the stops may not be necessary.
In that case, be sure to not give it power for very long -- once the button is pushed, the motor will stop, but if the power doesn't stop, the motor will start drawing far more current than normal, which will heat up fast and may burn out.
If it's a digital camera, you should be able to solder to the switch's terminals and just use a relay to trigger it. You may also find a solonoid to be a better solution than a servo, or at least an easier to deal with one.
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Bryan Heit wrote:

Oh..well whats it being controlled FROM..you could adapt a servo tester to move a servo to two defined locations depending on whether a button was pressed or not..or is it under radio control?
Servo testers are cheap enough..

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The Natural Philosopher wrote:

I am using the servo's to actuate buttons on a video camera within an underwater video housing I have made. The servo's themselves will be triggered by magnetic buttons (on the outside of the housing) which will engage reed switches inside of the housing.
Using a servo controller may also work. If I were building this from scratch I'd use solenoids. However, I am using a housing I made a while ago, and as such the space for the controls is very limited - much too small for the solenoids I've been able to find, and too small for the power supply (12V).
Bryan
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Bryan Heit wrote:

Right: I note you appear to be in canada or I'd recommend you get tow of this little servo tester kit, and replace the pot with a reed switches and four resistors..
http://www.micronradiocontrol.co.uk/easybuild.html
That honestly has to be the quickest solution.
Very smart idea to use magnetic switches on an underwater housing..I'll note that for possible future use.
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The Natural Philosopher wrote:

I think I'm going to go with some sort of servo controller/tester circuit. I've found a similar tester to the one you linked to that I can order here. Granted, using such a circuit will require that I change the control batteries more often, but that's only a minor inconvenience. Better then burning out the servos cause I'm too slow releasing a button...
As for the reed switch, that's pretty common in underwater gear, especially in stuff designed with deep diving in mind. Any hole in a housing represents both a weak point, a potential site for stress cracks, as well as a potential leak point. Getting around this - either with magnetics or fiber optics, allows control of the device without compromising the integrity of the housing.
Bryan
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Bryan Heit wrote:

Yup.. similar problems in RC submarine hydroplane operation..you want the actuators outside the pressure hull fully immersed..but the main gear inside.
I don't think a servo tester should draw more than a few mA by the way..its likely to be a 555 chip and not much else. All it is is a pulse between about 0.5 and 1. (or is it 1 and 2) ms every 20 ms or so..a realaxtuon oscillator with adjustobale discharge is all it takes. Heck a unijunction transistor, and 4 resistors and a capacitor would do it..
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