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We have been working with servo's in some our field equipment, however require a slightly different application. We will be deploying some underwater cameras for an extended period (3 months) and to prevent fouling of the camera face will need to make a wiper unit.

Is there a product, or does anyone have a schematic which will allow for timed control of the servos?

We are suggesting every hour for the servo to complete a return cycle or thereabouts.

I'm sure this would be very straight forward for the right person, however i'm a bit lost. I can make up a PCB if shown what needs to go where, but don't really understand the ins and outs of how it all works.

Thanks for your assistance.

Mark

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    \$\begingroup\$ Is this a hobby-style servo motor? Is the precision of the "one hour" critical or important at all? You're comfortable designing a PCB but not a small circuit? (don't expect an answer to have a schematic that you can drop directly into PCB software)... \$\endgroup\$ – W5VO May 11 '11 at 4:27
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    \$\begingroup\$ would a solenoid work? My thinking is that a servo has many more mechanical and electrical parts that may fail in an aquatic environment. Also the ease of use of a solenoid compared to trying to drive a servo motor, may make things less complex. \$\endgroup\$ – jsolarski May 11 '11 at 7:44
  • \$\begingroup\$ Provided you mean a PWM-type hobby servo, just about any micro-controller can do it. I'd be more worried about a shaft seal that's not going to be a slow leak over a period of months. \$\endgroup\$ – Chris Stratton May 22 '11 at 18:15
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Any of the popular small microcontroller boards would handle this easily. I don't know of anything prebuilt, but it should be a simple matter to find some e.g., Arduino code online to do what you need.

If you want a simple, drop in solution and can wait a week or two, let me know and I can build you one. I did something similar for lighting not too long ago. I'm just pretty busy for the next week or so.

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I would suggest using a 555 timer with a 1-5% duty cycle and a T of one hour. Some 555 are not capable of less than 50% duty cycle, in which case you'll want to make the duty cycle 95-99% and the servo be active low. This website is an excellent reference for 555 timers with schematics - http://www.kpsec.freeuk.com/555timer.htm#astable

It's a fairly simple circuit and you can easily play with it on a breadboard before making your PCB.

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By taking an hour to complete the full cycle, it sounds like you want the setup to continuously wipe at the speed of once an hour?

If you design the wiper to just rotate, it would be easy to just use a low speed DC motor to continuously turn and gear down to a very slow speed. This gives you the torque to push through the long rubber water seal. On the inside part of the shaft, you can mount a sensor to detect position. Now let a micro count to the time, turn on for a certain time, then stop when the sensor is tripped, meaning the wiper is not obstructing any sensors or cameras.

You can do the same thing with a servo, but it seems like the accuracy of position control and complexity of a servo is over kill in this situation.

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