Need help on bidirectional pwm for dc motor

Discussion in 'General Electronics Chat' started by syntaxerror, Dec 8, 2011.

  1. syntaxerror

    Thread Starter New Member

    Dec 8, 2011
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    Hi everyone,

    I want to make build a radar sort of thing where the radar dish would slowly move from side to side.

    To achieve that I want to,
    1. slow down a 12v dc motor using a pwm
    2. reverse direction in a set amount of time (say every 5s motor changes direction)

    I found a way to build a pwm using a NE555 timer, just not sure how to change direction.
    I don't know how to use micro controllers but I can build a basic circuit using a schematic.
    Any ideas or schematics? Can I use a relay or an H-bridge to do this? As you can see I don't know a whole lot about electronics.

    Thanks!
     
    Last edited: Dec 8, 2011
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Might suggest a gearhead motor to bring speed down to around 12 RPM. If you use a crank on the motor and a slightly longer crank on ANT. with a connecting rod, then the motor never needs to reverse as ANT moves from side to side. Otherwise use an H bridge with a 555 for timing & use PWM to control power to H br.+ or ground side.
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    If you use a servo like those for model airplanes, a 555 with variable duty cycle can make it go back and forth. 50% makes it stop, less than 50% goes one way and more than 50% makes it go the other. A second 555 running at a very low frequency can be used to vary the duty cycle.
     
  4. BillB3857

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  5. BillB3857

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  6. thatoneguy

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    Feb 19, 2009
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    For a 180 sweep radar, an RC Servo can't be beat on size or cost.

    Just send it a 0.5-2 mS pulse every 20mS, 1.5mS is center, short pulse is left, long pulse is right. I guess it depends on how you are looking at the servo to define L/R.

    Anwyay, they have a gearhead motor that is very responsive, and by knowing the pulse width you sent, you know the direction of whatever you are seeing.

    There are several robots that use the dual transducer ultrasonic rangefinder (SRF-08?) on a servo since they are so expensive, while the robot moves, the rangefinder goes left/right/left about 0.25 seconds per sweep, though that could be slowed down.

    Some BASIC programming languages have a RCOUT or SERVO command as well, so integration is very easy.

    http://www.youtube.com/watch?v=_RyodnisVvU

    --ETA: Another version, more of a real scanning/mapping robot Scanning head mapping robot
     
    Last edited: Dec 9, 2011
  7. syntaxerror

    Thread Starter New Member

    Dec 8, 2011
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    Thanks for all the replies guys, but err..gee this stuff sounds complicated. I was hoping there was a way to do it using a NE555. Oh and also, I was trying to get like a 45 degree angle left and right (left 45, right 45, middle 0).
     
  8. thatoneguy

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    You can do that with a few 555s, and logic gates with enable.

    Possibly not even that complicated, our resident 555 expert may have a deal already. Check out Bill Mardsen's Blog (his link is in my blog, if you click under "Blogs" and the linked number). Bill's blog should be first, which will give you a bunch of links, but he knows and has done everything that can be done with a 555, including servos.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Have you considered using the windshield wiper motor/linkage out of an automobile?

    It was designed to do the back-and-fourth motion you want from the beginning, and the motor is pretty strong.

    The old radar systems I worked on in the McDonnell/Douglas F-4J Phantoms had a dish antenna that would scan ±60° in azimuth from the aircraft centerline, and ±60° in elevation. It would take from between 1 to 1-1/2 seconds to scan in one direction or the other; I can't remember the exact timing now, as that was quite awhile ago. The antennas were hydraulically driven with 1,200 lbs of pressure supplied by the aircraft.
     
  10. djsfantasi

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  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    How about some specs on motor & antenna. You can directly drive a small motor from a PWM connected 555 & motor does not need to reverse to produce back & forth action.
     
  12. syntaxerror

    Thread Starter New Member

    Dec 8, 2011
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    Thank you all for the replies. A windshield wiper would be way too powerful. And for specs, it's actually a small radar dish. About 6 inches in diameter. I was hoping to use a small dc motor. I just wanted to build it as a model.
    Bernard, could you please explain a bit on how to use a 555 and a PWM?
     
  13. Bernard

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    On title page showing All About Circuits, upper right is a Find Box, put in Bill Marsden"s Blogs & a good discription of a 555 PWM ckt can be found. Just checked a small DC gearhead motor, 73 RPM @ 12V, @ 4.5V with 200 Ω s in series & brought it down to one rev. in 5 sec @ 15mA; torque suffered; but shows that a small motor could be run from a 555- a little more reduction would be better to improve torque. Motor is PN: G18213- The electronic Goldmine. I see that it is out of stock, but look at G16468, G16300, or G14728. Another source might be American Science & Surplus.
     
    Last edited: Dec 11, 2011
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