Help with Servo Please

Discussion in 'The Projects Forum' started by JDR04, May 13, 2013.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hello folks. Could somebody please help me with this as I have never worked with servos before.

    Would it be possible to modify the attached circuit so that it will control the servo? I would like to get the servo to continousely move backwards and forwards, like windscreen wipers.

    I do realise that the maximum voltage for the servo is 6V. Could I possibly insert a NPN and a PNP transistor in place of the LED's to control the voltage to the servo??

    Or could you suggest another circuit for me please.

    Thanks a lot-JDR04
    Spec:
    Torque: 1.2kg/cm @ 4.8v, 1.4kg/cm @ 6v
    Speed: 0.10/60deg @ 4.8v, 0.09/60deg @ 6v
    Voltage: 4.8v~6v


    Weight 10g
    Length 22mm (32.5mm with mounts)
    Height 30mm (without horn)
    Thickness 12mm
    Lead length 17cm
    Standard futaba / jr connector
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,088
    3,027
    The one spec missing for your servo is the current it requires. The 555 circuit would run basically the same at 6V as at 9V, so you could just use a 6V supply for both the timer and the servo.

    If the servo's current requirement is less than the rating of the 555 (200mA, I think), you may not need any additional transistors, although they may help with getting better switching.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    A standard RC servo has three connections, power, ground and control.
    The control signal is a PWM signal. You can experiment with this using a 555 circuit with a variable pulse width adjustment.

    For the servo to move back and forth you will have to generate a triangular wave and then output a PWM signal.
     
    JDR04 likes this.
  4. wayneh

    Expert

    Sep 9, 2010
    12,088
    3,027
    Oops, I was thinking servo = motor. Nevermind.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    What is the servo?
     
  6. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
  7. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hello Mr Chips,How does one go about doing that?
     
    Last edited: May 13, 2013
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Current Drain (4.8V): 5.4mA/idle and 150mA no load operating
    Current Drain (6.0V): 5.5mA/idle and 180mA no load operating
     
    JDR04 likes this.
  9. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    Tell us a bit more about what you are trying to move back and forth.
    What is the time for each cycle, i.e. how long does it take to move forward and back?

    One solution is to generate a square wave with a 555 timer circuit and feed this into an RC integrator to create an approximation of a triangular wave. Next we feed this into an analog comparator to give a PWM signal.

    A simpler solution would be to use a simple microcontroller IC.
     
  10. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    OK, here's more info on what I am trying to achieve.

    Check the attachment out and at the bottom is a method of controlling the servo by way of two switches. I am assuming that one switch will make the servo move clockwise and the other will make the servo move anticlockwise,I think:confused:

    So, if I replaced the switches with transistors and used the previous circuit to alternatively close each "switch now transistor" I would then get the servo to move in a backwards and forwards motion, or have a I missed something.

    Thanks for your time guys-JDR04
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Do you have the servo controller board shown in the picture?
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    How fast do wish to " wipe"; Max natural speed is 1/10 sec for 60 deg, so 1/10 X 3 [180 deg ] = 3/10 sec, X 2 full cycle = 6/10 sec or 100 CPM. Could use natural period with brief pauses at and od each half cycle.
    You need a clock of about 50 Hz, 1 ms + pulse 0 deg, & 2 ms fpr 180 deg. I am using something similar in post by MLD, to control a servo that lifts a heavy target. Also my first experience with servos.
     
    JDR04 likes this.
  13. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Yes, I have the controller board. It has a NE556 IC on it if that helps at all.
     
    Last edited: May 13, 2013
  14. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hi Bernard, not too fussy about the speed. I would estimate anything from 5 to 30 seconds to complete 60 degrees. I think thats what the servo I have bought does, 60 degrees, sound OK??
     
    Last edited: May 13, 2013
  15. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    I think the only way you will get a servo to go that slowly is if you accept having it move in steps. They're normally designed to go from one point to another as fast as possible, so if you insist on slow operation, you're imposing something on them that they aren't made for.
     
    JDR04 likes this.
  16. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks John P, its really not critical how fast it moves at this stage. Thanks for the info-JDR04
     
  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    Better recheck the specs- should do 60 deg. in less than 1/3 sec. under load. Under rated load, current draw will be well over one amp.
    Taking another look- torque is about 16 oz in, draw prob. less than 1 A.
     
    Last edited: May 14, 2013
  18. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hi Bernard, thanks for that. Come to think about it, its OK if the servo does 60 degrees in a 3rd of a second, that wont be a problem. Also, the load/weight will be very light.

    All the servo will be doing will be to tilt a balanced tray in an upward and downward stroke which will be also very short. Does this make any sense???
    JDR04
     
  19. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Since you already have the control board for the servo, I don't understand why you want another controller. It seems to me that all you need to do is simulate the action of a 10k pot on the input to the controller you already have. And since you don't need a slow movement of the servo, why can't you just use fixed resistors as shown in the datasheet you posted?

    Maybe I'm missing something. Bernard certainly has more knowledge than I have.
     
  20. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hi tracecom, I dont think I have explained myself sufficently.

    I have a tray that is balanced in the centre. What I want to do is position the servo underneath with an arm so it will push the tray up slightly and pull it down slightly. I need it to do this repetatively without me having to turn the pot backwards and forwards. I should have explained this in the begining,sorry guys. JDR04
     
Loading...