555 servo tester. What Pot?

Discussion in 'The Projects Forum' started by Fenris, Aug 21, 2008.

  1. Fenris

    Thread Starter Active Member

    Oct 21, 2007
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    hi all

    I've just knocked up a circuit based on the 555 IC. The circuit works fine except for the limited throw of the servo arm. To get over this I have changed the variable resistor of the circuit from the 10K specified to a 50K pot. This has increased the throw brilliantly From less than 45D total with the 10K to a massive 140D with the 50K. Now only the first half of the pot actually operates the servo so from 0ohm to approx 20K (measured with a meter). It is proportional in response but I need it to operate either side of the pots center. Am I correct in reasoning that I should now purchase a 22K pot (nearest value) and this would then function either side of it's centre. I.E. 11K each way. Or do I simply need to adjust R1 and R2 of the circuit to get the 'around centre' response? I intend to build the circuit to drive the eye-stalk of a dalek using the 14Kg torque servo I'm currently testing the circuit with. I intend to scavenge and modify the gimbal from an old RC radio transmitter to control it by fitting the servo drivers pot in place of the one thats already on the gimbal. Answers to both options would be gratefully recieved.

    Regards

    Fenris
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
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    Hi there

    built that one first :) it was indeed better with its throws but was very slow. The circuit I have used is functioning faster and I have a massive throw rate now I have changed the pot. I have fiddled a bit and changing R2 for a 470ohm resistor has moved the neutral point to the mid-point of the pot. I have the same 140D total throw but with dead points at each end of the pot so I am assuming that R1 will adjust the 'working' range of the pot. How am I doing so far with the lateral thinking?

    Regards

    Fenris
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Hi Fenris,
    You didn't mention on which "end" of the pot you ran out of adjustment on.
    Try replacing R1 with a 200k resistor, and see if that improves your range. Then try replacing it with a 180k resistor, and see how that does.

    By reducing the value of R1, you will be allowing reduction of the "on" time of the signal, while retaining much of the "off" time capability.

    I suspect that right now, you have too much adjustment range in the "off" time and not enough in the "on" time.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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  6. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, give the attached a whirl.

    It's an adaptation of Bill Marsden's PWM circuit. It'll give you from practically 0% PW to nearly 100% PW.

    It even has fewer parts than TVR's circuit.
     
  7. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Evening/Morning gentlemen

    Heres a pic of the bread board I have marked in red the original working range and position. In yellow is the current range and position on the pot. Thinking about it when I build this proper the gimbal cage will allow less movement of the pot anyway which is probably fine as I only need -10 degrees and anything up to +70/80 degrees so I may be over worrying about the 'dead' ends. I saw the circuit with the diode and may yet try it. The one I have built was chosen purely on what I had in my bits box. So no diode or 2k7 pot at the moment. The 180K didn't change anything btw. Smaller pot? 22K and 47K are options though I have neither :( thanks for looking :)

    Just saw your post Sgt. I haven't got the pot or diodes but I can get them later to try it out. Cheers

    It would have helped if I had attached the image!!:rolleyes:

    Regards

    Fenris
     
    Last edited: Aug 21, 2008
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you could use your 50k pot if you changed the cap (C1) to around 20nF/0.02uF.

    You might get by with other diode types like 1N4001-1N4007.

    The photo of your breadboard didn't attach - it might've been too large. Try cropping it down a bit. You may be able to reduce the size by saving it as a .PNG type. As a last resort, try converting it to a .JPG.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Bill,

    I think I've seen that circuit elsewhere on the net, it might not be an original concept from DPRG. With the net though, one can never know.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    When it comes to the 555 very little is new. The chip has been around so long it is pretty throughly explored, which is to say it is dang useful. Having said that, I'm still learning some of its quirks, such as sourcing an amazing amount of current both ways, but not being truely rail to rail.
     
  12. hemidenis

    New Member

    Nov 3, 2009
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  13. Wendy

    Moderator

    Mar 24, 2008
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    What kind of servo (make and model). If you have a link, a spec sheet would be nice.
     
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