The "infamous" 555 servo controller

Discussion in 'The Projects Forum' started by abc27, Dec 16, 2010.

  1. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    Hey everyone, i'm relatively new to electronics so please bear with me. I'm trying to make a simple 555 controller where a servo is controlled by a potentiometer linked to a 555 timer.

    I've etched a PCB for all of this already and that's ready to go and the layout's fine e.t.c. but I wanted to give it a shot on a breadboard first of all to see how things go. Needless to say, things didn't go nearly as smoothly as i'd had hoped for. I amn't sure whether it's the fact that it's my first ever time using a breadboard or that my entire schematic is fundamentally flawed.

    My first attempt at this yielded the following results

    The minute I connect the servo to the header pins, it starts humming and begins to move extremely slowly in one direction. Sometimes it bursts for about half a second after I plug it in. Turning the pot does not change direction at all, all it seems to do is completely stop the servo from moving at all. I'm personally unsure as to whether or not it's my poor breadboard usage or my schematic that is causing the circuit to work like this.

    I've attached a photo of my schematic and my breadboard with all the components connected.

    [​IMG]
    [​IMG]
    Disregard the right half of that schematic, it doesn't really need any testing.

    To recap, all I really want to know is whether or not my schematic is solid. My breadboard problems I can work on.
     
    Last edited: Dec 16, 2010
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,535
    Welcome to AAC!

    I recognize the schematic,

    http://wolfstone.halloweenhost.com/Motors/svoint_RCServos.html#Driving

    The link that site referred to is gone, so this is it. I suspect the design basically works but is too simple. There is a second schematic on the site that might work better.

    http://forum.allaboutcircuits.com/showpost.php?p=104945&postcount=4

    Do us a favor, shrink that image about 80%. Makes viewing the page on a standard browser awkward. Or you could link to this image...

    [​IMG]

    I have some interest in how this turns out. Please keep us posted.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    What brand of servo are you using? If you are taking the output header and just plugging into it, then for the vast majority of servos, you have the wrong connections. Center is + (usually red or red-orange), one side is - or ground (usually brown or black), and the other side is signal (i.e., connected to pin 3 of the 555, usually white or yellow).

    A capacitor from pin 5 to ground may also help. See datasheet. Finally, I would try a larger timing capacitor and smaller resistor. Slow movements can be due to wrong repeat rate, which should be 50 Hz.

    John
     
    Last edited: Dec 16, 2010
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    You are also missing decoupling caps
     
  5. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    Ok first of all i'd like to thank everyone for their input. Anyway I found a datasheet for my servo and sorted out the pinouts.

    There I was, thinking I had all my problems sorted out. Now I have an even bigger problem. Overnight the capacitor managed to fall out of the breadboard. I hadn't noticed that it was missing and I plugged the servo in. Instantly the servo made a very loud pop and smoke started billowing out of it. Now normally for most people that shouldn't be a major setback; just buy another servo. My problem however is that it will take 4 weeks or so for a new servo to arrive and I need this circuit done and dusted by this week.

    Now I need to repair my servo, anyone know where to begin with something like this?
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    If you would give us a small clue, like the brand and model number, we might be able to help. Chances are, however, you have neither the equipment nor experience to trouble shoot and repair a PCB with SMD's.

    If you can find the same servo with broken gears, you could swap the amplifier board. Otherwise, buy a new servo.
     
  7. ayashifx55

    New Member

    Dec 17, 2010
    21
    0
    I have made a circuit for you. This is what we use at school. You can replace R2 or put another potentiometer if you want it to be even more adjustable.

    I'm not VERY good with servo but my basic understanding is servo takes a signal and based on the pulse width / frequency, it will move a certain degree according to it. My basic 555 timer is fully adjustable. In class, we used it to count from 0 to 9999

    [​IMG]
     
  8. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    Well i've just had a look at the servo's amp board. It does indeed appear to be completely toast. One single capacitor blew and somehow managed to charr a fairly large section of the PCB and cracked it straight down the middle. The surround insides of the plastic casing looked as if they melted slightly as well with the heat generated.

    Does anyone have any idea what may have caused all of this to happen? What was the fatal flaw? Surely a missing capacitor on the timer circuit couldn't have caused so much damage.

    Is it likely that any of my other components have been fried?
     
    Last edited: Dec 18, 2010
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Reversed polarity?

    John
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,535
    Could be there was a continuous DC level where there should have been a short pulse. This might be a RMS issue.
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    The one thing that blows a servo almost instantly is reversed polarity. That is why they (excluding a few stubborn Germans) started making the middle wire V+.

    John
     
  12. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    The servo that blew had the same pinout

    1 - Negative
    2 - Positive
    3 - Signal

    And I think you're right actually :(

    For some unbelievably stupid reason it looks as if I have reversed the polarity of the servo. Now all I need to do is find a new servo to retest my circuit.

    I've a few CPU fans lying around, afaik they use PWM signals as well for speed control. I'll give one a shot.

    Edit: Looking good, the CPU fan works but turning the pot has no affect on speed. I amn't 100% sure that CPU fans have a PWM based tachometer (And even if, within the signal range as my servo).

    Edit 2: Scratch that! It appears that it is indeed PWM based in some way or form. Turning the potentiometer slows down the CPU fan and at it's lowest value the motor whines and starts reversing very slowly. If only I had looked at the servo pinouts before blowing the servo, I would have been finished.

    Edit 3: I'm mistaken. There is no control, it's just a direct power connection to the fan and the fact that the connector is loose is what is altering the speed. I've confirmed this by removing the 555 chip from the breadboard. The fan still runs.
     
    Last edited: Dec 18, 2010
  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Where are you????

    Servos are cheap or free. A friend just threw out a dozen old ones. I have half a dozen that I am tearing apart to reverse engineer the amplifier boards. In other words, if you talk to active hobbyists, you probably can get a lot of servos to mess with.

    John
     
  14. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    Ireland. Over here there isn't really much of a RC scene and the few local shops here that sell servos sell their cheapest for around €12.

    Oddly enough I can get stepper motors easily enough from the numerous broken printers, floppy drives e.t.c that I have lying around. Servos are much harder to source.
     
  15. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Shops need to stay in business, so their prices may be a bit higher than mail order.

    Clubs are another thing. Here are some sites that list clubs in Ireland:

    http://www.greenhobbymodel.com/movi...s, Flying & soaring locations, personal links

    http://www.rc-airplane-world.com/rc-flying-clubs-uk.html

    http://www.rc-airplane-world.com/rc-flying-clubs-ireland.html

    I don't think Irish modelers are any different than Americans. If you show up at a club, lots of people will want to help. If you are young, you will have even more help. Most clubs have swap meets where you can get stuff really cheap. Good luck.

    John
     
  16. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    Ok i've done a bit of research and it looks as if the cap that blew was a polarized cap hence explaining why it blew when I reversed the polarity. I can't afford having the servo blow out on me from something as simple as reversed polarity, I need to make it secure and "idiot-proof". Knowing that, would simply putting a diode on the positive line prevent any more servo blowouts? A 1n4148 (The only diodes I have currently available? I know it will drop the voltage by about 0.6V but I can stomach a small loss in torque for reliability.
     
  17. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Diodes can be wired to make plugging in idiot proof, but the plugs are also polarized to make them idiot proof. Center V+ is also idiot proof. How many idiots are we trying to protect against?

    Just wire center positive and forget it. Those plugs can be reversed without problems, except for not working. :D It does cause crashes, for those who don't do a pre-flight.

    As for the capacitor, I suspect it was a tantalum cap. They tend go out in a spectacular fashion.

    John
     
    Last edited: Dec 18, 2010
  18. abc27

    Thread Starter Member

    Dec 16, 2010
    50
    0
    No good. I just got two servos (After four weeks, mind you :() off hobbyking. Still the exact same issue as earlier. All I get is a jump the moment I connect the battery, I can even disconnect pin 3 and the behaviour remains the same. Anyone have any ideas?

    If I plug in the servo socket fully there is no movement whatsoever. If I keep a marginal connection I can get the servo to do seemingly random movement back and forth. I'm at a complete loss as to what I should do.
     
  19. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Is the schematic in post#1 still your current schematic, except for the pins to the servo?

    If so, I will try to get it going tonight before NCIS.

    John
     
  20. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
    899
    Just noticed that the wiper on your pot is not shown connected to anything. Double check how you have it connected. The center pin should be connected to one end.

    John

    Edit: Seems to work OK for me. Pulse range 1.02 mS to 2.90 mS. No capacitor on pin 5 seems to be needed. The problem may be as simple as not having the pot connected correctly.

    John
     
    Last edited: Jan 11, 2011
Loading...