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330 ohm resistor between Stamp I/O pin and servo signal wire. — Parallax Forums

330 ohm resistor between Stamp I/O pin and servo signal wire.

Martin_HMartin_H Posts: 4,051
edited 2010-04-13 12:47 in Robotics
All the servo control I've done with a micro-controller has you wire the signal wire directly to the stamp I/O pin. But Scribbler's hacker port has a 330 ohm resister between the pin and the port. While this protects the pin from shorting, it also means that it will be between my servo and the signal coming out of the pin.

Now I could just hook it up and see what happens, but I'm a bit nervous about making the magic smoke come out of Scribbler. My instincts tell me that the absence of a resistor is more likely to do damage then the presence of a resistor. It is likely to either not be a problem, or not work. But searching with Google hasn't turned up anything definitive.

Does anyone know what effect the 330 ohm resistor will have?

Comments

  • ercoerco Posts: 20,256
    edited 2010-04-07 02:14
    The resistor just stops a short circuit, or any large currents flowing in or out of the Stamp. Should work fine with a servo, no big currents are flowing in the control wire.

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    ·"If you build it, they will come."
  • Martin_HMartin_H Posts: 4,051
    edited 2010-04-07 02:33
    Thanks erco, I'll give it a try then.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2010-04-07 16:01
    While there aren't big currents on the servo signal line, some servos can inject electrical 'noise' into the signal line that can cause problems with a micro controller. The 'extra' resistance can help attenuate this noise so that it becomes less of a problem. With multiple servo's and/or lengthy runs, the noise issue is only exaggerated.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Martin_HMartin_H Posts: 4,051
    edited 2010-04-07 23:47
    That makes sense. I suppose anything with a motor can produce noise. I'll try a servo with my hacked Scribbler this weekend.
  • MikerocontrollerMikerocontroller Posts: 310
    edited 2010-04-08 00:00
    To those using the HomeWork Board·: All sixteen·I/O pins are tied to 330 ohm resistors.·
  • ercoerco Posts: 20,256
    edited 2010-04-08 03:12
    According to·http://www.parallax.com/StoreSearchResults/tabid/768/List/0/SortField/4/ProductID/133/Default.aspx?txtSearch=homework+board, they are series 220-ohm protection resistors, but·the exact·shouldn't matter in most low-current applications.

    I get hosed when I try to drive a 5V 20 mA relay directly off a HW board pin, since the resistor has a substantial voltage drop in that high-current case and the relay can't activate. In this situation, a BoE works, but·the HW board doesn't.

    The good news is that for many transistors like 2N2222's, 220 or 330 ohms is just about right for a bias resistor. In that case, you can use a HW board output directly connected to the transistor base, no additional resistor required.

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    ·"If you build it, they will come."
  • MikerocontrollerMikerocontroller Posts: 310
    edited 2010-04-08 04:06
    Sorry erco, you're right. My HomeWork Board is still alive thanks to those resistors. P.S. That's a good tip about the transistor interfacing.
  • ercoerco Posts: 20,256
    edited 2010-04-08 04:39
    MC: T'ain't about right, wrong, or sorry. It's all about blowin' stuff up PROPERLY! [noparse]:)[/noparse]

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    ·"If you build it, they will come."
  • Martin_HMartin_H Posts: 4,051
    edited 2010-04-13 00:26
    I tried this yesterday and I was able to control the servo/gripper with no problems. I also found servo ramping code which was much simpler than I thought. The only funny thing is I had a bug which confused the gripper state and I ramped it the wrong way at one point which caused it to snap open and then slowly close.
  • DutchBotDutchBot Posts: 24
    edited 2010-04-13 08:52
    What about high torque servo's? I suppose they run on much higher currents? How would that reflect resistor wise?
  • allanlane5allanlane5 Posts: 3,815
    edited 2010-04-13 12:47
    The "control line" doesn't use high currents, the high currents go through the Vcc and Gnd supply wires. So, with a low-current control signal, you're able to control a servo using high-currents on the supply wires.
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