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How to connect a servo to breadboard of BOE Bot? — Parallax Forums

How to connect a servo to breadboard of BOE Bot?

henry151henry151 Posts: 8
edited 2009-08-16 18:57 in Robotics
How would one safely connect a servo to the breadboard of the BOE Bot?

I would like to use Vservo to power extra servos on my project, I am using a 7.2v (6 AA NimH) power supply. With Vin selected.
Recently, while testing, I managed to fry pin 12 on my BS2 chip. I connected Vservo direct to the breadboard, it worked for a while, then pin 12 died. Interesting that the servo was connected to pin 11??

Or:

Is the 6 cell NimH too much for the BOE and stamp? Am I risking further damage to my robot?

I have now lost all confidence in my abilities and would welcome any help.

Comments

  • W9GFOW9GFO Posts: 4,010
    edited 2009-08-16 05:08
    henry151 said...


    Is the 6 cell NimH too much for the BOE and stamp? Am I risking further damage to my robot?

    I have now lost all confidence in my abilities and would welcome any help.

    With the exception of a few specialty servos, servos are not designed to be run on more than 6 volts.

    Servos are powered through the center pin on the connector. On the BOE that center pin either gets regulated 5 volts or whatever voltage your battery pack is supplying if you change the jumper to VIN. If you connect a servo via the breadboard it won't matter what you have set that jumper to. You would use three breadboard rows and three jumpers to make the connections. One jumper goes to VSS, one goes to a pin and the center will go to VIN or VDD.

    Rich H
  • PrettybirdPrettybird Posts: 269
    edited 2009-08-16 05:52
    So far I have been using 7.2 volts for servos. No problem yet. I admit I had to upgrade the original Parallax (futuba) servo to a heavy duty servo because of torque issues and it was ratted from 4.5 V to 8.4 V. Ii you worry, just put 2 diodes in series with the 7.2 V nicad battery pack or the center pin. A .6v X 2 will drop it down to 6 v. 7.2V has always been a standard for R/C stuff. They have instructions in the boe bot book for hooking up the Parallax homework. It has no 3 pin plugs.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-08-16 07:08
    Prettybird said...
    I had to upgrade the original Parallax (futuba) servo to a heavy duty servo because of torque issues and it was ratted from 4.5 V to 8.4 V.

    Could you show me where it says the Futaba servo is rated to 8.4 volts? I have looked at the specs of countless servos but I certainly won't claim to have seen them all.
    Prettybird said...
    7.2V has always been a standard for R/C stuff.

    Just because so many RC toys run off 7.2 volt packs doesn't mean that the servos operate at that voltage.

    The Homework Board drives servos off a 9V battery, it will drain it quickly. I can think of two reason why it works out okay; One, the expected usage of the servo is light - the servo won't be trying to draw much current for the exercises. Two, the 9V battery isn't capable of supplying much current even if the servo wants it. That's my speculation - I haven't tested it. They do caution not to use a 'battery replacer' or other power supply when using a servo with the Homework Board.

    A 7.2V battery pack on the other hand can supply lots of current to a servo that wants it, risking damage.

    Rich H
  • henry151henry151 Posts: 8
    edited 2009-08-16 12:35
    Thats the problem as far as I can tell. My servos are standard, they can take 7.2v, but they draw too much current and thats what fried one of my pins on the stamp.

    I am looking for a safe solution. So, how to connect via breadboard, whilst protecting the stamp against too much current?
  • Mike GreenMike Green Posts: 23,101
    edited 2009-08-16 13:55
    The only way a servo could damage a Stamp I/O pin is if there's a short of some sort inside the servo. A correctly functioning servo should not be able to damage a Stamp I/O pin. If you have too many servos (drawing too much current overall) or if you have a single servo that's stalled, thus drawing up to 1A and you've connected the servo to Vdd so it's operating off the Stamp's regulator, that might cause the regulator to shut down. It won't damage the regulator, but the Stamp will stop working while the servo is connected. In that case, the Stamp should work properly when the servo is unplugged.

    Something else must have happened if your Stamp I/O pin is damaged. It's difficult to make a suggestion without knowing what really happened.

    A good general way to protect Stamp I/O pins from mishaps is to connect a 220 Ohm resistor in series with the I/O pin. This is the way the Homework Board is designed. There are few circumstances where a 220 Ohm resistor will interfere with the normal use of the I/O pin, yet it will protect the Stamp I/O pin from a lot of potential kinds of damage. For some kinds of use (like driving LEDs) where you have to figure in the extra resistance into your circuit.
  • PrettybirdPrettybird Posts: 269
    edited 2009-08-16 18:57
    Diodes as a easy cheap electronics trick to drop any kind of DC voltage in .6V increments for anything if you worry so much. You can measure voltage drop on a diode with any inexpensive digital meter.
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