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View Full Version : RE: control of a servo with output-pin (3.3volt)



nomad
02-04-2007, 10:35 PM
hi,
first excuse my verry bad english.
I build a quadrabod with 3DOF.
the servo is a highTurque-Servo with Ncm 274 (Newton-cenitmeters) and a input voltage of
6.0volts.
now, l am looking for simple device (transistors ???и or Operation-amplifier)
and schematics to control the servo with a PropStick_USB-pin (3.3volt)

outputPin --------> [device]----------> [signal_In Servo]
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ииииииииииииииииииииииииииии 5.0voltиииииииииииииииии 6.ovolt

Thank you for your help.

greetings
nomad

и
и

inservi
02-04-2007, 11:11 PM
Hello,

Your English is not less good as mine. Mine is very poor.

Is your servo need 6.0 volt for the power and for the signal line ? OR how y think the servo need 6.0V for power and 5.0v for the signal line ?

In the second case, it's very simple to use a MOSFET transistor.

The BS250 can draw 600mA and work practicaly as a switch.

try this :


outputPIn -----> to the Gate of the BS250 (or similar)

+5V or + 6V -----> to the Source of the BS250

and the Drain to the signal line of the servo





+ 6V
| Source
|-----|
Gate | |
outputPin ----------------|---->| mosfet BS250 or similar one
|
|-----|
| Drain
--------------> servo's signal line
|
-----
| |
| | <-- this resistor is probably not necessary
| |
-----
|
|
Ground






and maybe a 1K resistor between the Drain and the ground (normaly not necessary)

This must work with just one transistor and no more. The signal at the servo's signal line is high when the outpunPin is low (It's an inverter).

dro

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in medio virtus

Post Edited (inservi) : 2/4/2007 3:31:31 PM GMT

Mike Green
02-04-2007, 11:43 PM
Most servos can be controlled directly from a Propeller. Even though the servo motor and electronics are powered by 6V, the control line is a logic signal, usually considered low when below approximately 0.7V and high above that voltage. You need a protection resistor in series with the Propeller I/O pin, anywhere from 470 ohm to 4.7K usually although theoretically you shouldn't need it (this limits the input current if the I/O pin is accidently connected to a voltage higher than 3.3V or to some other output pin).

nomad
02-05-2007, 05:56 PM
hi inservi & mike,

thank you for your quick answer.
now i want buy the stuff and then .... testing.
when i have troubles or antother questions.
i come back
greetings
nomad

John R.
02-05-2007, 10:51 PM
In the case of the servos, the 4.7k resistor is meant to protect the Propeller from back EMF from the servo motor. Again, this probably isn't absolutely needed, but is a nice precaution. For some servos, you may have to lower the resistor value, or go with the transistor amplifier like the one described above. See the this thread: http://forums.parallax.com/showthread.php?p=627842 for another discussion on this topic.

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John R.
Click here to see my Nomad Build Log (http://share.crustcrawler.com/JohnR/)

nomad
02-06-2007, 08:25 PM
hi john,
thanks for your answer
greetings
nomad