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View Full Version : Question Need advice for receiver/servo experiment...


pb1592
Jan 20, 2009, 02:38 PM
Hi.

In short, what I want to do is connect a very small 3.7V motor to a
receiver where a servo would go and allow the transmitter to control
the direction and speed of the motor. Anyone know a way to make this
happen? Below is a description of why I want to do this...

I am trying to piece together a hybrid heli out of an existing one and
some old parts. I have a 3-channel Symma S002 and, although its only 3
channel, I love flying it and like the way the body looks. This heli
has a horizontal tail rotor for forward/backward motion. The major
drawback to this heli, obviously, is lack of a gyro which makes it
difficult to control.

I have an old 3-in-one receiver/esc/gyro from a 4-channel Walkera 5#5
(which uses the exact same battery/motors as the S002). I want to put
the 3-in-one in the Symma S002. This heli has no servos and I want to
somehow use one of the servo outputs on the 3-in-one to control the
tail motor on the S002. The voltage output to control the servos are
more than enough for the tail motor but I can't figure out how to
allow for alteration of the current to make the rotor spin
forward/reverse/variable speed in response to moving the conrols. I
imagine in theory something could be constructed to do this. The three
pins for the servo ouput are V+, Vgnd and the variable signal pin to
control the pot in the servo. Any ideas on how this could be
accomplished with a tiny, cheap, homemade circuit?

Thanks!

BushmanLA
Jan 20, 2009, 06:43 PM
In general you can rip the guts out of a servo, attach just about any DC motor within reason or use the original one if it is fast enough, and replace the pot with two 2.5k resistors on either side of the center tap. (I think its 2.5k, better check).

Toss out all of the gears of course and attach your prop directly to the output shaft of the dc motor.

You now have a hacked servo that can give you some amount of speed control and forward/reverse direction.

The trick is getting a dc motor that will turn fast enough but not draw more current that the servo guts can handle.

david_dewit
Jan 26, 2009, 04:58 AM
http://www.rcgroups.com/forums/showthread.php?t=919303
http://www.rcgroups.com/forums/showthread.php?t=926662
http://www.rcgroups.com/forums/showthread.php?t=990007

Hope this helps.

Dave

earlwb
Jan 27, 2009, 04:24 PM
In robotics, many people on the simple robots take a pair of conventional servos and pull the tops off and carefully cut away the stop tab on the main gear or top of the case. Then they bypass the feedback pot with a couple of resistors, so that they have a neutral off, and forward or backwards. Then when you mount a wheel to the servo arm, you can use the servos as a drive wheel system. Reverse one servo too.
Here is a article on how they do it here:
http://dprg.org/projects/2003-05a/