Reversing a servo

K

Kevin Fitz-Gerald

Guest
I want to use a standard rc servo connected to a Dogtra remote receiver
(Dogtra RR) to trip a remote-controlled slingshot for dog training. Output
is something like 5-6 volts for about 2 seconds or so. The problem: I need
to reset the servo after pulsing it. Does anyone know of a simple way to
return it to its starting position? I know the polarity has to be reversed,
but it has to be done before the pulse is over. I'm not sure how to proceed.
I know it could be done with physical switches, but I am looking for a chip
that will do it in a neat package that could be inserted between the servo
and the receiver. Thanks for your help.

Regards,
Kevin

Remove first three characters of email: xyzkrfitz@defnet.com
 
"Kevin Fitz-Gerald" <xyzkrfitz@defnet.com> wrote in message
news:1062s2639c6tu4a@corp.supernews.com...
I want to use a standard rc servo connected to a Dogtra remote receiver
(Dogtra RR) to trip a remote-controlled slingshot for dog training. Output
is something like 5-6 volts for about 2 seconds or so. The problem: I need
to reset the servo after pulsing it. Does anyone know of a simple way to
return it to its starting position? I know the polarity has to be
reversed,
but it has to be done before the pulse is over. I'm not sure how to
proceed.
I know it could be done with physical switches, but I am looking for a
chip
that will do it in a neat package that could be inserted between the servo
and the receiver. Thanks for your help.
You don't want to reverse the polarity. You need to generate a new servo
Click to expand...
control pulse of the width corresponding to the required position. The
'pulse' train is a continuous train (it is never 'over', so long as the
transmitter is on...), usually comprising a pulse of about 1.5mSec (varying
in width according to the required position from 1mSec to 2mSec), repeating
about every 50mSec. Hence all you need is something like a NE555, set to
give the right width pulse, and repeat interval, and a second NE555, set to
give a couple of seconds delay, triggered by the switch. Then use the 'two
second' NE555 to control a logic gate to switch between the radios pulse
train, and the one from the NE555. When the two second 555, is triggered,
the servo will start receiving the train for the new position, and move to
this.

Best Wishes
 
On Wed, 24 Mar 2004 06:25:49 -0500, "Kevin Fitz-Gerald"
<xyzkrfitz@defnet.com> wrote:

I want to use a standard rc servo connected to a Dogtra remote receiver
(Dogtra RR) to trip a remote-controlled slingshot for dog training. Output
is something like 5-6 volts for about 2 seconds or so. The problem: I need
to reset the servo after pulsing it. Does anyone know of a simple way to
return it to its starting position? I know the polarity has to be reversed,
but it has to be done before the pulse is over. I'm not sure how to proceed.
I know it could be done with physical switches, but I am looking for a chip
that will do it in a neat package that could be inserted between the servo
and the receiver. Thanks for your help.

Regards,
Kevin

Remove first three characters of email: xyzkrfitz@defnet.com


Hello Kevin,
Click to expand...
the little radio control servos that I have played with require
a pulse roughly every 20 milliseconds of between roughly
1millisecond and 2milliseconds duration. Neutral position
being roughly 1.5 milliseconds.

You can make a little pulse generator to operate your servo.

Perhaps you could could make your Rx operate a relay.

When the relay is in one position the pulse generator
produces a 1millisecond pulse and when the relay clicks
it switches a capacitor or resistor in the pulse generator
to produce a 2millisecond pulse. So the servo will be
driven hard left or hard right as the relay clicks.

Google up radio control circuits and servo tester
for a circuit of the pulse generator. Oops better
make that schematics, you will get more hits with
that word. :)

Cheerio,
John Crighton
Sydney
 
On Wed, 24 Mar 2004 12:38:00 GMT, john_c@tpg.com.au (John Crighton)
wrote:


You can make a little pulse generator to operate your servo.
Click to expand...
Hello Kevin,
I am following up my own post.

A quick look on the web turned up this diagram
of a suitable pulse generator/servo tester.
http://members.aol.com/hprinzler/9610-03a.gif
http://members.aol.com/hprinzler/kape_e04.htm

You can modify that circuit a little bit to give you
the two different pulse widths that you require.
Make R3 a trim pot.
Add an additional trimpot (same value as R3)
between R3 and R1
Have a pair of relay contacts short out the new
additional trimpot so that pulse width changes
1 mSec to 2 mSec.

You will need an oscilloscope to measure the
pulse width as you play around with the two
trimpots.

Have fun,
John Crighton
Sydney
 
Thanks for the ideas. Some further clarification; the output from the
Dogtra receiver is pure +7 vdc (no wave form of any kind) that lasts 1-2
seconds. I'm still trying to find out exactly how long it lasts. The servo
has had its circuit board removed and is simply a motor and gear train in a
case. This is why I wanted to reverse the polarity.

Kevin

Remove first three characters of email: xyzkrfitz@defnet.com
 
On Wed, 24 Mar 2004 19:07:42 -0500, "Kevin Fitz-Gerald"
<xyzkrfitz@defnet.com> wrote:

Thanks for the ideas. Some further clarification; the output from the
Dogtra receiver is pure +7 vdc (no wave form of any kind) that lasts 1-2
seconds. I'm still trying to find out exactly how long it lasts. The servo
has had its circuit board removed and is simply a motor and gear train in a
case. This is why I wanted to reverse the polarity.

Kevin
Click to expand...
You could make a motor reversing setup with limit switches and
small relays. a flip/flop chip or a decade counting chip could
alternate the direction on each button push. A simple motor
revrsing setup at the bottom of the below page.

http://www.geocities.com/zoomkat/switch.htm
 
"Si Ballenger" <shb*NO*SPAM*@comporium.net> wrote in message
news:4062343c.173930228@news.comporium.net...
You could make a motor reversing setup with limit switches and
small relays. a flip/flop chip or a decade counting chip could
alternate the direction on each button push. A simple motor
revrsing setup at the bottom of the below page.

http://www.geocities.com/zoomkat/switch.htm
Click to expand...
Hi Si - Thank you, that's the idea. Now I just need a small neat package (a
chip?) that I could either plug in-line with the receiver/servo, or I could
put into the servo case. And I don't want to pulse the servo again to reset
it; I can do that now with a switch built into the case to reverse polarity.
I need the servo to go forward for half the +dc pulse, and reverse the other
half of the pulse. -Kevin
 
On Thu, 25 Mar 2004 06:24:24 -0500, "Kevin Fitz-Gerald"
<xyzkrfitz@defnet.com> wrote:

"Si Ballenger" <shb*NO*SPAM*@comporium.net> wrote in message
news:4062343c.173930228@news.comporium.net...
You could make a motor reversing setup with limit switches and
small relays. a flip/flop chip or a decade counting chip could
alternate the direction on each button push. A simple motor
revrsing setup at the bottom of the below page.

http://www.geocities.com/zoomkat/switch.htm

Hi Si - Thank you, that's the idea. Now I just need a small neat package (a
chip?) that I could either plug in-line with the receiver/servo, or I could
put into the servo case. And I don't want to pulse the servo again to reset
it; I can do that now with a switch built into the case to reverse polarity.
I need the servo to go forward for half the +dc pulse, and reverse the other
half of the pulse. -Kevin
Click to expand...
You probably can use the motor reversing setup with a single
small 6 volt double throw, double pole relay. Wire the servo
motor leads to the two center common leads on the relay. Cross
wire the outboard relay contact leads in an "X" type fashon. Then
connect the one side of the "X" wiring to the motor power source.
Now with no power on the relay coil, the set of normally closed
relay contacts applys power to the servo, rotating the servo
until the limit switch opens the circuit stopping the servo in
that direction. This is the resting state of the servo. When
power is applied to the relay coil, the relay switches over and
connects the motor leads to the other side of the "X", which now
supplys opposite voltage to the servo, moving it in the opposite
direction until the other limit switch opens the circuit. When
power is removed from the relay coil, the relay goes back to its
deenergized state connecting the motor leads to the origional
side of the "X" once again reversing the voltage to the servo.
The relay rotates back to its origional position where the limit
switch opens the circuit. This might work quite well with your
~1-2 sec blip of 7v power, having it energize the relay coil.
This "X" setup is often used with spring centering on-off-on
position toggle switchs for running motors in two directions
(raise/lower outdrive on a boat and such). The diodes in the
reversing circuit bypass the limit switches and allow enough
voltage to flow in only one direction to get the motor off the
limit switch letting it close.
 
"Si Ballenger" <shb*NO*SPAM*@comporium.net> wrote in message
news:40639cc3.266213424@news.comporium.net...
You probably can use the motor reversing setup with a single
small 6 volt double throw, double pole relay. . . . The diodes in the
reversing circuit bypass the limit switches and allow enough
voltage to flow in only one direction to get the motor off the
limit switch letting it close.
Click to expand...
Thanks Si, I will try this out.

Regards,
Kevin

Remove first three characters of email: xyzkrfitz@defnet.com
 
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