I'm trying to do what is in that link you gave me. In the big picture,
I'm trying to control a servo so it will rotate a touch screen out of
the dash of my car; up to viewing angle. So, I will want to have it
run either when I give power to it or switch it on and then stop at the
specified angle and then again go back down when it is switched off or
So, do you know how to build electronic circuits ? (Solder wire and chips)
Do you think your RC servo will move the LCD display ?
An RC servo has about 270 degree movement, will that move the LCD to
where you want it ??
This sounds like more then your initial request.
Have you built the hardware that holds the LCD display yet ?
Do you have any pictures ?
No, I don't know how to build circuits - I guess that is really what I
want to understand. I understand the concept behind using the 555 ic
with the servo, just not sure how to build the entire thing.
I'm pretty sure my servo will move the LCD - it's a small touch screen
that weighs just over 1 lb.
I really only need the servo to move the LCD setup about 90 degrees,
which should not be a problem with this servo (it moves about 180
I just got the basis of the hardware done for the screen today. I
still have to clean it up and put padding in it and such (it's made of
aluminum and thus both light weight and has sharp edge, etc.). I made
the pivot point out of a hollow aluminum rod which brings me to my
other question right now: how will I go about attaching the servo to
the end of the rod to turn it? But, that was not intended to be
answered here, but would be very helpful.
Thanks for your help.
You need a strong servo for this, unless your touch screen is very small
and light. Most weigh at least 9-10 ounces, and several weigh a pound or
The GWS S06 servos come with a version that offers some 110 oz-in of
torque. That might be a minimum for you, as long as the servo is merely
rotating the display about its center, and not actually lifting it.
You can spend a lot of money on servos, and the stronger ones are bulky.
Have you considered not doing it the "Star Trek" way (which is cool, I
admit), and use dashpots instead? You can get surplus dashpots for maybe
$5 each. With springs from the hardware store they'll provide a smooth
gliding action of opening the LCD into view. You then push it back
manually into a locked position.
I've looked into using an actuator already and found that the servo
would be the best way to do it because of the size. Something much
bigger would not fit into the space I have available. I'll be the
first to admit if this doesn't work I'll back up toward something less
active, something more like you suggested using a spring and a latch or
The servo I have is rated close to that 110 oz-in, so it might do the
To throw some math at the problem, if the screen weighs one pound and
measures eight inches from the rotation axis, the servo will initially
have to lift the full weight of the screen at the center of gravity, or
4 inches * 16 ounces = 64 oz-in minimum torque. A plain-vanilla hobby
servo is about 50 oz-in, so he'll need to look at larger ones that offer
I don't think he's rotating around the center. But the 110 oz-in servo
could do the trick, if the screen isn't heavier than we're guessing.
Some of the standard (non-digital) servos, in the 1/4 scale range, are
actually pretty cheap for 100+ oz-in. I'm of course looking at
http://www.servocity.com which has pretty much everything a servo user
could want. Including shaft adapters, to address his earlier question
about mounting the servo to a pivot tube.
One more consideration: this is a touchscreen, and when the screen is
being poked the servo will be fighting back. Also, the same thing will
be happening during acceleration and deceleration. One too-hard of a jab
or braking and the servo gears could be stripped out. Using a servo of
course means that to keep the screen upright, the servo would need to be
powered at all times. Otherwise the touchscreen will flop over the first
time it's poked.
I like the dashpot idea, actually. Might provide a little bit of shock
Using math, it looks like I will need more torque if my setup stays the
24 oz * 5.5 in. from pivot point to top of touch screen = 132 oz-in.
As for the stability of the servo, I think it will do the job beacause
I'm planning on keeping it powered, and if I get one that has metal
gears it should be fine, correct?
For servos, there's no real guarantee on longevity. They are designed
for high performance over short durations, maybe a 20 minute flight. I
don't have numbers on hand but I had thought they were usually under 50
hours expected usage. The gears aren't the only factor, other wear parts
include the potentiometer and the motor itself, which is usually driven
as hard as possible.
Please also realize that the servo is going to try to snap your screen
to the vertical position in about a quarter second. Unless you have some
kind of programmed proportional sweep of the servo input, you are going
to risk breaking the LCD or even launching it who-knows-where.
On 19 Jan 2006 09:54:29 -0800, email@example.com wrote:
If you are just looking for open/close action, you could use the
geared motor from a $8 6v B&D cordless screwdriver from walmart,
and a motor reversing circuit. Some simple diagrams for circuits
at the bottom of the below page. Use limit switches to stop the
action on each end of movement.
When you do the mechanical design on this, it would be a good idea to
design it so that the screen rests on something in both the open and
closed positions. If the servo is supporting it in either position in a
moving car, you will probably see double the forces you calculate for a
stationary system due to bumps and vehicle dynamics. The vibration will
also beat things up over time.
Well, this seems to be turning into something more iffy and more
complicated than I expected. At this point I kind of just want to
scrap it and go with something I can just manually open. Maybe the
auto open idea is more of a pipe dream.
Thanks for all your comments, I'll have to think about it.
The electronics isn't too hard -- except that you appear to be a
novice. One thing to think about: if you turn the power off, how will
the servo close? You really need to have the logic control the power so
that you can keep the power on long enough to move the servo and then
turn itself off. Also as somebody pointed out you have to ramp the
servo so that you don't try to move the screen too fast. Not hard to do
if you do it in software.
The real battle is the mechanical side. Even something like mounting
the screen to the servo will be difficult. Most RC servos I've seen
just have a nylon horn with small teeth on the *inside* diameter. Not
exactly a strong mechanical connection. Plus as others have mentioned
you'll want some sort of locking mechanism. I've seen screens that do
what you're trying to do, and although I didn't look inside, I would
suspect that they use geared DC motors with limit switches. (Look
inside a CD-ROM tray and see how that works, same exact thing.) Those
motors are much stronger (think a cordless drill) and more reliable,
and you can control the velocity by changing the gearing appropriately.
If you think you can handle the mechanics, the electronics is something
you could get a friend (or maybe even someone here) to help you make.
Do a Google search for "servo tester" and "servo 555" (both without
quotes) There are many examples of building a 555-based tester for R/C
servos. I remember seeing some that were pretty basic, including
explanations on how to assemble the board. These testers are simple
enough that you can create them on a basic solder board that you can buy
at Radio Shack.
The first hit I got doing a search for "servo 555" is this one:
Seems to have a lot of what you're looking for.
Seems to me a worm gear setup with limit switches would be the desirable
situation. Worm gears, if properly lubed, last a long time and resist
external forces once they are turned off. A simple double relay circuit can
be wired to handle the logic with a push of a button and cause the motor to
retract by pushing the same button again. A power mirror motor gotten from
a junk yard would probably do the trick. If not that - go for a power
antenna motor. (note - a power antenna motor often has the proper relay
already supplied - but the stops may need to be adjusted).
I don't think you want a hobby servo motor because of what others have
said - they are expensive, they are always on, they require supporting
electronics. On the other hand... when worm driven motors stop they STOP.
There is no more power needed to maintain their position. There is a great
resistance to move in either direction. This configuration is what I would
use for your suggested application.
You can adapt these motors to servo electronics - but they are a bit
sluggish and power hungry when used this way.
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