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<div class="moz-cite-prefix">Hi Lucas,<br>
Please "reply all" to this msg to let us know if the
information so far has given you any ideas for a solution, and if
so, what your implementation scheme will be. Also, if you're OK
with sharing the details, we're eager to know what the string is
connected to, if the string release happens periodically, etc.
If circumstances have changed or something else has made this
project of yours moot, no worries, and I don't want you to feel
any pressure to reply. It's just that some of us are very curious.
:-)<br>
I'd like to summarize a few high points I remember from the
msgs so far. Folks: I've only read a few msgs of this thread, so
forgive me if this is partially or entirely redundant.<br>
1) Raspberry Pi gpio pins are *VERY* weak, only being able
to source/sink a few milliamperes of current, in a strictly "3.3v
power supply" context. That is, that pin cannot tolerate negative
or >3.3v potentials, cannot be expected to hold a large current
at ground, properly drive a high current load, etc. Part of the "0
< voltage < 3.3" requirement is that no inductive load can
be allowed to dump it's back EMF from a magnetic field collapse
into the Raspberry Pi pin: that will almost certainly render that
part of your Raspberry Pi inoperable if it doesn't destroy the
whole chip. So, for multiple reasons an RPI can only directly
sink/source ("provide a ground for"/"drive") a few devices such as
another integrated circuit or bare transistor or a (low voltage)
LED with a suitably large value resistor for current limiting.<br>
2) Driving something like a solenoid, stepper or plain DC
motor, relay, etc, requires a circuit that can "translate" the
Raspberry Pi signal/sink into a relatively high current and
usually higher voltage capable of running the device.<br>
3) Servo motors are special. In the context of this thread
(i.e in regards to the "hobby-type" servos Rod Radford mentioned
vs an industrial servo), they are self-contained motors that have
a very low voltage, low current drive requirement easily met by a
Raspberry Pi and conveniently works in conjunction with off the
shelf software capable of causing the servo motor to hold
position, move a fixed distance one way or the other, rotate
continuously in one direction or another, etc.<br>
"String grabbing" would seem to be a special case of the "rope
grabbing" someone described vis a vis sailboats, so emulating the
sailboat device on a scale appropriate for your string and weight
would seem sensible. Using a servo motor for the mechanical end of
things would allow you to focus on your application and not get
bogged down with hardware/software details.<br>
I look forward to seeing you and the others who have contributed
to this thread at a future meeting. The next mtg at NCSU is
Monday, October 8th.<br>
<br>
Best Regards,<br>
Pete (home from UNC hospital and recovering from step one of a
long-planned two part heart-remodeling project)<br>
<br>
On 9/23/2013 6:57 PM, Lucas Rumney wrote:<br>
</div>
<blockquote
cite="mid:CACCReN+vHPbVJDQR0g4c7tPm9hSXA9uomAY5itSLLaWBng=CaA@mail.gmail.com"
type="cite">
<div dir="ltr">I am trying to make something that would achieve
the following end:<br>
<br>
raspberry pi being able to release a weight on a string. <br>
<br>
My idea was that the pi could send a signal to disengage some
kind of clamp and release the weight, but I am not sure what
kind of clamp or what I should use here?<br>
<br>
Does anyone have any better ideas how to release a weight on a
string controlled by a pi?
<div><br>
</div>
<div>Thanks, I can't attend this meeting because of my
courseload, but I still want to participate in general
discussions. </div>
</div>
<br>
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