<div dir="ltr">Pete,<div><br>I just realized I was not using "Reply All". So much for my internet skills I replied twice with only "Reply" so they never made the list. Both are copied below.</div><div>
<br></div><div>I love the Bob Pease reference on driving the solenoid using a capacitor. (I got an autograph a copy of "Troubleshooting Analog Circuits" from him at seminar 10 years ago). Note that the author of the solenoid_drivers.pdf has a typo in their schematic. All of the reference to calculating a resistor value are actually referring to the resistor in parallel with the capacitor. The resistor with the capacitor is always calculated based on the DC resistance of the solenoid coil. The DC resistance of the solenoid and the resistor in parallel with power capacitor were labeled several different ways and could confuse someone who was not familiar with the application. </div>
<div><br></div><div>-Shane</div><div><br></div><div><br></div><div><<<span style="font-family:arial,sans-serif;font-size:13.333333969116211px">Like most things in engineering, it depends. How much weight and how much control do you need (milliseconds or seconds between the signal and the load dropping?) A small electromagnet could release a weight. A hobby servo or solenoid could pull a pin to release a weight. The hobby servo would require minimal hardware. The others would require at least a transistor and diode to power the magnet or solenoid. Below is a link for a tutorial on the hobby servo. </span></div>
<div style="font-family:arial,sans-serif;font-size:13.333333969116211px"><br></div><div style="font-family:arial,sans-serif;font-size:13.333333969116211px"><a href="http://learn.adafruit.com/adafruits-raspberry-pi-lesson-8-using-a-servo-motor/servo-motors" target="_blank">http://learn.adafruit.com/adafruits-raspberry-pi-lesson-8-using-a-servo-motor/servo-motors</a><br>
</div><div style="font-family:arial,sans-serif;font-size:13.333333969116211px"><br></div><div style="font-family:arial,sans-serif;font-size:13.333333969116211px">Is the weight load small enough to hold with a clothes-pin? If so, you might be able to put a cam on a hobby servo and use it to open the clothes pin to release the load. For that matter you could hang the load from a pin or rod attached to a hobby servo and rotate the servo to drop the load. It all depend on the load and the level of control that you need. More detail will get you better guesses! I hope something here is useful to you.>></div>
<div style="font-family:arial,sans-serif;font-size:13.333333969116211px"><br></div><div style="font-family:arial,sans-serif;font-size:13.333333969116211px"><<The Adafruit solenoid looks very nice if you have the power to drive it. Use a decent logic-level N-FET along with a kickback diode in parallel with the solenoid and you should be all set. I would add a 100 ohm resistor in series with the gate of the FET just for good measure.</div>
<div style="font-family:arial,sans-serif;font-size:13.333333969116211px"><br></div><div style="font-family:arial,sans-serif;font-size:13.333333969116211px">As Rodney mentioned, you need to be careful of noise induced or conducted to the power rails of your microcontroller. I recommend using a twisted-pair cable (phone or Ethernet cable would be fine) to connect your gate drive signal and logic ground to the FET and only connect them at the FET with the FET mounted as closely to the solenoid as possible.>></div>
</div><div class="gmail_extra"><br><br><div class="gmail_quote">On Thu, Sep 26, 2013 at 10:19 AM, Pete Soper <span dir="ltr"><<a href="mailto:pete@soper.us" target="_blank">pete@soper.us</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div>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)<div><div class="h5"><br>
<br>
On 9/23/2013 6:57 PM, Lucas Rumney wrote:<br>
</div></div></div>
<blockquote type="cite"><div><div class="h5">
<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>
<fieldset></fieldset>
<br>
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