[TriEmbed] Question concerning a mechanical release

Thu Sep 26 10:25:54 CDT 2013

Hello,

I had fairly decent luck controlling a 24V solenoid just using a arduino
and a F12N10L power mosfet (http://www.fairchildsemi.com/ds/RF/RFP12N10L.pdf).
This particular mosfet has a back EMF protection diode built in.  They are
also pretty cheap.

Thanks,
Charlie West

On Thu, Sep 26, 2013 at 10:49 AM, Shane Trent <shanedtrent at gmail.com> wrote:

> Pete,
>
> 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.
>
> 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.
>
> -Shane
>
>
> <<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.
>
>
> http://learn.adafruit.com/adafruits-raspberry-pi-lesson-8-using-a-servo-motor/servo-motors
>
> 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.>>
>
> <<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.
>
> 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.>>
>
>
> On Thu, Sep 26, 2013 at 10:19 AM, Pete Soper <pete at soper.us> wrote:
>
>>  Hi Lucas,
>>    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. :-)
>>    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.
>>       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.
>>      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.
>>      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.
>>    "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.
>>   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.
>>
>> Best Regards,
>> Pete (home from UNC hospital and recovering from step one of a
>> long-planned two part heart-remodeling project)
>>
>>
>> On 9/23/2013 6:57 PM, Lucas Rumney wrote:
>>
>> I am trying to make something that would achieve the following end:
>>
>> raspberry pi being able to release a weight on a string.
>>
>> 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?
>>
>> Does anyone have any better ideas how to release a weight on a string
>> controlled by a pi?
>>
>>  Thanks, I can't attend this meeting because of my courseload, but I
>> still want to participate in general discussions.
>>
>>
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>
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