[dorkbotpdx-blabber] where to get and learn about solenoids

[Greg Borenstein]

Greg,

That was really helpful! I'd love to take you up on that offer for the  
rotary solenoids if you've still got them. Any leads on where I can  
look to learn more about "opto isolators"?

-- Greg

On Dec 2, 2008, at 11:38 PM, Greg Grunest wrote:

> Greg,
>
> Solenoids come in all different varieties.  Typically, you find the  
> 12 V,
> linear, push or pull solenoids in everyday consumer electronics (disk
> drives, hard drives, tape players, that old and now useless Zip-100  
> drive
> has a really cool 6v one in it but it's *really* hard to get it out,  
> etc...)
> but every so often you can find surplus rotary solenoids too.  For  
> moving
> the drumsticks, a rotary solenoid might be the thing.  They  
> typically move
> through a 90 degree arc when energized and then return to where they  
> started
> when power is removed.  However I've had some before that sort of  
> ratchet
> through 15-30 degree increments each time power is applied eventually
> spinning all the way around.  They were connected to a little  
> mechanical
> counter and appeared to do nothing more than count how many times  
> this thing
> had fired.
>
> The pros:
> 	They're cool!
> 	They typically work forever - millions of cycles.
> 	You can do things with an $8.00 solenoid that you could otherwise
> spend hundreds of dollars trying to replicate.  (I once built a remote
> controlled golf cart and used Ford starter solenoids ($8.00 from  
> Napa) to
> switch the 600 Amp battery connections to the motor.  I used a $3
> optoisolator to drive the solenoids and an 8051 to drive the  
> optoisolator.
> Controlling 600Amps from a microprocessor for $11 - Woot!)
> 	You can get feedback from them by measuring how much current they
> draw (voltage drop across an in-line resistor) and tell if they are  
> hitting
> anything.  They tend to draw a lot of current (like a stalled electric
> motor) if you hold them against their will.
>
> The cons:
> 	They suck current like mad and therefore are not good for battery
> driven applications.
> 	It's basically a coil so you have to worry about re-induction when
> the magnetic field around the coil collapses so you don't blow up  
> sensitive
> circuitry (like the Arduino or a transistor).
> 	They are typically not driven by any control signal that is easy to
> generate in a microprocessor world, 9-50V, 500ma-2A.
> 	(Because of these two drawbacks, I highly recommend a simple 8 pin
> optoisolator to drive them.  It's cheap and completely protects the
> Arduino.)
> 	You can't control how fast they move like you can with a servo.
> They are either in or out, turned or not.  In fact, if you try to  
> slow them
> down by limiting the voltage they draw a LOT of current.
> 	They are entirely digital - On or off, in or out.
>
> 	And the number one drawback is........ can you say "latch up"?
> (sometimes they stick in the "energized" state.)
>
> Just my 2 cents.
>
> BTW:  If you're interested, I think I have a couple of the 90 degree  
> rotary
> ones left.  Enough for you to play with and try the drums anyway.
>
> - Greg Grunest
> greg at grunest.com
>
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