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

[Greg Grunest]

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