[sdiy] Kinda OT: electro-mechanical instruments/ solenoids

[Cornutt, David K]

From: Ken Stone [mailto:sasami at hotkey.net.au]
> When the
> solenoid is activated, the momentum of the bead causes it to 
> hid the metal
> bar, and the flex of the cane will lift it clear. It sure 
> beats trying to
> electronically time the drive pulse to prevent the striker 
> from damping the
> bar after it hits.

Have you ever seen how pinball machines regulate the flippers?
The power to the solenoid is routed through a leaf switch that
is placed at a certain point along the shaft's travel, so that
when the shaft passes that point, the leaf switch opens and
the spring pulls the shaft back until the switch closes again.
It's an electromechanical regulator.  The purpose is to give
the solenoid full power when the button is first pressed so
the flipper has enough oomph to push the ball back, but then
reduce the power when the solenoid is held at its travel limit
(as when the player is using the flipper to hold the ball),
so the solenoid doesn't burn out.  

The main problem with it is that it's maintenance intensive;
the cycling of the switch wears the contacts rapidly, and when
the contacts get worn the whole arrangement starts to buzz.
(Some of you old-timers may recall mechanical automotive voltage
regulators that had the same problem.)  However, if you could
build some inertia/hysteresis into the arrangement, then you
could turn the buzz into a cycling at any desired rate.  That
would give you an auto-repeat mechanism, which with a xylophone
or the like would tend to make it sound like a gamelan.  Could
be interesting.

BTW, you'd be amazed at some of the things those old theater
organs did.  Some of them had mechanically operated guitars,
banjos, and violins with solenoid-operated "fingers" to stop
the strings.