Re: science teacher needs help

On 19 Oct 2005 21:15:31 -0700, "semidemiurge" <rick.fetters@xxxxxxxxx>
wrote:

>I need a simple solution for a classroom science project. I am a
>science teacher and I am constructing apparatus to demonstrate that a
>projectile drops at the same rate as an object falling in the vertical
>axis alone.
>I have the target released by energizing a solenoid, this is suppose to
>happen when the projectile exits the muzzle of a blowgun. My problem
>is how to energize the solenoid by the passage of the dart out the
>muzzle. One thought was using a NO microswitch but it causes a
>deflection in the path of the dart no mater what I do. I was thinking
>about using a small piece of alluminum foil that would be severed
>(breaking contact) but I would need some technique/circuit that would
>start the current flow to the solenoid when the foil circuit was
>openned. I don't have a suffecient background in electronics to solve
>this on my own, any suggestions? thanks rick

Your foil carries current to a relay that holds a set of contacts open
until the foil breaks . . . not much of a circuit. I think you'd want
to try that idea for problems too. I imagine the foil may deflect the
projectile also.

You might ram the projectile out of the barrel with a pneumatic
cylinder and have it make a connection when it protrudes past the
barrel.

Use a different projectile - a ball bearing drops down a long groove
and is deflected upward by a bend in the groove and shorts a set of
contacts inlaid into the guide groove. (or inlaid into the barrel of
your air gun if the projectile can be conductive)

A photo switch is the obvious choice.

Surplus center has a nice one for $19.95 Tiny physical / beam size
and good range.

http://www.surpluscenter.com/item.asp?UID=2005102007371005&item=11-3059&catname=

* Supply voltage 10-30 VDC
* 150 mA current switching rating
* Visible red beam
* NPN sinking output
* 8 m (26 ft.) max. range
* Two bolt mounts on 1" centers
* Size of each: 15/16" x 1/2" x 1-3/8"

The circuit is maintained while the beam is intact and opens when
blocked so you'd still need a relay - of 80 ohms or greater coil
resistance if run at 12 volts

Surplus Center puts out a pretty good paper catalog with all manner of
interesting tinker toys. American Science and Surplus is another
inexpensive toy source. I don't see any photo switches though.
http://www.sciplus.com/ Good paper catalog & interesting tinker toys.
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