Model Forum / Radio Controlled / Water Models / October 2006

You may want to use a magnetic "reed switch" to detect the hatch opening.
Glue a small magnet to the edge of the hatch so it comes very close to the
glass envelope of the switch.  Here's a description of how they work:
http://en.wikipedia.org/wiki/Reed_switch The glass envelop makes these
naturally water proof - you can waterproof the connection with a couple of
coats of model enamel, but in fresh water it's not critical.

Radio Shack used to carry a couple of these, but I don't know if they still
do, except as the plastic-cased ones used in burglar alarms. You can
often find them very cheap at electronic surplus stores like this:
http://www.sciplus.com/singleItem.cfm?terms=12671&cartLogFrom=Search
The smaller ones are probably better for your application - one whose glass
section is about an inch long.

If you're lucky, you can find a switch that is "open" when the magnet is
close.
That means it would turn on your LED when the magnet moves away. These
aren't that easy to find, though, since the most common type closes when the
magnet is close. In some cases, it's easier to find "SPDT" types which have
two contacts - one of which is open when the other is closed. These have
three wires instead of two. (The common variety is SPST - single pole,
single throw.)

If you can only get the SPST type, you could use a small relay with SPDT or
DPDT (double pole, DT - just use one set of contacts) to turn on the LED
when the reed switch opens. Or just assume a lit LED means the hatch is
closed.

By the way, for the resistor value to use with your LED, here are a
few suggestions that should give you plenty of light. (Each color LED
has slightly different needs, but they sort of fall into two categories.)

Red, green or yellow LED:  430 ohms for 12V, 300 for 9V, 180 for 6V
White or blue LED:  360 ohms for 12V,  240 for 9V, 130 for 6V

1/4 watt resistors are fine, those are standard values so you should have no
trouble finding them. You can always pick the next higher or lower value
(which will give you just a tad less or more light). Use one per LED if you
have more than one LED. (You could use three or four so you'd be sure
to see it no matter which direction the boat is facing. The extra battery
drain
will be negligible - you could use a 9V battery just for the LEDs; an
alkaline
one should last 10-12 hours even with 4 LEDs.)

The long leg of the LED goes to + and the short leg to - (or, if the legs
are
the same length, the body usually has a flat side next to the - leg).
Doesn't
matter which leg you put the resistor on.

I'd suggest a colored LED - red or green - they're cheaper and a little
more robust as well as easier to see if there are reflections of the moon
or nearby lights on the water. You can get some really bright ones too.

-Wm