TILT!!!
DAMMIT
Dual Sensor Single Display for Rebreather
BY TOM ROSE
REBREATHERS CAN AND DO KILL...
I AM NOT RESPONSIBLE FOR YOU...
YOU MAKE YOUR OWN DECISIONS...
AND REAP THE CONSEQUENCES
If you are planning to duplicate this project, it is assumed you know just how crazy you must be to dive a RGU or any other
rebreather. These things can and do kill people...smart people...dumb people...experienced people...beginners alike...Always
know your PPO.
“… ever since he was a young boy, he played the silver ball…….”
Oh, how I hated that word tilt when I played the pinball machine in my dads
restaurant. Even though I played the machine with red painted quarters that we
got back, the game was over. The active component in the machine that caught
me trying to put unacceptable influence on the motion of the ball was a little device called
a tilt switch. In those days it consisted of a ball in a cage that made
contact when the machine was tilted. They are still used today in toys, trunk lids,
and hood lids....and now in the display of my rebreather. You can find them
in old thermostats or buy them for about a buck on the web.
I had several in my “toy” box when I got an idea of how to switch the signal
from one oxygen sensor from another without a hole through the display housing.
Holes usually mean leaks...if not today...some day in the future. Now I could
just use one big display saving battery life and space for some other
electronics I was going to add. The first step was to solder two tilt switches together butt to butt.
That way one would turn off as the other turned on. This was pretty neat so far. You can see the simple circuit on
the right.
I played with it for a few minutes listening to the satisfying clicks as the balls
inside the tilt switches rolled back and forth. Feeling very satisfied with myself,
I soldered a lead to the middle to serve as the lead to the display.
The game was afoot...
Then I plugged the newly built double tilt switch into a bread board to
make the rest of the connections in the prototype easy.
I attached the leads one of the oxygen sensor across the ends of a 10K pot.
The sweeper lead from the pots go to the end leads of the tilt switches.
I repeated the process for the other sensor. The switch selected output goes to
the Liquid Crystal Display that has been modified with an adjustable gain.
For my test I did not make the outputs of the two oxygen cells the
same allowing me to demonstrate to my satisfaction that it worked. You can
see that it reads differently when tipped forward and backwards. In the actual unit,
I will adjust the output of the high cell to the value of the lowest. I will
adjust the gain pot on the display to read .209 in ambient air. If they do not
agree, I will go with the one that is worse and abort the dive.
It took longer to write this page than to make the unit.
I planned to put in my standard Otter box with lots of room for additional goodies now
that only one display is needed.
I wonder if I put another tilt switch in to turn off the power to the display.
That would really solve the problem of battery life if the display was a LED. Maybe two
side by side switches for safety. Hmmmm...what do you think?
I made a quick plexiglass box so it would be ready for Walkers Cay. It worked great but as the dive master said, it looked like an invention....His nice way of saying it looked like crap.
Then along came Dr. Bob...the guy who always pushes me into
trouble.
"Why don't you really do a good job for a change and machine it in delrin."
Between them, I decided to go ahead and make it a little more presentable.
I bought delrin, I started to learn to really use my milling machine, I spent days thinking about it and a few hours making it between thinking about it. I machined the
cavity in the delrin for the battery and meter, the "O" ring, and the holes for the stainless machine screws. I tapped a hole in the end for a 1/4 inch pipe fitting and screwed the waterproof cable into it. A few solder connections and I was done.
I had implemented the three switch design. One senses if the display is hanging down and keeps the power off. Lifting the display turns it on and tilting it changes which sensor is activated.
To the right you can see the other end of the waterproof cable. The two sensors are under the molded rubber caps on the PVC "T" at the junction to the counterlung.
It no longer looks like an invention...it is both rugged and has the look of real stuff. Oh yeah, along the way I pressure tested it to 80 psi or about 160 feet of depth in my paint pot pressure pot.
Enjoy,
Tom
I knew some
of you would ask so here is a source of tilt switches.