unhd (Roger Gonzalez ) (11/25/90)
I was so impressed with the dozens of inclination sensor ideas, that I'm going to throw a question out myself: I need a cheap, fast, and small sensor to detect when its been submerged in (fresh) water. I assume that this sort of thing exists for turning on automatic bilge pumps. We need one for our EAVE vehicle, but I was also considering an automatic waterbowl filler for my cats... -Roger -- "The question of whether a computer can think is no more interesting than the question of whether a submarine can swim" - Edsgar W. Dijkstra rg@unhd.unh.edu | UNH Marine Systems Engineering Laboratory r_gonzalez@unhh.bitnet | Durham, NH 03824-3525
llw@corwin.eng.yale.edu (Louis L. Whitcomb) (11/27/90)
Greetings:
The switches used to control bilge pumps typically are mercury
switches which, attached to a float, get tilted when immersed in
water. The mercury completes the circuit, and can carry several amps
of current no problem. You can get them for $15 to $30 depending on
the brand. I use the "rule-a-matic" in my boat. Be warned: they have
two problems... 1) they measure water presence indirectly (they
actually measure displacement due to buoyancy) and will only function
properly if mounted upright... 2) they tend to make intermittent
contact ("bounce" in ee terms) when jostled around even when the water
level is low (this is because the mercury sloshes around inside the
switch.)
More fancy bilge pumps use two different techniques: 1) they spin
the bilge pump moter occasionally and measure the motor load (back emf
and current, I imagine) and shut down when the pump cavitates (runs
dry.) 2) they have an ultrasonic sensor to measure the level of the
water in the bilge.
Water seperating fuel filters have a "water sensor" consisting of
two bare metal contacts. A small op-amp circuit can measure the
resistance across the two electrodes to detect water.
Perhaps you could be more precise in *what* you want to detect about
water?
By the way, what IS the "Marine Systems Engineering Lab" ?
The Best,
-Louis.
--
............................................................................
Louis L. Whitcomb llw@corwin.eng.yale.edu ph: (203) 432-4311
Yale Robotics Laboratory fx: (203) 432-7481
Department of Electrical Engineering, 1968 Yale Station, New Haven, CT 06520jpexg@rice-chex.ai.mit.edu (John Purbrick) (11/27/90)
In article <1990Nov24.211134.28944@uunet!unhd> rg@unhd.UUCP (Roger Gonzalez ) writes: > >I need a cheap, fast, and small sensor to detect when its been submerged >in (fresh) water.......... OK, how about: Set up an oscillator, a 555 should be fine. Use a small capacitor; I'd start with 100pF and see how it goes. In parallel with this, arrange two plates where water can get between them. Now, what will happen is that this forms (obviously) an extra capacitor. But the dielectric constant of air is small. Water, however, has a large dielectric constant, so when the capacitor gets flooded, it becomes much larger and the circuit's frequency of oscillation will go down. If the water ISN'T pure, you may get more of a resistive than capacitive effect, and it's not clear what will happen. Some change in circuit behavior, anyway. --John Purbrick ps Note that the water doesn't need to touch the electrodes--it only has to fill most of the space between them.
hbg6@citek.mcdphx.mot.com (11/29/90)
In article <1990Nov24.211134.28944@uunet!unhd> rg@unhd.UUCP (Roger Gonzalez ) writes: > .... >I need a cheap, fast, and small sensor to detect when its been submerged >in (fresh) water. I assume that this sort of thing exists for turning >on automatic bilge pumps. We need one for our EAVE vehicle, but I was >also considering an automatic waterbowl filler for my cats... > .... Dig out the National Semiconductor "Special Purpose Linear Devices" data book and look up the LM1830 (page 5-123 in my copy). The 1830 is a one chip fluid detector. The fluid to be detected must be a little conductive to work. I've used them with city water, rain water and aquarium water with no problem. In a nut shell, the device uses two probes; one is connected to the circuit ground and the other to the chip. There is a TTL level output indicating 'fluid detected'. Two capacitors are all that is needed to make the chip work. Even the most novice hardware type should be able to get it working. I have another solution that uses a float, 3 Lbs. of salt, a mouse, a hungry cat, 9 feet of twine and a lit candle but I'll save it for sci.rube.goldberg. :-) Later, John ..................................................................... reply to 'from' address; hbg6@citek.mcdphx.mot.com NOT the 'sender' line address! Someday my sysadm will decide this is a 'real' problem. :-) ..................................................................... All opinions expressed are mine and not Motorolas, their loss. .....................................................................
paulc@hp-lsd.COS.HP.COM (Paul Carroll) (11/30/90)
> I need a cheap, fast, and small sensor to detect when its been submerged > in (fresh) water. I assume that this sort of thing exists for turning > on automatic bilge pumps. We need one for our EAVE vehicle, but I was > also considering an automatic waterbowl filler for my cats... > > -Roger Okay, I'll bite. What's wrong with a simple float attached to a lever or string? This would work if some part is going to be underwater enough for the float to be moved. It would be important to make sure that the float is able to move freely. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + Paul Carroll "Schizophrenia + + HP Logic Systems Division And I thought I was really sick + + paulc%hp-lsd@hplabs.hp.com I am beside myself." + + -- Spiffi + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++