Float switch circuitry without floats?

Something simple is just a tube with one in closed that hooks around out of the tank for you to see.

The idea is that the water pressure will push water into the tube and compress the air inside.

Its simple and no electronics needed. You calibrate it by pouring in a known amount of water, mark on the tube the water level, then repeat with more water. You can do this several times to get a "ruler" for the water level. No calculations or anything else is needed. (although the air might be to compressible and temperature dependent so you would use something else such as oil)

If your serious about using electronics there are a half million ways to do it. Another simple method is to use a long resistive rod with a float on one end. The rod acts as a variable resistor and as the water rises it moves the rod upward(or I suppose it could be turned upside down) shortening the length of the rod between the float and a contact. Resistance is proportional to height which is proportional to amount of water(which by controlling the resistance you might get it matched well). You can hook up a cheap ohm meter and use that as your guide. Probably cost just a few dollars and very simple method. (although I doubt its very clear what I mean)

| | | | | | | | |* |* | | | - |

So - and * form a circuit and the part of the rod inbetween is a resistive load - is at the water level and * is fixed(say at the top of the tank). As the water rises it shortens the distance between - and * (which are connected to an ohm meter or some other device) reducing the resistance. Hence smaller resistances means larger height and vice versa(and its linear).

Anyways, just 2 ideas to consider....

Capacitive level probe. the fluid is the dialectic. the container is one side of the cap while the probe is the other.. Use it to alter a VFO that gets converted to an analog voltage derived from the freq. The probe should be coated with some thing to protect it like teflon etc..

Or, buy one .

Even easier: run a clear tube out of the bottom of the tank and up the side. The level in the tube is exactly the level in the tank.

John

. It will work fine. You may need a transistor switch at each level (the current may not be high enough to light an LED). You could use the kettle as the common electrode. Using electrical conduction for level detection is common.

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I can't stop thinking :) As so often, there are more ways to skin a cat and yours will work though is has some drawbacks. The resistance of the water will not be constant. Pure water has a very high resistance, so high it is considered an insulator. Pure water is rare, so I think your water will have low enough a resistance but it still can be pretty high. As you have to count for the worse scenario, you will indeed need an amplifier (transistor) to make sure you get a reliable signal. Another problem can be caused by the current. Although the current will be very small, it can solve (part of) your wires. This does not only depend on the current but also on the minerals solved in the water. Not to forget the direction of the current. Nevertheless, I've seen long poles of teflon with conducting rings on the outside and wires on the inside used to measure waves and tides in the Northsea. I remember from the explanation that they used special metal and AC-current for the measuring to achieve an acceptable livetime in the aggressive seawater.

To make a long story even longer, I propose two solutions without electronics.

1. Take a plastic tube and put reed contacts in it on selected distances. Make a floater with a hole in it and a magnet glued on it. No need to say you'd better seal both ends of the tube with some silicone -or other sanitair kit. As you have some experience with circuitry you can fill in the use of the construction. yourself.
2. Get some clear flexible tubing with a length over three times the height of your kettle. Mount one end inside the kettle as close to the bottom as possible.Gide the tube upward, over the edge, down to litlle below the bottom of the kettle and then up again to end somewhere above the kettle. Once you filled this tube fully with water, you can always see the fluid level on the outside unless the kettle runs fully dry. If that's the case, you can fool this meter by placing a little tin inside the kettle and make the inside end of the tube end up in this tin.

success

petrus bitbyter

I interpreted that statement to mean that the kettle was too high for him to see over the lip inside, without standing on a ladder or equivalent. So, is the kettle actually so high that it disappears into the ceiling?

At any rate, I made a suggestion that might work. If it won't work for him, it's for him to decide. Your following me around, barking at my feet like some crazed Chihuahua, is just silly.

John

Don Lancaster's "CMOS Cookbook" has just what you need (page 283 in my old 1977 edition). He calls it a "Booster Tank Level Sensor" for fire engines. The tank is grounded, and there are sensor electrodes at various depths, insulated from the tank. Each electrode goes to the input of a CMOS inverters and is pulled high by a large-valued resistor when there is no water covering it. When the water hits it, it pulls the input low and the inverter output goes high. (He shows the inverter driving an LED directly, without even a series resistor. He might be relying on some internal current limiting, or this might be a circuit error. You can use the inverter output to drive an NPN driver through 10K to its base, ground the emitter, and put your LED and series resistor from the collector to the positive rail.)

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

known

Oh, did you say anything funny? I must have missed that.

BTW, did you miss that my response was to JS's liquid-filled tube suggestion?

OK, post a design that responds to the OP's needs, something a beginner could put together successfully in a hour maybe. I have one in mind that takes 12 parts total, and a probe he could assemble in 5 minutes.

All you seem to do lately is whine at other peoples' suggestions. You probably aren't having fun.

John

I would use a float switch instead. You could have the actual switch outside of the pot, and have a float connected to a stick, attached to the side of the pot. When the float gets to the fill line, the other end of the stick connects to the switch:

Switch 1k o------\\/\\/\\/\\----LED----GND 5V----o--__ o- | | --------------| | | | | Float moves, causes switch to close | | | | Inside Pot | | | | | | | | | | | | | | | | | | | | .---.| Float '---'| | Water Level ^ | | | | | (created by AACircuit v1.28.6 beta 04/19/05

FWIW

Regards, Bob Monsen

Common solutions: sight glass (a glass tube mounted alongside the kettle, joined at the bottom to the kettle and open at the top). Breakable, might need cleaning attention.

Float: a calibrated stick visible over the pot will tell you what the float level is. Loose parts, need to lift it out to clean.

Bubbler: a tube with a slow bleed of gas; when you hear the bubbles, it's got the end under water. Loose parts, easy to clean. Can use a pressure sensor instead of listening.

Ultrasonic level sensor: no contact, easy to keep clean. Sonar, basically.

Capacitive dipstick: two close-spaced electrodes, capacitance goes up when liquid fills the space. Capacitance is analog to liquid level. Affected by liquid composition.

Capacitive sensor: two electrodes at a target position, capacitance shifts when liquid reaches that position. Capacitance gives over/under level indication only. Can be combined with the sight tube. I've seen these offered for chemistry labs, the exact liquid composition doesn't matter much.

Pressure sensor/switch: depends on density of the liquid times depth.

Force sensor: ten gallons is HEAVY, hang the pot from a spring scale (or put a scale under the stove).

A simple conductance sensor relies on electrical current in the liquid, which is unreliable (the electrodes can crud up even if the liquid is a great conductor).