Liquid level sensor and controller (auto top-off for aquarium)--- make money online

I built a liquid level controller that senses the level of water in my aquarium and powers a water pump when necessary to compensate for evaporated water. This is known as an "automatic top-off" system. The controller is based around a Keyence FS-V11, which sends light out a plastic fiberoptic, and measures the amount received via another fiberoptic.

The optical probe makes use of total internal reflection to control how much light is returned to the Keyence based on how far the probe is immersed in the water. It is also completely passive, non-metallic, and has no moving parts for high reliability. The Keyence triggers a solid state relay that supplies power to a standard AC outlet. The pump is then plugged into the outlet, and moves water from a reservoir into the aquarium on command.

hey everyone here's a project that built

a while back that resurfaced recently

this is a liquid level controller that I

originally built for my aquarium so let

me show you how it works the probe is

here and when the probe is in water you

can see the status light has gone off

over here and as soon as the probe is

removed from water the light comes back

on and so the idea is that you can plug

a water pump into this electrical outlet

and when the light is on the pump will

be running so it will pump water into

the aquarium and when the level gets

high enough it shuts the pump off so

this is handy because aquariums

evaporate a lot of water surprisingly of

the high amount so for reef aquariums

that have high intensity lighting it can

be a cup or two per day so this is

actually a really nice time saver so as

you can see this part of it at the

bottom here is a commercially available

unit and I got this one surplus at Hall

Ted but later I needed more of these and

got them on eBay for fairly cheap in

fact I have got a whole brick of them

but I'll talk about these in a minute

this is a Kean's fSV 11 and it's got

this little thing here there's the

nameplate and the idea is that this

thing generates a light signal and sends

the light out via a plastic fiber-optic

and then the light returns via another

plastic fiber optic and this thing

measures how much light comes back so

when the probe is in water a low amount

of light is coming back and when the

probe is out of water much more light

comes back and then this thing uses the

difference to decide whether to throw a

switch or not what's pretty cool is that

the little controller here is very

configurable so I can use these up/down

keys to set what my threshold is going

to be can dial this thing up and down I

think it has a resolution of 0 to about

4,000 and so when the probe is in the

water you can see we're getting an

arbitrary value of like 70 back or 65

and when it's in air we're getting 650

so we could set the threshold anywhere

between those values and get it to work

and then this thing has a whole bunch of

other modes too you can set it can

output like a pulse or something and you

can check whether the Reta the output

switch to beyond when there's a lot of

light coming back or vice-versa when

it's not a lot of light and it gets

fairly complicated

one thing that this thing does which is

pretty nifty is it it actually outputs a

pulse of light and then measures the

return and then subtracts the return

from when and when the pulse is not on

so it's sort of a differential sensor if

this thing just outputs steady steady

light and then measure the return value

if I wave this around near a light

source the the value would be going up

and down it's changing a little bit now

because if you bend these fibers you'll

get a a difference in the return but the

susceptibility to ambient light would be

much worse if it didn't pulse the output

and then do a differential reading just

so that it's only seeing the light from

its output pulse the sensor itself is

made of a piece of acrylic that I turned

on the lathe and I just cut a 45 degree

half angle so the total thing is ninety

here and then drilled some holes farther

up where I could push the plastic fibers

into this sort of a conical prison

almost so the light comes out of one

fiber you can almost see it see it

coming out there and reflects internally

to the other face of the cone and then

back out this receiving fiber so when

this is dunked in water or almost any

liquid the light scatters off into the

into the liquid when it's in air the

light is reflected due to total internal

reflection

so this sensor makes use of the fact

that the love that the properties at the

surface between the plastic and the

water change optically because of the

index of refraction there so it's a very

robust sensor it's actually hard to fool

this because it's it's sort of sensing

in this entire area of the cone here so

the the level that the water has to come

up to to make the light diminish to the

threshold value is very repeatable and

there's no moving parts in here and this

thing is essentially sealed so there's

it's it's a very high quality robust

sensor you might notice a little bit of

crud here this was installed in my

aquarium and so there's a lot of calcium

deposits and sort of dead algae on there

surprisingly that doesn't even affect

the sensor very much because the algae

in the

calcium deposits don't cause the light

to be scattered away like the water so

it doesn't cause the sensor to be

sensitive to false positives so this

type of fiber-optic has a one millimeter

diameter acrylic core it's PMMA poly

methyl methacrylate and then it has a

jacket on the outside that's some other

kind of plastic but it's very easy to

snip this with a pair of scissors and

then polish the end using sandpaper you

can buy this type of fiber optic at you

know Edmond scientific or a lot of the

suppliers like that but you can also buy

the cheap Toslink audio cables on ebay

you can get really long cables for for

very very cheap and if you cut one of

those Toslink cables up you'll find that

there's a much fatter outer rubber

jacket and then inside it's basically

the same as the stuff that I've got here

at this point it's kind of the same

thing and for the money I mean you

really can't beat it this is what the

Toslink end looks like and if you just

chop that off that's what you've got

inside I don't know if these are still

available on eBay but for a while these

are fairly cheap on there - and what you

do is you just open this thing up and

push the fibers in here and then clamp

it shut and set your threshold and you

do all kinds of interesting sensing with

these so for example you can have the

two fibers aiming out in the same

direction and then if you've got like a

an automated production line or

something like that and you want to

sense whether an object is going by you

on the conveyor belt when the reflection

hits the receiver fiber you can have

this trigger something if you do want to

prep the end of one of these fibers one

thing that helps a lot you can just cut

it with a pair of scissors and then if

you make a polishing puck like this what

you can do is put the the fiber through

the polishing puck so that it's coming

out the other end just slightly and then

put that down onto a piece of sandpaper

and with water move the puck around like

this to polish the end of the fiber and

the puck will make sure that the fiber

stays perpendicular to the surface of

the sandpaper and you can use just

standard wet/dry sandpaper probably

start with something fairly coarse like

320 because when you cut this with a

scissor you'll have a very rough edge

and then work your way all the way up to

1500 or even 600 these things are not

super high optical quality the fact that

the fiber is a whole millimeter in

diameter means that you don't have to

get very good and kind of see when I aim

the fiber up the fluorescent light on

the ceiling you can kind of see it

coming out the end there

these Qian's devices have an open

collector output so what I did is I

wired this up with a solid-state relay

and the relay just triggers this

electrical outlet here and inside here

there's a low-voltage power supply these

things will run 12 to 24 volts DC and

the solid-state relay can accept 3 230

volts on its input side so there's

really very little circuitry inside

there there's just an AC power cord and

then the fiber optics alright hope that

was helpful

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