An MCU-Based Monitor For A Communal Well
Jeff‘s Microchip Technology PIC12F510-based system monitors the
water filtration system in an underground well. The design
indicates when the communal filtration system's brine tank is out
of salt.
We are beginning to see some signs of public panic due to water
issues. I have read about towns whose water sources （e.g.， wells）
are being threatened by pollution， both natural and man-made. I
have also read about cases where upstream users are diverting
increasingly large amounts of flow for their own （gluttonous）
needs. Perhaps Mother Nature will choose to frustrate humanity by
redistributing her life-giving precipitation. While solutions on a
local level might be an initial necessity， regional， national，and
even global resolutions might better promote unity. The Earth‘s
resources do not respect our concocted territorial boundaries. Do
we have the rights to water， coal， or any other resource under our
feet？ What about when a resource just passes through （e.g.， a
river）？
We must respect the planet and our neighbors， and we should be
willing to compromise in order to avoid major conflicts.
Unfortunately， some people don‘t think this way. As a result， it
may not be long until a gallon of water costs as much as a gallon
of oil. Only time will tell. I do know， however，that during the
next few years engineers like us will be tasked with developing new
systems to facilitate better water management， delivery，
filtration，and monitoring. I recently used my engineering skills to
tackle a local water issue. In this article， I‘ll describe how I
built an MCU-based monitoring system for a water filtration tank.
WATER SUPPLYUrban dwellers depend on municipal water works to provide an
unending supply of fresh， albeit treated， water. As urban sprawl
increases，more rural homesteaders are tied together by local
community wells and even private single-home water systems. While a
private system might be outside the jurisdiction of town
regulations， I think it‘s safe to say that no one wants bad
tasting，smelly， or toxic drinking water. As the landscape changes，
we want to be assured that our water supply has not been
compromised. Periodic water testing relieves these concerns. Once
we find that our underground aquifer is not being polluted， our
attention shifts from supply to quality. Just because the water
isn't toxic doesn‘t mean it is palatable. The rotten egg smell from
sulfur， the laundry staining rust from iron，or the numerous other
byproducts of natural but nontoxic minerals have a direct impact on
the quality of the water we pump out of the ground. Extracting
undesired material can be a fairly automatic process. Water
filters/ softeners mine minerals from the water by various means.
Filtering is used to remove particulate. These can easily be seen
and removed by simply straining the water，using a material with
restricted openings. Filters are available down to 1 μm （0.01× the
size of the human hair）。 The smaller the pores， the slower the flow
through the filter， and the quicker it will become clogged （needing
to be replaced）。 A filter of 10 or even 50 μm may be adequate
unless you require protection against water parasites. Carbon is
highly effective in removing chlorine， organic contaminants，
chemicals，and undesirable tastes and odors. Like the filter
cartridge， you must replace carbon periodically because it has a
finite amount of absorption and will cease functioning once full.
As water travels through the Earth，minerals are absorbed and water
becomes harder. The extent of hardness is measured in grains per
gallon（GPG）。 A grain is a unit of weight（approximately 0.000143 lb，
with 1 GPG = 17.1 ppm）。 Water with an excess of 5 to 10 GPG is
considered very hard. The dissolved minerals can accumulate on
surfaces in the form of a hard scale. The buildup will eventually
clog pipes and may damage water-using appliances. These minerals
also affect the ability of soap to clean surfaces， dishware，and
laundry. While these are not generally harmful to the body， a water
softener can remove them and protect your plumbing.
A water softener that will remove these is based on the exchange of
ions. An ion is a molecule that has lost or gained one or more
valence electrons，giving it a positive or negative electrical
charge. My system has two basic parts， the brine tank and the resin
tank with a controller （see Figure 1）。The tall tank is filled with
resin beads typically made from styrene or divinylbenzene. As well
water passes through the tank， mineral ions are attracted to the
resin， which in turn gives off salt ions. Like other filtering
devices， once the surface area of the beads is covered with mineral
ions，the exchange ceases. Unlike other filters，the system has the
distinct advantage of being able to clean or regenerate itself via
a periodic regeneration cycle. The tank‘s controller measures the
amount of water used to determine when a cleaning cycle is
necessary. When necessary， the controller initiates a cleaning
cycle.
During a cleaning cycle， well water temporarily bypasses the filter
while a brine solution-sodium chloride（NaCl） or potassium chloride
（KCl）and water-flushes the mineral ions from the host resin. Thus，
the resin is regenerated by the exchange of brine ions for mineral
ions. The host resin itself does not need replacing.
However，because the brine is removed and replaced with water each
cleaning cycle， the NaCl or KCl in the brine tank needs
replenishment. The brine tank can hold a few hundred pounds of NaCl
or KCl pellets. When the brine is replaced with water， the pellets
will be dissolved by the water until saturated， at which point the
remaining pellets sit in the brine. Eventually， when all of the
pellets have been dissolved and you‘ve forgotten to add to the
brine tank， the regeneration cycle becomes useless and the removal
of minerals ceases.
My house and seven others in the neighborhood receive water through
a single community well that just happens to be on my property. A
perfectly working pump and water softener are key to keeping the
neighborhood one happy family. Guess who gets the call when
something acts up？
The pump， storage tank， and water softener are housed in a
below-ground well house. Generally， this all worked without much
intervention. Nevertheless，I came up with an idea several months
ago. I wanted the ability to know-without having to periodically
enter the dungeon pit-when the brine tank is empty and when the
system fails.
TWOFER
Knowing the depth of the pellets left in the brine tank without
having to venture underground is a big improvement. Because the
water level in the brine tank isn‘t constant，weighing the tank to
determine the potassium level won't work unless I know how much
water is in the tank and can subtract its weight. Using electronics
inside the brine tank isn‘t safe （due to corrosion）， so I keep any
electronics on the outside. I can sense the level of the water from
the outside with a magnet floating on the water's surface. Sensors
measure the water level. I use the same sensors to measure the
position of a magnet that rests atop the pellets.
The system in Figure 2 is what measures the pellet and water
levels. The brine tank has a float switch associated with the water
refill cycle that prevents the water level from rising too high and
overflowing the brine tank. This float is mounted inside a
4″diameter pipe inside the tank. To measure the water level， I use
a foam float with a magnet in the brine tank. To measure the pellet
level， I use a piece of 0.5″ plastic egg crate with a magnet in the
brine tank.
点击查看Figure2HALL MONITOR
In a previous column on electromagnetics，I discussed how a
magnetic field is produced around a conductor that has a current
passing through it（"Electric Motor Technology： Theory，Construction，
And Requirements，" Circuit Cellar 216， 2008）。 When this conductor
is within an external magnetic field， the two magnetic fields
create a force that tries to move the conductor. This force also
has an effect （discovered by Edwin Hall） on the relative position
of the current within the conductor （see Figure 3）。 If current
flows through a conductor （xaxis） and you measure across the
conductor（y-axis）， you would measure no potential. As an external
magnetic field increases through the conductor（z-axis）， the current
through the conductor would be forced off center and a potential
would be measured across it. A Hall sensor is designed to measure
this effect using a thin current-carrying element in a bridge
configuration. With a constant current flowing across the element
in one axis （x）， the other axis （y） will be balanced while no
magnetic field is present. Any imbalance is proportional to the
strength of the magnetic field passing through the element
perpendicular to its other axis （z）.
Additional circuitry within the Hall-effect sensor configures its
function（see Figure 4）。 Hall sensors are available to measure the
magnetic strength， to sense the presence of a magnetic field， or to
flip-flop with a magnetic field‘s polarity reversal. Another import
use for the Hall-effect sensor is to indirectly measure the current
in a wire by measuring the magnetic field produced by the current
in that wire. In this project， I want to sense the presence of a
magnetic field from a magnet over a minimum distance of
approximately 1″ to 2″.
For this operation， I chose the Allegro MicroSystems A3113， a
micropower，ultra-sensitive， omni-polar Halleffect switch. Typical
switching characteristics（with hysteresis） are 40 gauss（G） on and
32 G off. Note that gauss is the unit of magnetic flux density
（B）。The Earth‘s magnetic field is approximately 0.5 G. The local
hardware store had a small display of various magnets and I picked
up a pair of ceramic magnets about the size of a domino. I
connected one of the Halleffect devices to a 5-V supply and
monitored the device's open-collector output （with a 47-kΩ
pull-up）。 I found the magnet triggered the device right around 2″
from it. Perfect. This verified that the devices were acceptable
for my design criteria.
To prevent any contamination or reaction from the ceramic magnets，
I dipped each into Plasti Dip， a liquid plastic coating used on
many tool handles. This turned out to be more difficult than I had
anticipated because of the magnet‘s strong attraction to the can's
metal side. I taped a thread to the magnet so I could immerse it
and hang it to drain and dry. The pull of these magnets simply
ripped off the thread as it jumped to the can‘s side，creating the
mess I was trying to avoid！ I should have poured some of the liquid
into a plastic cup first.
EGG CRATE AND FOAM
On my last trip to the hardware store， I seized the opportunity to
search for the items I needed for this project. In the insulation
aisle， I found some 1″ high-density foam. This became my brine
float. This foam doesn't crumble like the block foam that comes as
most packing material. In the lighting aisle， I found some 2′ × 4′
plastic egg crate grills for fluorescent fixtures. They became my
pellet float.
Using the plastic brine tank‘s lid as a pattern，I cut a disk out of
the foam and the egg crate. I also cut out a 4.5″ hole along one
edge， which keeps the disks from spinning within the tank while
floating atop the brine and riding atop the salt. After a couple of
trial fittings in the tank， I attached magnets to the foam disk and
the egg crate disk （see Photo 1）。 The magnets are vertically
aligned to one another on each of the disks so when they are in the
tank they pass directly behind the Hall-effect PCB strips mounted
on the tank's exterior. The brine can easily pass through the holes
in the plastic egg crate； the pellets cannot. Therefore，the egg
crate rests atop all of the undisolved pellets（see Photo 2）。 While
a cleaning cycle is active， all of the brine may be withdrawn from
the tank. But it is quickly replaced by the end of the cycle. To
prevent the two disks from attracting each other， I placed spacers
on the egg crate to keep the two disks at a minimum of 3″apart. The
normal level for the water is quite high.
To keep the maximum length of this narrow PCB reasonable， I used a
length of 8.1″。 This allows sensors to be evenly spaced at every
inch and each PCB to report in a single byte（one sensor/bit/inch）。
Because the circuit allows daisy chaining， you can use a number of
8″ sections， as long as you tell the software how many boards
（bytes） are in the chain.
Each PCB has its own parallel-toserial shift register. The serial
shift register bus contains five signals： 5 V，SDA， SCK， load， and
ground. All eight parallel input bits are sampled using the load
control line. The inputs can then be shifted out using a SPI （or in
this case， the lines are bit banged）。 All Hall-effect outputs are
high （1） unless they are sensing a magnetic field. Then the out is
pulled low （0）。 The data is read most significant bit to least
significant bit and refers to sensors positioned at 31″ down to
0″（using four sensor boards and requiring 4 bytes）.
ATINY APP FOR A TINY MICRO
This tiny application is written for a tiny microcontroller. At
less than a buck， these eight-pin （and six-pin）devices can be used
for many interesting projects. However， because they are
inexpensive， they generally don‘t have many of the peripherals that
larger devices have. The eight-pin Microchip Technology PIC12F510
microcontroller I used for this project does not have SPI support，
so the serial shift routine must be software-driven（see Figure 5）。
The clock is toggled and data is read by assembly instructions as
opposed to using a hardware peripheral that could do this in the
background automatically.
Limited memory can be problematic for two reasons. The obvious
problem：you have limited space in which to cram your application.
Writing in assembler is sometimes the only way your application can
fit because a higher-level language might require overhead that can
put you over the space available. Another problem arises because
they generally have few or no hardware peripherals. This means you
can chew up precious memory just supporting that function， which
then leaves even less for the application.
This application requires only approximately 30% of the 1-KB
program space available to the PIC12F510. That includes the
software SPI and integer division routines. The application
periodically samples the Hall-effect sensors （by reading in 4 bytes
of data） and stores the data to a small table （see Figure 5）。Two
variables， salt and water， are used to indicate float levels. They
are initialized to 32， one more than the maximum level of any
sensor （in this case 32 sensors， 0″ to 31″）。 The table is scanned
LSB （0） through MSB （31） to find a low. If a low is found，
indicating the sensor is seeing a magnetic field，then the bit
counter value is stored in either the salt or water variable. The
salt variable is tested to see if it has been set. If any value
other than 32 is found， the second variable water gets the counter
value； otherwise，salt gets the counter value. This gives salt the
first counter value when the first low is found（lower level） and
water the last counter value when the second low is found （upper
level）。 If a float magnet is seen by two adjacent sensors，the
lowest sensor is used for the variable salt and the highest is used
for the variable water.
The counts in salt are displayed by the red LED. The LED blinks
once per second for each ten‘s digit and then again for each unit's
digit. A digit of zero leaves the LED on for a longer period of
time. The green LED then repeats the display of level count for the
water variable （see Figure 6）。 The LEDs are mounted under the roof
overhang outside of the locked well house so they can be viewed by
looking out of my living room window. Now I can keep track of the
system without having to go below ground.
SIMPLIFYING THE SIMPLISTICThe basic premise of the project is to indicate when the water
filtration‘s brine tank is out of salt. I could have done this with
a single sensor properly positioned at the bottom of the tank，but
this project gave me the opportunity to demonstrate the use of
Hall- effect devices to do measurement and not just indication.
While the actual level of water in the brine tank isn‘t important，
measuring more than one level using the same strip of sensors is a
good application. If the filtration system breaks down， the level
of water in the brine tank will be an indicator of potential
trouble. Because I know that a cleaning cycle normally takes place
every "x" days， if the level of the brine hasn't changed in， say，
2x， then I know there must be a problem.
As it is， I use a regulated 5-V wallwart supply to power this
circuit（plugged into a GFI）。 If I control 5-V power to the
Hall-effect sensors （leave them unpowered when not being
sampled），the circuit current will be low enough to run with
batteries. I could have used Allegro‘s A3214 for this project. Its
less frequent sampling period（60 ms instead of 240 μs for the
A3213） translates to a much lower operating current. But it wasn't
available at the time I bought these parts.
I will let this circuit run for a while. I will watch its
performance. I already have some thoughts on tying this information
to other data I might collect in the well house. "Collect"being the
key word here. I can think of a few technologies I want to
investigate before taking a second look into the pit.
Jeff Bachiochi （pronounced BAH-key-AHkey）has been writing for
Circuit Cellar since 1988. His background includes product design
and manufacturing. He may be reached through the
magazine（jeff.bachiochi@circuitcellar.com） or his web site
（www.imaginethatnow.com）.
PROJECT FILES
To download code, go to
ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/217.
SOURCESA3113 and A3213 Hall effect switches
Allegro MicroSystems, Inc.
www.allegromicro.com
PIC12F510 Microcontroller
Microchip Technology, Inc.
www.microchip.com
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