There are two traditional methods for finding the level of
water in a tank: (1) tapping down the side of the tank until the sound suddenly
changes; and (2) removing the tank cover and dipping in a measuring stick. The
first method is notoriously unreliable, while the second method can be awkward
After all, who wants to clamber up on top of a tank each time
you want to find out how much water is inside it?
That's where this simple circuit comes in. It uses five green
LEDs arranged in a bargraph display to give a clear indication of how the water
supply is holding up. The more LEDs that light, the higher the water in the
tank. A sixth red LED lights when the tank level drops below a critical
There are no fancy microcontrollers or digital displays used in
this project. Instead, it uses just a handful of common parts to keep the cost
as low as possible.
Fig.1 shows the circuit details. It's based on an LM3914 linear
LED dot/bar display driver (IC1) which drives five green LEDs (LEDs 1-5). Pin 9
of the LM3914 is tied high so that the display is in bargraph mode and the
height of the green LED column indicates the level of the water in the tank.
Fig.1: the circuit is based on an LM3914 dot/bar display driver (IC1) which drives LEDs 1-5. Its output depends on the number of sensors covered by water - the more covered, the higher the voltage on Q1's collector and the greater the voltage on pin 5 (SIG) of IC1. LED6 provides the critical level warning.
The full-scale range of the bargraph depends on the voltage on
pin 6. This voltage can be varied using VR1 from about 1.61V to 2.36V. After
taking into account the voltage across the 390Ω resistor on pin 4, this gives a
full-scale range that can be varied (using VR1) between about 1.1V (VR1 set to
0Ω ) and 2V (VR1
set to 470Ω ).
By the way, if you're wondering where all the above voltages
came from, just remember that IC1 has an internal voltage reference that
maintains 1.25V between pins 7 & 8. This lets us calculate the current
through VR1 and its series 1kΩ resistor and since this same current also flows through the
series 1.5kΩ and
we can calculate the voltages on pins 6 and 4.
As well as setting the full-scale range of the bargraph, VR1
also adjusts the brightness of LEDs 1-5 over a small range. However, this is
only a secondary effect - it's the full-scale range that's important here.
IC1's outputs directly drive LEDs 1-5 via 1kΩ current limiting resistors.
Note, however, that an LM3914 has 10 comparator outputs but we only need five
steps for this application. That's done by wiring the outputs of successive
comparator pairs in parallel - ie, pins 1 & 18 are wired together, as are
pins 17 & 16 and so on.
Table 1:Resistor Colour Codes
||4-Band Code (1%)
||grey red yellow brown
||grey red black orange brown|
||blue grey yellow brown
||blue grey black orange brown|
||green blue yellow brown
||green blue black orange brown|
||orange orange yellow brown
||orange orange black orange brown|
||red red yellow brown
||red red black orange brown|
||brown black yellow brown
||brown black black orange brow|
||grey red orange brown
||grey red black red brown|
||red red red brown
||red red black brown brown|
||brown green red brown
||brown green black brown brown
||brown black red brown
||brown black black brown brown|
||orange white brown brown
||orange white black black brown|
Water level sensor
Fig.2: follow this diagram when installing the parts on the PC board. Note that some parts have to be omitted for 12V battery operation - see text.
The input signal for IC1 is provided by an assembly consisting
of six sensors located in the water tank and connected to the indicator unit via
light-duty figure-8 cable. This sensor assembly relies on the fact that there is
a fairly low (and constant) resistance between a pair of electrodes in a tank of
water, regardless of the distance between them.
As shown in Fig.1, sensor 1 is connected to ground, while
sensors 2-5 are connected in parallel to the base of PNP transistor Q1 via
resistors R5-R1. Q1 functions as an inverting buffer stage and its collector
voltage varies according to how many sensor resistors are in-circuit (ie, how
many sensors are covered by water).
When the water level is below sensor 2, resistors R5-R1 are out
of circuit and so Q1's base is pulled high by an 82kΩ resistor. As a result, Q1 is off and no
signal is applied to IC1 (ie, LEDs 1-5 are off). However, if the water covers
sensor 2, the sensor end of resistor R5 is essentially connected to ground. This
resistor and the 82kΩ resistor now form a voltage divider and so about 9.6V is applied to Q1's
Fig.3: this is the full-size etching pattern for the PC board. Check your board carefully before installing any of the parts.
As a result, Q1's emitter is now at about 10.2V which means
that 0.8mA of current flows through the 2.2kΩ emitter resistor. Because this same
current also flows through the two 1kΩ collector load resistors, we now get
about 0.8V DC applied to pin 5 (SIG) of IC1. This causes pins 1 & 18 of IC1
to switch low and so the first green LED (LED5) in the bargraph lights.
As each successive sensor is covered by water, additional
resistors are switched in parallel with R5 and Q1's base is pulled lower and
lower. As a result, Q1 turns on "harder" with each step (ie, its collector
current increases) and so the signal voltage on pin 5 of IC1 increases
accordingly. IC1 thus progressively switches more outputs low to light
Note that Q1 is necessary to provide a reasonably low-impedance
drive into pin 5 (SIG) of IC1, while keeping the current through the water
sensors below the level at which electrolysis becomes a
Critical level indication
The power socket and RCA connector are both mounted directly on the PC board. Make sure that all parts are correctly oriented and that they are in the correct locations.
IC2 is a 555 timer IC and it drives LED6 (red) to provide a
warning when the water level falls below the lowest sensing point; ie, when all
the green LEDs are extinguished. However, in this role, IC2 isn't used as a
timer. Instead, it's wired as a threshold detector and simply switches its
output at pin 3 high or low in response to a signal on its threshold and trigger
inputs (pins 6 & 2).
It works like this: normally, when there is water in the tank,
LED5 is on and its anode is at about 2V. This "low" voltage pulls pins 6 & 2
of IC2 low via a 100kΩ resistor, so that these two pins sit below the lower threshold voltage.
As a result, the pin 3 output of IC2 is high and LED6 is off.
However, if the water level falls below sensor 2, LED5 turns
off and the anode of LED5 "jumps" to +12V. This voltage exceeds the upper
threshold voltage of IC2 and so pin 3 switches low and LED6 turns on to give the
critical low-level warning.
The PC board in secured to the bottom of the case using two 10mm standoffs at one end, while the RCA socket provides the support at the other end.
Note that the control pin (pin 5) of IC2 is tied to the
positive supply rail via a 1kΩ resistor. This causes IC2 to switch at thresholds of 0.46Vcc
(5.5V) and 0.92Vcc (11V) instead of the usual 1/3Vcc and 2/3Vcc and is necessary to ensure that IC2
switches correctly to control LED6.
Power for the unit is derived from a 12-18VAC plugpack supply.
This drives a bridge rectifier D1-D4 and its output is then filtered using a
electrolytic capacitor and applied to a 12V 3-terminal regulator (REG1). The
output from REG1 is then filtered using a 10μF electrolytic capacitor and used to
power the circuitry.
Note that a regulated supply rail is necessary to ensure that
the water level indication doesn't change due to supply
Construction is straightforward, with all the parts installed
on a PC board coded 05104021 and measuring 80 x 50mm. This is installed in a
standard plastic case, with the LEDs all protruding through the lid.
Fig.4: the water level sensor is made by threading six lengths of 1mm enamelled copper wire through 8mm OD clear PVC tubing (see text). The six sensors should be evenly spaced down the tube.
Fig.2 shows the parts layout on the PC board. Begin the
assembly by installing the resistors, diodes and capacitors, then install the
ICs, transistor Q1 and the 3-terminal regulator (REG1). Make sure that the
diodes and ICs are installed the right way around.
The same applies to the electrolytic capacitors - be sure to
install each one with its positive lead oriented as shown on Fig.2.
This is the author's completed water level sensor. A weight can be attached to the bottom end to keep the plastic tube straight when it is immersed in the tank.
Trimpot VR1 can now be installed, followed by the RCA socket
and the 2.5mm power socket. The two sockets are both PC-mounting types and mount
directly on the board.
The LEDs are fitted last and must be installed so that the top
of each LED is 33mm above the PC board. This ensures that the LEDs all just
protrude through the lid when the board is mounted in the case on 10mm spacers.
Make sure that all LEDs are correctly oriented - the anode lead is the longer of
You can easily convert the LM3914 (IC1) from bar to dot
operation if that's what you prefer. All you have to do is cut the thinned
section of track immediately to the left of the 0.1μF capacitor and install a wire link
between the two vacant holes at the top of the board near IC1. Alternatively,
the link can be omitted (ie, pin 9 can be either pulled low or left open
Improved Water-Level Sensor
For a long-life water level sensor, Bob Barnes of RCS Radio
suggests that the probe be made out of 19mm plastic conduit fitted with
stainless-steel radiator or fuel pump hose-clamps for the sensors.
Suitably sleeved nichrome or stainless steel wire ("up the
spout") can then be used to make the connections between the clamps and the
You will need to use Multicore Arax cored solder or Litton Arax cored solder
(available from Mitre-10) when soldering nichrome or stainless steel wire (ie, a
corrosive flux is needed). You can buy nichrome wire from Dick Smith Electronics
or from Jaycar, while stainless steel wire should be available from boating
Fig.5: this full-size artwork can be used as a drilling template for the front panel.
If the unit is intended for 12V battery operation in a mobile
home or caravan, regulator REG1 and diodes D2, D3 & D4 are omitted. Both D4
and REG1 are then replaced by wire links - ie, install a link instead of D4 and
install a link between the IN & OUT terminals of REG1.
D1 remains in circuit to protect against reverse battery
If the tank is of made of metal, you can dispense with Sensor 1
and connect the tank directly to the circuit ground. You must also ensure
sensors 2-6 do not touch the walls of the tank. This can be done by slipping a
length of 25mm-OD clear PVC tubing over the completed probe, securing it at the
top so that the water inside can follow the level in the tank.
The PC board is mounted in the bottom of the case on two 10mm
standoffs and is secured using 3mm machine screws, nuts and washers. Note that
the corners at one end of the PC board must be removed to clear the pillars
inside the case.
You will have to remove these corners yourself using a small
hacksaw and rat-tile file if this hasn't already been done.
Fig.6 shows the locations of the two board mounting holes in
the bottom of the case. You will also have to drill two holes in one end of the
case, so that they line up with the RCA socket and the power socket when the
board is installed (see Fig.6).
The front-panel artwork (Fig.5) can be used as a template for
drilling the front panel. There are six holes to be drilled here - one for each
LED - and these are all 5mm-dia. It's a good idea to countersink these holes
from the underside of the lid using a 6mm drill, so that the LEDs slip easily
into position when the lid is fitted.
The top of the water level sensor can be secured to the tank using a suitable bracket,
The sensor assembly is made by threading six lengths of 1mm
enamelled copper wire through 8mm OD clear PVC tubing - see Fig.4. This tubing
should be long enough to reach the bottom of the tank, with sufficient left over
to fasten the top end securely. The reason for using 1mm wire is primarily to
make it easy to thread it through the plastic tube.
The top sensor (S6) is placed about 100-150mm below the
overflow outlet at the top of the tank, while the other sensors are spaced
evenly down the tube.
Begin by using a 1.0mm drill to drill holes through the tube
wall at the appropriate points, including a hole for the bottom sensor (S1) to
hold it in place securely. That done, you can thread the wires through by
pushing them through the drilled holes and then up the tube. You will find that
the wire goes in more easily if the PVC tube is bent at an angle so that the
drilled hole is in line with the bore of the tube.
The end of each wire should also be smoothed before pushing it
into the tube, to avoid scratching the enamel of the wires already in the tube.
Leave about 150mm of wire on the outside of the tube at each point.
It's a good idea to trim each successive wire so that it
protrudes 20mm further out of the top of the tube than its predecessor. This
will allow you to later identify the individual wires when attaching the
When all six wires have been installed, the next step is to
solder the wire for S1 to the "earthy" side of the figure-8 lead, cover it with
insulating sleeving and pull the covered joint down about 50mm into the 8mm
tube. This done, the resistors can be soldered to their appropriate wires.
Fig.6: this diagram shows the drilling details for the plastic case.
Push about 15mm of 2.5mm sleeving over each wire before
attaching its resistor. This sleeving should then pulled up over the joint and
the bottom end of each resistor after it is soldered. Once all the resistors
have been soldered, the wires should be pulled down so that the joints are just
inside the 8mm tube, as shown in the photo.
When this process is complete, there will be five resistors
protruding from the top of the 8mm tube. Their remaining leads are then twisted
together, soldered to the other side of the figure-8 cable and covered with
heatshrink tubing. The other end of the figure-8 cable is fitted with an RCA
plug, with the resistor lead going to the centre pin and the sensor 1 lead going
to the earth side of the connector.
The next step is to scrape away the enamel from the 150mm wire
lengths at each sensor point and wind them firmly around the outside of the
tube. A 30mm length of 12.5mm copper water pipe can be pushed over sensor 1 to
add weight and increase the surface area if desired.
Note: on no account should solder be used on the submersible
part because corrosion will result from galvanic action.
Finally, the end of the plastic tube and the holes can be
sealed with neutral-cure silicone sealant. However, don't get any silicone
sealant on the coiled sensor wires, as this will reduce the contact area (and
perhaps render them ineffective).
Now for the big test. Apply power to the unit and check that
the red LED comes on and that there is +12V on pin 3 of IC1. If all is well, the
unit can now be tested by connecting the sensor assembly and progressively
immersing it (starting with sensor 1) in a plastic dish that's full of water.
When sensor 1 and sensor 2 are immersed, LED6 should extinguish and LED5 should
Similarly, when sensors 1, 2 & 3 are immersed, LEDs 5 &
4 should be on and so on until all five green LEDs are lit.
Finally, trimpot VR1 must be set so that the appropriate LEDs light as the
sensors are progressively immersed in water. In practice, you should find the
two extremes of the pot range over which the circuit functions correctly, then
set the pot midway between these two settings.
1 PC board, code 05104021, 80 x 50mm
1 plastic case, 130 x 67 x 44mm
1 PC-mount RCA socket
1 RCA plug
1 2.5mm PC-mount power socket
1 12V AC 500mA plugpack
1 100gm spool 1.0mm enamelled copper wire
1 length 8mm-OD clear PVC tubing to match height of tank plus 200mm
2 3mm x 20mm screws and nuts
2 10mm spacers
1 LM3914 linear dot/bar driver (IC1)
1 NE555 timer (IC2)
1 BC558 PNP transistor (Q1)
1 78L12 12V regulator (REG1)
4 1N4004 diodes (D1-D4)
5 5mm green LEDs (LEDs1-5)
1 5mm red LED (LED6)
1 100μF 35VW PC electrolytic
1 47μF 16VW PC electrolytic
1 10μF 16VW PC electrolytic
1 0.1μF greencap
Resistors (0.25W, 1%)
1 820kΩ 1 82kΩ
1 680kΩ 2 2.2kΩ
1 560kΩ 1 1.5kΩ
1 330kΩ 9 1kΩ
1 220kΩ 1 390Ω
1 100kΩ 1 470Ω trimpot
Light-duty figure-8 cable, 2.5mm PVC sleeving, heatshrink tubing.