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Build A Water Level Indicator

Low-cost circuit lights a LED bargraph to indicate the level in a rainwater tank.

By Allan March

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 and time-consuming.

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 threshold.

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.

Circuit description

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.

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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 390Ω resistors, 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
No. Value 4-Band Code (1%) 5-Band CodE(1%)
1 820kΩ grey red yellow brown grey red black orange brown
1 680kΩ blue grey yellow brown blue grey black orange brown
1 560kΩ green blue yellow brown green blue black orange brown
1 330kΩ orange orange yellow brown orange orange black orange brown
1 220kΩ red red yellow brown red red black orange brown
1 100kΩ brown black yellow brown brown black black orange brow
1 82kΩ grey red orange brown grey red black red brown
2 2.2kΩ red red red brown red red black brown brown
1 1.5kΩ brown green red brown brown green black brown brown
9 1kΩ brown black red brown brown black black brown brown
1 390Ω orange white brown brown orange white black black brown

Water level sensor

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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 base.

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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 additional LEDs.

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 problem.

Critical level indication

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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.

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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 100μF 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 variations.

Construction

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.

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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.

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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 the two.

Dot operation

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 circuit).

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 resistors.

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 suppliers.

Battery operation

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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 connection.

Metal tanks

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.

Final assembly

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.

Sensor assembly

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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 resistors.

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.

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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).

Switching on

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 come on.

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.

Parts List
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

Semiconductors

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)

Capacitors

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

Miscellaneous

Light-duty figure-8 cable, 2.5mm PVC sleeving, heatshrink tubing.

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