Some water tanks are undoubtedly a good idea. Why pay for water when you can catch it free?

You can have the greenest garden in the street, along with the cleanest car, while you thumb your nose at the water restrictions now in place in most capitals and many regional centres.

But once installed, how can you determine how full (or how empty!) your tank really is?

There are several traditional methods for finding the level of water, among them: (1) tapping down the side of the tank until the sound suddenly changes; (2) on a hot day feeling down the tank for a change in temperature; (3) pouring boiling water down the side of the tank and looking for the line of condensation and (4) removing the tank cover and dipping in a measuring stick.

The first two methods are notoriously unreliable, while the last two also have their problems. Only the last is accurate. But 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 a row of ten coloured 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.

The LEDs are arranged in the familiar "traffic light" colours of green, yellow and red to instantly indicate relative levels at a glance (green is good, yellow not so good and red is bad!) as well as the specific levels represented by the individual LEDs.

A further red LED lights when the tank level drops below a critical threshold. This can simply be to warn you of impending localised drought (hey, your tank’s empty!) – or it (or indeed any of the ten-LED "string") could be used to trigger an audible alarm, turn on a pump etc, as we will discuss later.

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.

It can be used in a traditional metal tank or one of the new slimline plastic jobs. As long as you can get very access inside the tank from the top to the bottom, this circuit will work.

Circuit description

Fig.1 shows the circuit, which only has a few differences to the April 2002 circuit. As in that design, it is based on an LM3914 linear LED dot/bar display driver (IC1) which in this case drives not five but ten LEDs (LEDs 1-10).

Pin 9 of the LM3914 is tied high so that the display is in bargraph mode and the height of the LED column indicates the level of the water in the tank. However, (and this is one of the minor tweaks we’ve made), this pin can be easily isolated, turning the display into a dot type, thus saving power. If you’re running from a battery supply in the bush, often every milliamp is sacred!

The PC board mounted inside the UB5 Jiffy Box. It's held in by the sensor socket at one end and the gaps in the vertical ridges.

Indeed, the PC board pattern has been arranged so that a miniature switch could be included to swap between bar and dot modes.

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 0W) 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 1-10 over a small range. However, this is only a secondary effect – it’s the full-scale range that’s important here.