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.

This is the legacy website; for newer content, please see the new website. Issue: 163 Published: 1 April, 2002 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. 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.

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