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Green Technology: Build an Electronic Rain Detector to Conserve Water

Difficulty
Time Required Average (6-10 days)
Prerequisites Some experience with electronics would be helpful. If you are new to electronics, plan to spend some time learning about the subject prior to making the circuit. The workbook included with the kit has introductory information.
Material Availability The procedure calls for an electronics kit that can be purchased online. See the Materials and Equipment list for details.
Cost Average ($50 - $100)
Safety No issues

Abstract

"Waste not, want not" is a common saying. These four words sum up an important principle: if you "waste" an important resource, you might find you don't have enough of it when you "want" it. One of the most important resources we have is clean water. In many parts of the world, including the western part of the United States, water shortages pose serious problems. In this science project, you will build an electronic circuit that can 1) detect when it is raining and 2) shut off power, such as to a sprinkler system (don't worry, you don't need a sprinkler system to do this science project!).

Objective

Build a circuit that detects rain and can shut power off to a sprinkler system when it is raining. Important Note: This is an environmental engineering project. Most, but not all, science fairs accept engineering projects completed using the engineering design process. If in doubt, you should check with your fair before you follow the engineering design process within this science project, instead of the scientific method.

Credits

David B. Whyte, PhD, Science Buddies

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Last edit date: 2013-03-15

Introduction

You might already know that conserving water is a good idea, but what exactly are the benefits of water conservation? One benefit is energy conservation. Water-pumping, delivery, and wastewater-treatment facilities consume a significant amount of energy. In some regions of the world (like California) over 15 percent of total electricity consumption is devoted to water management. Saving water conserves this energy for other uses. Another benefit is habitat conservation. Overuse of fresh water can lower the levels of lakes and rivers, causing significant environmental problems. Minimizing human water use helps preserve freshwater habitats for local wildlife and migrating waterfowl, as well as reduces the need to build new dams and other water-diversion infrastructures.

There are few things as wasteful as a water sprinkler system running during a rain storm. The goal of this environmental engineering science project is to build an electronic circuit that can detect when it is raining and that can shut off the power to an automatic sprinkler system.

The circuit will be modified from an example in an electronics kit, making it relatively easy to build. The circuit contains an electronic part called a 4011 integrated circuit (IC). The 4011 IC has four nand gates (the word nand is derived from "not and," reflecting the fact that the "nand" output is the reverse of the "and" output). You will just be using one of them for this science project. A nand gate is able to turn things on or off, depending on the kind of input it receives. The nand gate has two inputs, labeled 1 and 2 in Figure 1, below. The input is "on" when it is at 9 volts (V) and "off" when it is connected to ground. Note: The workbook that accompanies the kit explains what "ground" and other terms mean, and also describes the symbols used in the circuit diagram.


Electronics and Electricity science  project <B>Figure 1.</B> A rain detector circuit.
Figure 1. A rain detector circuit. When the sponge is wet, no current flows from input 3 to ground, so the circuit shuts off power. When the sponge is dry, the circuit allows current to flow through the LED to ground (the LED is a substitute for the sprinkler; when it is on or off, the sprinkler would be on or off). The circuit "senses" rain because there is an electrical connection between the wires when the sponge is wet. When this electrical connection is made (when the sponge is wet) the voltages at inputs 1 and 2 become high (9 V, "on") and output goes to 0 ("off"). To turn off a sprinkler system, the power that controls the sprinklers would be connected through the circuit.

Here is how the output is controlled by the two inputs. When inputs 1 and 2 are both at 9 V (that is, 1 and 2 are both "on"), the output from input 3 is "off." For all other combinations of the states of inputs 1 and 2 (off/off, on/off, off/on) the output from input 3 is "on" (9 V). You can put this relationship in a truth table, as shown below.


1 23LED lights up Rain
On On Off No Yes
On Off On Yes No
Off On On Yes No
Off Off On Yes No
Table 1. Truth table for the nand gate. The output (3) is "off" (0 V) only when both inputs (1 and 2) are "on" (9 V). This is the opposite of an "and" gate truth table, where the output is "on" only when both 1 and 2 are "on." The table could also use "high" vs. "low", or "1" vs. "0," rather than "on" vs. "off."

This science project involves making the circuit shown in Figure 1 and demonstrating that it shuts off power to the light-emitting diode (LED) when the sponge is wet. The LED represents the sprinkler system. If you choose, you can add the circuit to a real sprinkler system (see the Variations). In an automatic sprinkler system, the water is turned on and off by a solenoid. When the solenoid is powered by a voltage, a part (called a diaphragm) moves so that the water can flow. When the power is turned off, the part falls back to its original location and the water flow is blocked.

The circuit will be modified from a circuit that turns off the power when a touch sensor is touched. For this engineering science project, the touch sensor will be replaced with a "water sensor" in the form of a sponge. If the circuit were used outside, the wet sponge would keep the power off until it was allowed to dry out. It is important to note that the circuit allows electricity to flow only when two conditions are met: the power switch is "on" (as it would be when the sprinkler system is turned on) and the water detector does not sense water. In the circuit, turning the power on just means moving the power switch to the "on" position. For a sprinkler system, the power would most likely be turned on by a timer. When the water sensor is wet in a sprinkler system, the electricity is not allowed to flow, even when the power is turned on.

Terms and Concepts

  • 4011 integrated circuit
  • Nand gate
  • Ground, in circuitry
  • Truth table
  • Light-emitting diode (LED)
  • Solenoid

Questions

  • How much water does the average household use per day?
  • What are some reasons to conserve water?
  • How many nand gates are in the 4011 IC?
  • What does the truth table for an "or" gate look like?
  • How can you combine nand gates to make an "or" gate?

Bibliography

Materials and Equipment

  • Electronic Sensors Lab kit; available from Radio Shack at www.radioshack.com, item # 280-0026

  • Included in the kit:
    • 4011 integrated circuit
    • R1 and R2: Resistors,100K-ohm (2)
    • R3: Resistor, 10M-ohm
    • R4: Resistor, 470-ohm
    • LED, red
    • Wire of various lengths to make circuit connections
    • Breadboard
  • Battery, 9-V
  • Sponge, rectangular, type you'd use in your kitchen
  • Scissors
  • Lab notebook

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Experimental Procedure

Note: This engineering project is best described by the engineering design process, as opposed to the scientific method. You might want to ask your teacher whether it's acceptable to follow the engineering design process for your project before you begin. You can learn more about the engineering design process in the Science Buddies Engineering Design Process Guide.
  1. Attach the 9-V battery, following the directions that came with the sensor board.
  2. Push the power and display switches off.
  3. The circuit you will use is shown on page 52 of the workbook.
    1. The circuit is modified to make it a water detector.
    2. Instead of using a touch sensor to control the circuit, you will use a sponge that turns the circuit off when it is wet.
  4. Insert the 4011 IC across the slot in the breadboard, with pin 1 at F5.
    1. See the diagram on page 16 of the Sensors Lab workbook for how to position the IC across the slot.
  5. Insert R1 into F1. The free end will be part of the water sensor.
  6. Insert R2 into G1. The free end will be part of the water sensor.
  7. Insert R3 across G4 and L7.
  8. Insert R4 across H3 and N3.
  9. Insert LED across N4 (anode) and L4 (cathode). The anode is the longer lead on the LED.
  10. Connect L1 to ground with a white wire.
  11. Connect F7 to +9V with a white wire.
  12. Connect F4 to +9V with a white wire.
  13. Connect J1 to K1 with a white wire.
  14. Connect K2 to L2 with a white wire.
  15. Connect G10 to H10 with a white wire.
  16. Connect K10 to L10 with a white wire.
  17. Connect H9 to K9 with a white wire.
  18. Connect L9 to ground with a white wire.
  19. Connect the unused pins of the 4011 (pins 5, 6, 8, 9, 12 and 13) to ground.
  20. Push the power switch on.
  21. The LED should light up. If it does not light up, check your wiring carefully.
  22. Cut the sponge into two parts (square shapes).
  23. Soak one piece of sponge in tap water. Keep the other piece dry.
  24. Touch the wet sponge to the two free leads from resistors R1 and R2. What happens?
  25. Touch the dry sponge to the two free leads from resistors R1 and R2. What happens?
  26. For your science fair display board, note that turning on the power to your circuit is equivalent to the sprinkler system's timer starting the sprinkler. The LED shows whether or not electricity is flowing. When the LED is on, the sprinkler would be on.

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Variations

  • Use a multimeter (available at Amazon.com) to measure voltages at various points on the circuit when the sponge is wet or dry. This will allow you to show that if when the sponge is wet, there is a 9-V drop from both positions 1 and 2 on the chip, compared to ground.
  • Put the circuit in a box and install it in a sprinkler system. You might want some help from a professional electrician for this.
  • Modify the circuit on page 72 to make a rain detector. This circuit uses an operational amplifier. The current for the sprinkler would run through the green LED.
  • How would you modify the circuit to turn something on when it rains, such as a buzzer?
  • What is the maximum distance the two probe wires can be separated? Does this distance vary with distilled water vs. rain water?

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