What it shows
A laser beam passes through a sample bottle to an LDR; cloudy water scatters light and the LDR reading falls. The Arduino converts the reading to a clarity scale calibrated with known mixtures and shows it on an LCD, a low-cost version of a turbidity meter.
The science
Suspended particles scatter and absorb light, so less of the beam reaches the detector on the far side. The LDR resistance rises as light falls, and the divider voltage on A0 drops. The relationship is not linear, so the sketch stores readings for clean water (100 percent clear) and a very cloudy sample (0) and maps between them. A dark tube around the path stops room light changing the baseline. Real meters measure scattered light at 90 degrees; this simpler version measures the straight-through beam.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An Arduino Uno or Nano clone and USB cable
- A 5 mW laser diode module (3 to 5 V)
- An LDR and a 10 kΩ resistor
- A 16x2 I2C LCD
- Identical clear plastic bottles or test tubes
- A cardboard tube to keep room light out
- Muddy water, milk and clean water for calibration
Safety
For this project: Never look into the laser or point it at anyone; mount it so the beam is enclosed in the tube. An adult must set up the laser. Keep water off the circuit.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
9 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Cut two holes opposite each other in the cardboard tube at the height of the sample; fix the laser at one hole and the LDR at the other, facing it.
Wire the laser to pin 7 through a transistor or directly if its current is under 20 mA; wire the LDR divider to A0 and the LCD to A4 and A5.
Place a bottle of clean water in the tube, switch the laser on in code, and record the A0 reading as the clear value.
Make a very cloudy sample with a few drops of milk and record it as the zero value.
Map readings between the two values to a 0 to 100 clarity score and show it on the LCD.
Prepare samples of 1, 2, 3 and 4 drops of milk in the same volume and record the score for each.
Plot score against drops to show the response curve for the report.
Test tap water, pond water and filtered water and rank them.
Switch the laser off between readings in code to extend its life and avoid stray light.
What you should see
A clarity score that falls steadily as more milk or mud is added, and a ranked list of real water samples from clearest to cloudiest.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is turbidity important?
Cloudy water may carry microbes on the particles and blocks light for water plants; it is a basic water quality test.
2Why enclose the path?
Room light reaching the LDR adds to the reading and shifts the calibration.
3Why do real meters look at 90 degrees?
Scattered light at the side rises from zero with particle count, which is more sensitive at low turbidity than a small drop in the straight beam.
4What unit do professionals use?
NTU, nephelometric turbidity units, calibrated against a standard formazin suspension.
Take it further
Add a second LDR at 90 degrees to the beam and compare the two methods.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.