What it shows
A DHT11 inside a clear box greenhouse reads temperature and humidity. The Arduino switches a 12 V fan when it gets too hot and a 5 V mist pump when it gets too dry, keeping the plants in a chosen band and showing the state on an LCD.
The science
Plants transpire best in a narrow range of temperature and humidity. The DHT11 reports both over its single data line. Two independent control loops run: temperature above a set point starts the fan, which draws in cooler room air; humidity below a set point runs the mist pump for a few seconds. Each loop has a gap between on and off levels so the equipment is not switched constantly. The LCD shows the readings and which device is running.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An Arduino Uno clone and USB cable
- A DHT11 module
- A 2-channel 5 V relay module
- A 12 V computer fan with adapter and a 5 V mini pump with a fine mist nozzle or pinholed tube
- A 16x2 I2C LCD
- A clear plastic storage box with a plant or two
- Jumper wires
Safety
For this project: The fan is 12 V and the pump 5 V; nothing from the mains passes through the relays. Keep the water tub outside the electronics area.
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 a fan-sized hole at one end of the box and tape the fan to blow outward, with a vent hole at the other end.
Wire the DHT11 data pin to pin 2 and hang the sensor inside the box, away from the mist nozzle.
Wire the relay INs to pins 7 and 8; wire the fan through relay 1 and the pump through relay 2 to their own supplies.
Wire the LCD on A4 and A5 and show temperature, humidity, FAN ON or OFF and MIST ON or OFF.
Set a fan-on temperature of 30 degrees and fan-off of 28; set a mist-on humidity of 50 percent and off of 65.
Write the loop to read every two seconds and apply both rules with the hysteresis gaps.
Limit each mist burst to five seconds followed by a 30 second pause so the sensor can respond.
Warm the box with a desk lamp from outside and watch the fan start; let it dry and watch the mist run.
Print a log line to the serial monitor each cycle and plot it with the serial plotter for the display board.
What you should see
The fan starts within seconds of the box passing the set temperature and the mist runs when humidity drops; the LCD shows the live readings and device states.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why place the sensor away from the nozzle?
Droplets landing on it would read 100 percent humidity at once and stop the mist too early.
2Why does misting also cool the box a little?
Evaporating water takes heat from the air.
3What is a set point?
The target value the controller tries to keep the variable near.
4Why not use delay() for the mist burst?
The fan loop and the display would freeze during the delay; millis() timing keeps everything running.
Take it further
Add an LDR and a 12 V LED grow strip that fills in light on cloudy days.
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.