What it shows
A DHT11 inside a model grain bin watches humidity; stored grain spoils and grows mould when air humidity stays high. The NodeMCU shows the reading on a phone page, sounds a buzzer and sends an alert when humidity stays above a limit for a set time.
The science
Grain is safe in storage when the air around it is below about 65 percent relative humidity; above that, moulds grow and insects thrive. The DHT11 reports humidity to the ESP8266. Because a brief rise when the lid opens is harmless, the sketch only alarms when the reading stays above the limit for more than two minutes, using millis() to time it. The web page shows the live value, and an HTTP request to a free notification service sends the phone alert.
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.
- A NodeMCU ESP8266 and USB cable
- A DHT11 module
- A buzzer and a red LED with a 220 Ω resistor
- A plastic container with some rice or wheat
- A 5 V power bank
- A phone on the same Wi-Fi
Safety
For this project: USB power only. Do not eat grain used in the demonstration after it has been dampened.
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.
Wire the DHT11: VCC to 3.3 V, GND to GND, DATA to D4.
Wire the buzzer to D5 and the LED through its resistor to D6.
Install the DHT library and the ESP8266 web server example; print humidity every two seconds.
Serve a page showing humidity, temperature and the words SAFE or DAMP.
Add a timer: when humidity goes above 65 percent note the time; if it stays above for two minutes, set the alarm.
On alarm, beep the buzzer, light the LED and send one notification using the free service described in the ebook.
Clear the alarm when humidity drops below 60 percent for one minute.
Hang the sensor inside the grain container; put a wet cloth in to simulate a leaking roof and watch the timer run.
Note the time it takes for the grain to dry back once the cloth is removed and the lid opened.
What you should see
Humidity sustained above 65 percent for two minutes triggers the local alarm and a phone message; a brief lid opening does not.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does damp grain spoil?
Moisture lets moulds and insects grow and the grain itself respires faster and heats up.
2Why wait two minutes before alarming?
To ignore brief changes such as opening the lid, and alert only on sustained dampness.
3What is the safe humidity for stored grain?
Roughly below 65 percent relative humidity for most cereals.
4Why place the sensor inside rather than on the lid?
The air between the grains is what matters; the lid surface can read differently.
Take it further
Add a second sensor outside and show the inside and outside readings side by side.
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.