Hygrometer for air humidifier on DHT11

It is a design of hygrometer for ultrasound air humidifier. This simple circuit uses a small microcontroller and cheap humidity sensor.



Construction:
The circuit was based on one-sided PCB. It contains a linear stabilizer 5V, multiplexed display, processor Attiny2313, two buttons and a relay that controls the operation of humidifier. The hardware is very simple.



DHT11 used in this design is an integrated humidity and temperature sensor. This sensor is really simple in construction, it is also not very accurate, but it also has very important advantages – it is digital and cheap. The sensor is connected to the processor by a one-wire bus. Master sends the command of temperature and humidity reading via a longer pulse of the ground, and slave responds by bits which can be then set to 5 bytes (2 bytes of humidity, 2 bytes of temperature and a byte of CRC).

Functions:
Program was written in WinAVR. Data from DHT11 are read every 1 second, multiplexing and time is measured by T0 counter, times of pulses given by DHT11 are counted on T1 counter. 3 times faulty read the values of the sensor (no response, incorrect control sum) causes switching to an error state (Er symbol on the display) and switching on the humidifier for constant. If the read is correct, the circuit operates in a normal way. Contacts vibrations are also carried out on Timer0. There is also a rapid auto-repeating when the button is pressed.

It is possible to set two values:

• temperature below which the humidifier will not switch on
• set humidity

Both these settings are stored in the EEPROM. During normal work, measured value of humidity is displayed. Flashing dot in the corner of the display indicates work of the humidifier. Pressing the S1 button switches to setting the temperature (for a second before modified value, “C” is displayed). Pressing the S2 button results in switching to setting the humidity (“rH” for a second). When you stop modifying the value of temperature or humidity, after 5 seconds the system switches to work, writes the settings to EEPROM and displays the current humidity.

Pictures:



Link to original thread - Higrometr do nawilżacza powietrza na DHT11