Precision hygrometer

This hygrometer was designed in early spring 2006. Click any image for a full view.


When storing cigars in a country different from where they were manufactured, it is necessary to control the environment so that the air has the same humidity as that of the country of origin (preferably Cuba). In northern Europe the air is much drier, and without taking special measures cigars will dry out, become brittle and lose their taste. Therefore, cigars are kept in a special box called a humidor, which is equipped with a humidification device (to be filled with water at regular intervals). The goal is to maintain a relative humidity of 70%.

Relative humidity is not easy to measure accurately, and analog hygrometers, such as those supplied with humidors, are very inaccurate devices. I have tried three different types which have all been unreliable or stopped working altogether.

This project is about getting continuous and trustworthy information about the climate inside my humidor. Digital hygrometers with better accuracy are available, but I wanted a solution where I could read the value without opening the lid, and more important - I wanted to do it in the overkill style that befits a true DIY'er!

Humidity sensor

The project is based on the digital humidity/temperature sensor SHT75, manufactured by Sensirion, Switzerland.

SHT75 (image from

Sensirion makes various sensors for industrial/laboratory use, and SHT75 is their high-end humidity sensor. Prior to delivery, each sensor is individually calibrated in a precision humidity chamber, and an accuracy of +-1.8% is stated in the device datasheet. Measurement data is read over a two-wire serial interface, making the wiring of the sensor to the host controller non-critical.

Controller and display

The sensor is the only thing which is mounted inside the humidor. The external circuit is rather simple, based on a PIC16F676 14-pin microcontroller. The PIC communicates with the sensor, reading the humidity about once every two seconds. The measurement is a 12-bit binary value, which is translated into relative humidity (%) by a 2nd-degree formula given in the sensor datasheet. The RH value is continuously displayed on a two-digit led display.


The sensor is connected to the controller/display by a thin ribbon cable. This cable is relatively discreet, and runs next to one of the hinges at the rear of the humidor. It is thin enough not to be in the way when the lid is shut.

The LED display looks nicer than a LCD one, but it rules out battery power. The hygrometer is powered by a regular wall-socket transformer.

If the relative humidity is outside the interval 65-75%, the value shown in the display starts flashing. This feature act as a discreet 'alarm' function, calling attention to the need for refilling (or removing) the humidifier.

Construction details

The schematic diagram is not complex. It consists of little more than the MCU and a two-digit LED display. The circuit needs a stabilized 5V supply. Refer to the SHT75 datasheet.

The controller/display unit is built on a small PCB. This is the board layout used (mirrored for UV exposure). Note that the LED display is mounted on the solder side.

Software for the MCU is available upon request.


Smoking is a really bad idea. That's why it should be limited to the occasional enjoyment of a cuban cigar under festive circumstances.

(c) Marcus Gunnarsson 2006

Back to Electronics