Humidity and Temperature Sensor keychain (idea)
I wish I had a small device that can fit on a keychain that displays temperature and humidity of the room. Winter is getting to an end and the problem of dry air will soon be over. But in times of warm weather there is problem of high humidity. So a sensor that can measure it is useful at all times of the year.
Building it should be a straight forward endeavor. For the main brain of the device I would use STM32F103, and it should be easy to connect sensor that work over I2C connection to it. On top of that as a way to display information one of those small 0.96 inch oled displays will be used.
They are cheap and for some reason I got 5 of them.
For the sensors there are a few options:
DHT11 - That's a sensor you get in Arduino kits, and it's bulky with it's casing, and I never saw one without it's casing.
TI HDC1080DMBR - Small sensor, in my opinion too small for hand soldering, and the pins are underneath the casing.
Bosch BME280 - Most expensive option, also too small, but great accuracy.
Choice is between HDC1080DMBR and BME280. With HDC1080DMBR there is a problem of library for Arduino. I can't seem to find it on Github. There is a library that can be installed on Arduino IDE, but link to github does not work. However, BME280 has a library by Adafruit, because they sell it with their board. And another good thing is that it measures pressure too. But it's price of 3.5 EUR opposite to other that are under 1 EUR. I guess for small batch the price is not important.
Design:
Design of a casing should not be much bigger than oled display board. Perhaps the board can be expanded to the left and right a little to achieve symmetrical edges.
And at one end there will be a hole where small keychain string can be attached. When holding a device and looking at the screen, the sensor will be placed on the back side, so casing will have small holes, so sensor can measure.
Now that I have written that down I figured a battery needs to go somewhere. The battery will be standard 3V coin battery.
And if possible it will be placed in between the oled and the main board.
That's it for the idea.
