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There are several instructables out there on how to build an infrared thermometer. It occurred to me that it could be interesting to add more versatility by adding a hygrometer sensor. This way, through code it could be possible to determine the dew temperature.
On average use, determining dew temperature is not really necessary, however if for example some paint job needs to be done, determining the dew temperature is important. The surface temperature should be some 3C (5F) above dew temperature to avoid the formation of a moisture film between the coating and the base that could compromise the quality of the paint job. For this, in a single instrument it is posible to sense the surface temperature contactless, and at the same time determine if the site conditions are appropriate for the job.
Apart from determining dew point temperature, the device can also display other parameters that the usual infrared thermometer can’t, such as relative humidity and apparent temperature (indoors).

In order to fit the circuit in a not too large holder, it is best to use a small processor. For this case I used the Arduino Nano. I strongly recommend the Arduino Nano V3 based on the FT232 chip and not the one with the generic CH340 chip.

I initially bought the latter one and I had several issues communicating with the Arduino application. I searched for solutions but never really got to solve the problem. Some times closing and opening the app again solved the issue; other times I had to restart the computer. Since I was constantly modifying and enhancing the code I had to upload regularly the sketch to the Board. Eventually something happened to the Arduino board and ceased to synchronize completely. Probably the bootloader got corrupted, so I went for the Arduino with the FT232 chip. It’s slightly more expensive but definitely saves a headache.

Another advise, if you can get a DHT22 instead of a DHT11 it would be even better. Both are very similar but the DHT22 is more accurate and has a greater sensing range.

Aside from that, the first step once all de supplies and tools are gathered is to assemble the circuit on a breadboard. This will guarantee that all the components are working well before the final assembly. I usually test continuity even for each wire independently ever since I got a dupont wire damaged. Once the circuit is assembled it will not allow us to fully test all the components until the program is loaded to the board.

The device was conceived in such a way that the user can switch between two modes: an object temperature mode, and a hygrometer mode. The device has two push buttons. One of them (the one connected to pin D7) will allow the user to switch between modes, while the other push button (the one connected to pin D5) enables the infrared sensor. Only while the button is pressed will de divise display the sensed temperature. If the button is released the display will freeze the last measurement taken. In addition to that, whenever the device is in the object temperature mode and if the corresponding push button is pressed, a LED lights up (in my case I chose a white LED connected to pin D12). Whenever the mode is switched, another LED lights up (in my case I chose a blue LED connected to pin D10). In the hygrometer mode however it is not necessary to keep any push button pressed. The device will update measurements every quarter of a second and the LED will remain on.

Another feature added to the apparatus is a battery level indicator. To achieve this, a voltage divider was added. From the above image, Z1 corresponds to a 2.7 kOhm resistor and Z2 corresponds to a 5 kOhm trimmer set at 2.98 kOhm (carefully calibrated with a multimeter). The voltage divider proportionally drops down the voltage of the 9V battery (with real charge up to 9.5V) to a scale of 0-5V readable by the analog pins of the board (Vout in the image is connected to pin A1 and Vin directly to the battery). Within the code a scale is set up which will in turn draw a battery icon on the display, showing the level of the battery.”

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