RHT Device - Relative Humidity and Temperature Sensor

RHT Device is a very simple relative humidity and temperature sensor that shows the data in the screen. The project was chosen to test the I2C communication of the PIC18F14k50.

Github Repository

You will need the following components (For V1 revision):

• 1x PCB v1 (Thanks to PCBWAY to sponsor this project!)
• 1x HTU20D(F)
• 1x PIC18LF14K50-I/PT or PIC18F14K50-I/PT
• 4x NPN 600mA
• 4x 220 Ohm resistor (0805 metrics)
• 4x 4K7 Ohm resistor (0805 metrics)
• 5x 10K Ohm resistor (0805 metrics)
• 1x Buzzer 5V
• 3x 100nF Capacitor (0805 metrics)
• 1x CR2032 Battery Holder
• 1x 4-Display 8 digits (3461AS)
• 1x Tactile Push Buttons
• 1x ZLLS410TA diode

You will find all the components, links to the components, Gerber and simulations files in the GitHub Repository.

As you may see, the design is pretty forward: The pic18LF14k50 drives some NPN transistors, which allow the display to light. Same for the buzzer.

The display has four numbers in parallel, so we will need to have a high-frequency blink in order to "trick" the human eye to see them all light at the same time. 

For the button, an RC filter made of a 10K resistor and 100nF will filter any bouncing effect.

Last but not least, the Humidity and temperature sensor is the HTU20D, which gives us the reading using I2C communication.

Solder paste can be applied with a syringe or stecil. Afterwars, the SMD components must be placed with the use of some tweezers. It can be soldered in a hotplate or using a heatgun.

All the TH components can be placed and soldered by hand. It is recommended to cut the leads from all the components

You will need a Tag-Connect 6-pins cable for the ICSP.

You will find the code here:

https://github.com/makingdevices/RHT-device/tree/main/Firmware

Or from the attached file. You should use MPLAB and C18++

Instructions of use:

• When the button is pressed, the temperature is shown. (d in last digit)
• Next time the button is pressed, relative humidity is shown. (h in last digit) It will change between temperature and humidity
• After 10s of no use, the screen will be off and the device enters sleep mode.
• One button click will wake up the device for another 10s

One of the main requisites for the board is to obtain a minimum power consumption so the device can live from a coin cell.

1. When the screen is ON we obtain: 10.69 mA.
2. When the screen is OFF we obtain: 6.79 uA.

With this data and a normal use of 5 reading a day, the battery should last for several years.

If you like this project and wants to support my work, you can follow me on instagram:

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