Temperature and Humidity Meter

This project is a combined temperature and humidity meter. It is an improvement on my PIC-based humidity meter and my PIC and AVR thermometers. I use an ATmega164, which as lots of I/O and memory.

The humidity sensor is a HS1101, made by Parallax (p/n 27920) and available from Digikey (27920-ND) for around US$10. The circuit for the humidity meter sensor is identical to the earlier project. I had made a mistake in construction, which resulted in erratic operation. The problem is that the HS1101 must be physically as close to the LMC555 as possible. I had run a 4-inch wire to the circuit board, and the unit became sensitive to noise picked up from the surroundings. In this version, I placed the sensor board at the top of the case, where the HS1101 can plug right in to the circuit. 

At the top of the unit you can see the TC1046 temperature sensor (Digikey TC1046VNBTRCT-ND) and the HS1101 humidity sensor. The board at the top contains the LMC555 circuit and the power supply regulator. A 6 conductor cable goes the the lower board, which contains the CPU. The middle board has the LCD display and 2 switches. The switches hold the board in place. There is a 26 conductor ribbon cable to the CPU board just below. Since the LCD uses little power, the unit is battery operated (with provision for an external power supply), another big improvement over the older projects. The battery and holder are at the bottom of the case. The power leads go to the on/off switch (SW1) and then to the sensor board regulator, and regulated power goes to the CPU board.

The LCD display is a Lumex
LCD-S301C31TR 3 digit unit (DigiKey 67-1788-ND) with no controller or backlight. I must admit that this LCD display is a pain to use. You need 24 connections to the CPU (hence the large ribbon cable in the photo), and the program must drive each segment to make up the display. However it is cheap (<C$2), and as mentioned, uses little power. I just wish there was provision for a backlight. I also want to note that this display is a static LCD, and normally requires an AC signal to drive the segments. A very good write-up on the subject can be found here. However I did not implement this, as I find that the Lumex display maintains its contrast for some time after power-up. I never leave the unit on for more than a few seconds at a time, so this is not a problem for me.

When you switch the unit on, the battery voltage is sampled via R5 and R6 and ADC0. For 2 or 3 seconds, the battery voltage is displayed. Then the display changes to temperature or humidity, depending on the setting of SW2. Other improvements are the choice of Celsius or Fahrenheit (JP1), and the ability to display negative temperatures. The temperature sensor is connected to ADC7 and uses the internal 1.1V Vref. A digital median filter is used to smooth out the temperature display. The humidity circuit generates a squarewave which is connected to T1. Timer0 is programmed to give 1 second intervals, where the T1 count is read and reset. An interpolating routine is used to get humidity from the measured frequency.

Download C source code for the project

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