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.
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