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First of all I would like to thank all of those who preordered the TrH Meter kits during the fundraising campaign that we ran on Tindie. It’s because of your support that we successfully launched this project. Please follow the soldering instructions provided below while building the TrH Meter.
TrH Meter project kit
This project is about building a microcontroller-based digital room thermometer plus hygrometer that displays temperature and relative humidity on 4 large (1 inch) seven segment LED displays which adjust their brightness level according to the surrounding illumination. It consists of a closed loop system that continuously assesses ambient light condition using an inexpensive light-dependent resistor (LDR) and uses that information to adjust the brightness of the display. An inexpensive DHT11 sensor is used to measure temperature and relative humidity. The microcontroller used in this project is PIC16F688, and it runs at 4 MHz clock generated from its internal source. A separate display driver chip (MAX7219) is used to control and refresh the display data on the seven segment LEDs.
TrH Meter: A DIY indoor thermometer plus hygrometer
This is a revised version of my LM35 based digital thermometer project that I posted last year. Although it is one of the simplest projects, it is very popular among newbies who are just starting to learn microcontrollers. There was a little flaw in the original project as pointed by some readers. I was using a 1.2 V reference for A/D conversion with PIC16F688 microcontroller. However, the PIC16F688 datasheet says Vref should be equal to or higher than 2.2 V to ensure 1 LSB accuracy of A/D conversion. Here, I am rewriting the same project but this time I am using a MCP1525 IC to generate a precise 2.5 V reference for A/D conversion. This will improve the accuracy of temperature measurements.
Interfacing LM35 temperature sensor with a PIC microcontroller
Interrupts are powerful concept in embedded systems for controlling events in a time-critical environment. In a typical embedded system, the embedded processor (microcontroller) is responsible for doing more than one task (but can do only one at a time). For example, let’s say in a programmable digital room thermostat, the microcontroller is assigned to monitor the room temperature, turn the AC or heater ON and OFF, control the LCD display, and respond to any new temperature setting from the user. Out of these the first three tasks are non-time-critical and are executed continuously in sequence one after the other, within the main loop. But when the user presses any button on the setting panel, the microcontroller should be able to read it before the user releases the button. So this is a time-critical event and the microcontroller should stop whatever it is doing and respond to this higher priority event. This is possible through the use of interrupts. This tutorial first describes the interrupt system in general and then illustrates how it is handled in PIC micrcontrollers.
Understanding interrupts in PIC microcontrollers
Microcontrollers are widely used in measuring various physical variables. The techniques involved in the measurements could be different for individual variable type and are mostly based on the characteristics of the variables to be measured. This tutorial describes some methods for measuring the capacitance of a capacitor using microcontrollers. The techniques use the characteristics of the capacitor itself and are therefore universal and can be easily implemented with any microcontrollers.
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