Nuts and Volts - September 2017

One drawback from using this approach is it will not stop the water flow if there is an irrigation valve failure. Another approach would be to add an irrigation valve that’s controlled by the Irrigation Watch’s relay that would shut off all water to the system if there is an alarm condition.

This project includes several interesting ideas such as the control of a 16 character by two line LCD display, an innovative approach to detecting three input key switches with only two I/O pins, and the interfacing of four 24 VAC signals from the irrigation control unit.

The project is built around the PIC16F1829, programmed using their MPLAB X IDE, XC8 C language complier in free mode and the MPLAB Code Configurator. Source code, software listings, and Gerber files are all available at the article link.

Hardware

The schematic for the Irrigation Watch is shown in

Figure 1. Port A of the PIC16F1829 is dedicated to in-circuit programming, reset and programming voltages, Notice the use of opto-isolators to interface the PIC16F1829 with the 24 VAC signals from the X-Core unit to the irrigation system water valves. Port pins RB4 through RB7 are configured with weak pull-ups for the open collectors of the LTV-814 opto-isolators. Using available PIC16F1829 features like weak pull-ups reduces the need for external resistor components, saving expense and printed circuit board (PCB) real estate. Port C of the PIC16F1829 provides control of the LCD display.

The LCD display and three user key switch inputs provide all the needed user control of the Irrigation Watch functions. The three user key switches provide ‘Left,’ ‘Right,’ and ‘Enter’ inputs to the PIC16F1829. Port A pins RA0 and RA1 are (like the port B inputs) configured as inputs with weak pull-ups.