Switch through custom data with 16×2 LCD screen and Raspberry Pi

Introduction

After starting my circuit testing with some LEDs and switches I moved up to a small 16×2 LCD screen (HD44780 controller) with the Raspberry Pi.  The configuration of the LCD pins and base code for output to the 16×2 LCD screen is taken from the excellent Raspberry Pi Spy website.  I’ve made a few modifications:

• Added some python code to obtain hostname, IP address, CPU load and memory usage.
• Introduce a switch that cycles through the data above (read in through a voltage divider and GPIO input).

Circuit Diagram

Using the excellent Fritzing tool my circuit diagram is shown below (again, very similar to the Raspberry Pi Spy website with a couple of additions)

There is a 10K potentiometer in the middle (attached to the contrast pin of the LCD screen) to control screen contrast.  In addition there is a voltage divider with the switch feeding into  GPIO 22.  This switch will be used to cycle through the data output to screen.  The voltage divider is required because the LCD screen is using the 5V rail but the GPIO pins should never receive an input above 3.3V.  In addition R₂ in the voltage divider is acting as a pull down resistor.

The calculations below were used to obtain the voltage out using some simple resistor values:

 (this is sufficient to provide a HIGH on the GPIO pin)

If you want a voltage closer to 3.3V then a 6.8K resistor will provide ~3.0V.

In addition to the voltage input above, the current dropped across R1 when the switch is pressed is ~0.5mA.  Both voltage and current readings were confirmed with a multimeter.

Code

Here is the python code for the switch cycling:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 2021

#!/usr/bin/python # # HD44780 LCD Script for # Raspberry Pi # Reads switch input and shuffles through # Hostname, IP address, CPU load # and memory usage. # The wiring for the LCD is as follows: # 1 : GND # 2 : 5V # 3 : Contrast (0-5V)* # 4 : RS (Register Select) # 5 : R/W (Read Write)       - GROUND THIS PIN # 6 : Enable or Strobe # 7 : Data Bit 0             - NOT USED # 8 : Data Bit 1             - NOT USED # 9 : Data Bit 2             - NOT USED # 10: Data Bit 3             - NOT USED # 11: Data Bit 4 # 12: Data Bit 5 # 13: Data Bit 6 # 14: Data Bit 7 # 15: LCD Backlight +5V** # 16: LCD Backlight GND #import import RPi.GPIO as GPIO import time import subprocess # Define GPIO to LCD mapping LCD_RS = 7 LCD_E  = 8 LCD_D4 = 25 LCD_D5 = 24 LCD_D6 = 23 LCD_D7 = 18 SWITCH_IN = 22 # Define some device constants LCD_WIDTH = 16    # Maximum characters per line LCD_CHR = True LCD_CMD = False LCD_LINE_1 = 0x80 # LCD RAM address for the 1st line LCD_LINE_2 = 0xC0 # LCD RAM address for the 2nd line # Timing constants E_PULSE = 0.000005 E_DELAY = 0.000005 def main():   # Main program block   GPIO.setmode(GPIO.BCM)       # Use BCM GPIO numbers   GPIO.setup(LCD_E, GPIO.OUT)  # E   GPIO.setup(LCD_RS, GPIO.OUT) # RS   GPIO.setup(LCD_D4, GPIO.OUT) # DB4   GPIO.setup(LCD_D5, GPIO.OUT) # DB5   GPIO.setup(LCD_D6, GPIO.OUT) # DB6   GPIO.setup(LCD_D7, GPIO.OUT) # DB7   GPIO.setup(SWITCH_IN, GPIO.IN) # Switch Input   cmd = "ip addr show wlan0 | grep inet | awk '{print $2}' | cut -d/ -f1"   cmd2 = "hostname"   cmd3 = "free | grep cache: | awk '{print $4}'"   cmd4 = "uptime | tail -c 17"   i = 1   # Initialise display   lcd_init()   while True:     time.sleep(0.05)     if GPIO.input(SWITCH_IN) == True:         if i == 1:             hostname = run_cmd2(cmd2)             fhostname(hostname)             i = 2             time.sleep(0.1)         elif i == 2:             ipaddr = run_cmd(cmd)             fipaddress(ipaddr)             i = 3             time.sleep(0.1)         elif i == 3:             cpuload = run_cmd4(cmd4)             fcpuload(cpuload)             i = 4             time.sleep(0.1)         elif i == 4:             memory = run_cmd3(cmd3)             ffreememory(memory)             i = 1             time.sleep(0.1)         else:             print("no function found") def fhostname(hostname):     # Send hostname information     lcd_byte(LCD_LINE_1, LCD_CMD)     lcd_string("Hostname",2)     lcd_byte(LCD_LINE_2, LCD_CMD)     lcd_string(hostname,2)     while GPIO.inputSWITCH_IN == True:         pass def fipaddress(ipaddr):     # Send IP info     lcd_byte(LCD_LINE_1, LCD_CMD)     lcd_string("IP Address",2)     lcd_byte(LCD_LINE_2, LCD_CMD)     lcd_string(ipaddr,2)     while GPIO.inputSWITCH_IN == True:         pass def ffreememory(memory):     # Send some text     lcd_byte(LCD_LINE_1, LCD_CMD)     lcd_string("Free Memory",2)     lcd_byte(LCD_LINE_2, LCD_CMD)     lcd_string(memory + " MB",2)     while GPIO.inputSWITCH_IN == True:         pass def fcpuload(cpuload):         # Send some text         lcd_byte(LCD_LINE_1, LCD_CMD)         lcd_string("CPU Load",2)         lcd_byte(LCD_LINE_2, LCD_CMD)         lcd_string(cpuload,2)         while GPIO.inputSWITCH_IN == True:                 pass #Capture IP Address def run_cmd(cmd):         p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE)         output = p.communicate()[0]         return output.rstrip() #Capture Hostname def run_cmd2(cmd2):         p = subprocess.Popen(cmd2, shell=True, stdout=subprocess.PIPE)         output = p.communicate()[0]         return output.rstrip() #Capture Hostname def run_cmd3(cmd3):         p = subprocess.Popen(cmd3, shell=True, stdout=subprocess.PIPE)         output = p.communicate()[0]         output = str(int(output) / 1024)         return output.rstrip() #Capture CPU Load def run_cmd4(cmd4):         p = subprocess.Popen(cmd4, shell=True, stdout=subprocess.PIPE)         output = p.communicate()[0]         return (output.rstrip()).lstrip()  def lcd_init():   # Initialise display   lcd_byte(0x33,LCD_CMD)   lcd_byte(0x32,LCD_CMD)   lcd_byte(0x28,LCD_CMD)   lcd_byte(0x0C,LCD_CMD)   lcd_byte(0x06,LCD_CMD)   lcd_byte(0x01,LCD_CMD) def lcd_string(message,style):   # Send string to display   # style=1 Left justified   # style=2 Centred   # style=3 Right justified   if style==1:     message = message.ljust(LCD_WIDTH," ")   elif style==2:     message = message.center(LCD_WIDTH," ")   elif style==3:     message = message.rjust(LCD_WIDTH," ")   for i in range(LCD_WIDTH):     lcd_byte(ord(message[i]),LCD_CHR) def lcd_byte(bits, mode):   # Send byte to data pins   # bits = data   # mode = True  for character   #        False for command

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