Pelican Crossing – Part 2: I2C MCP23017 & 555

Figure 1 - Working Breadboard Prototype

In Pelican Crossing – Part 1 software threads were used to provide timing to both the lights and the buzzer at the same time.  In Part 2, the I2C MCP23017 is used again but instead of using software to time the operations, the intermittent buzzer is implemented in hardware using a 555 timer.  This simplifies the code but makes the hardware more complex.  I learned something from both versions.

I found some great information on 555 timers from The Electronics Club – 555 and 556 Timer Circuits page.  This version creates an astable multivibrator running at 6.8Hz.  The timer is enabled & disabled on command from an output from the MCP23017 via a transistor.

Figure 2 below shows the schematic:

Figure 2 - Schematic

Below is the Python code:

import threading
import smbus
from time import sleep 

bus = smbus.SMBus(0)
address = 0x20 # I2C address of MCP23017
iodirA = 0x00 # Register for Bank A I/O Direction
iodirB = 0x01 # Register for Bank B I/O Direction
gpioA = 0x12 # Register for Bank A GPIO
gpioB = 0x13 # Register for Bank B GPIO

address = 0x20 # I2C address of MCP23017
bus.write_byte_data(address, iodirA, 0x00) # Set all of bank A to outputs 
bus.write_byte_data(address, iodirB, 0xff) # Set all of bank B to inputs 

# GPIOA0 - Red
# GPIOA1 - Amber
# GPIOA2 - Green
# GPIOA3 - Buzzer
# GPIOA4 - Ped Red
# GPIOA5 - Ped Green

# GPIOB0 - Button

delayOnWalk = 4;
intraChangeDelay = 1
delayButtonServicing = 0.2

def sequenceLights():
    bus.write_byte_data(address, gpioA, 0x12)
    bus.write_byte_data(address, gpioA, 0x29)
    bus.write_byte_data(address, gpioA, 0x2A)
    bus.write_byte_data(address, gpioA, 0x14)

    bus.write_byte_data(address, gpioA, 0x14)
    while True:
        mybutton = bus.read_byte_data(address,0x13)
        if mybutton == False:
except KeyboardInterrupt:
    print "Caught Keyboard"
    bus.write_byte_data(address, gpioA, 0x00)

Leave a Reply

Your email address will not be published. Required fields are marked *