I'm trying to connect a small one line relay to my Raspberry Pi but the relay doesn't react. The red light on the relay is on the whole time.

import RPi.GPIO as GPIO
import time


# init list with pin number

pinList = [27]

# loop through pin and set mode and state to 'high'

for i in pinList: 
    GPIO.setup(i, GPIO.OUT) 
    GPIO.output(i, GPIO.HIGH)

# time to sleep between operations in the main loop

SleepTimeL = 2

# main loop


  GPIO.output(27, GPIO.LOW)
  print "FOUR"
  print "Good bye!"

# End program cleanly with keyboard
except KeyboardInterrupt:
  print "  Quit"

  # Reset GPIO settings

Photo of Pi and Relay

  • What connections have you made between the Pi and the relay board?
    – joan
    Sep 19, 2015 at 16:58
  • it is so weird because it works if I use wiringPI :/ but I don't know c so I would like that it works using python. The connection is: Pin 1 ---> VCC Pin 6 ----> GND Pin 13 ----> IN. Maybe there is a conflict between python code and c code so it doesn't work?
    – user30407
    Sep 19, 2015 at 17:28
  • I'm getting such alert in my terminal after I try to execute this code above: script1.py:14: RuntimeWarning: This channel is already in use, continuing anyway. Use GPIO.setwarnings(False) to disable warnings.
    – user30407
    Sep 19, 2015 at 17:40
  • Ignore the warning. It's not relevant. Does the relay operate when you connect a ground pin or a 3V3 pin (pin 1 or pin 17) to IN?
    – joan
    Sep 19, 2015 at 19:05

1 Answer 1


My 8 relay module is set up to control the heating and air conditioning in a remote building. I run 5v to all the relay coils - that is the high side. I run GPIO pins to the low side. The GPIO pins are normally high - 3.3v. When a GPIO pin is made to go down it looks like ground to the relay to which it is attached and the relay operates/closes.

#      GPIO09 -  Turn on or off a specified relay so
#                it stays on or off after the program ends 
#                at power on all pins are in input state ready to sense hi or lo

import RPi.GPIO as GPIO                      ## Import GPIO library
GPIO.setmode(GPIO.BOARD)                     ## Use board pin numbering

devarr=["Air Conditioning","Heater","Unused","Unused","Unused","Unused","Unused","Unused"]

##  Use a dictionary as a lookup table to give a name to the GPIO_function return code
pin_use = {0:"GPIO.OUT",1:"GPIO.IN",40:"GPIO.SERIAL",41:"GPIO.SPI",

for i in range(0,8):                         ##  set pins to be outputs and at 3.3 volts  
    GPIO.setup(pinarr[i], GPIO.OUT)

for i in range(0,8):
    print "\n"
print("GPIO09 Starting")

##for i in range(0,8):                         ##  Print current relay usage   
##  fno = GPIO.gpio_function(pinarr[i])      ##  get function number
##  print "Relay " + str(i+1) + " pin " + str(pinarr[i]) + " usage is " +  pin_use[fno]
while 1 == 1:
    for i in range(0,8):                         ##  get and display pin on/off status  
        func = GPIO.input(pinarr[i])
        print "Relay " + str(i+1) + " " + devarr[i] + " pin " + str(pinarr[i]) + " is " + dirarr[func]

    print " "

## Ask user for relay #
    relno = raw_input("Enter number of relay to change: ")
    if not relno:
        print "exiting"
        raise SystemExit

    if(int(relno) < 1):
        print "Bad choice - exiting"
        raise SystemExit
    if(int(relno) > 8):
        print "Bad Choice - exiting"        
        raise SystemExit

    relno = int(relno)
    ans = raw_input("ON or OFF ?  1 or 0: ")

    if (ans == "1"):
        print "Turning on relay " + str(relno) + " Pin " + str(pinarr[relno -1])
        GPIO.output(pinarr[relno - 1],False)         ## Switch off pin = conduct
        for i in range(0,2):
            print "\n"

    if (ans == "0"):
        print "Turning off relay " + str(relno) + " Pin " + str(pinarr[relno -1])
        GPIO.output(pinarr[relno - 1],True)          ## Switch on  pin = non-conduct
        for i in range(0,2):
            print "\n"

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