relay always on

How come my (Arduino compatible only,) low level trigger 5V relay with Rpi is always On?

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import RPi.GPIO as GPIO
import time

#set GPIO Pins
GPIO_IN = 21

GPIO.setwarnings(False)     # To avoid same PIN use warning 
#set GPIO direction (IN / OUT)
def distance():
    # set Trigger to HIGH
    GPIO.output(GPIO_TRIGGER, True)
    # set Trigger after 0.01ms to LOW
    GPIO.output(GPIO_TRIGGER, False)
    StartTime = time.time()
    StopTime = time.time()
    # save StartTime
    while GPIO.input(GPIO_ECHO) == 0:
        StartTime = time.time()
    # save time of arrival
    while GPIO.input(GPIO_ECHO) == 1:
        StopTime = time.time()
    # time difference between start and arrival
    TimeElapsed = StopTime - StartTime
    # multiply with the sonic speed (34300 cm/s)
    # and divide by 2, because there and back
    distance = (TimeElapsed * 34300) / 2
    return distance

if __name__ == '__main__':
        while True:
            dist = distance()
            print ("Measured Distance = %.1f cm" % dist)
            time.sleep(1) #the time befor it moves forward tothe next step
            if dist >= 5:
                GPIO.output(GPIO_IN, GPIO.HIGH) #turm pump on
                print("water pump is on")
            if dist <= 6:
                GPIO.output(GPIO_IN, GPIO.LOW) #turn pump off
                print("water pump off")
    except KeyboardInterrupt: # Reset by pressing CTRL + C
        print("Measurement stopped by User")
        GPIO.cleanup() #clean up all the porta i"v used

hi guys im connecting to the R-pi an ultrasonic sensor to mesure distanse and a relay to control a motor but the 5v relay doesnt seems to work.

  1. why the relay turn on in GPIO.setup(GPIO_IN, GPIO.OUT) line? this is a line to define GPIO pin
  2. why the reraly allways “on”? GPIO.output(GPIO_IN, GPIO.LOW) line does nothing, this line supos to turn the relay off but nothing happans

thank you.raspbianpi-3b+relayShareEditFollowClose 1Flagedited 42 secs agotlfong013,63133 gold badges77 silver badges2222 bronze badgesasked 12 hours agoLiron Aviv1 New contributor

  • The relay might not be suitable for use with the Pi. Does it switch on if you (CAREFULLY) connect a wire from a Pi 3V3 pin to the input? Does it switch off if you (CAREFULLY) connect a wire from a Pi ground pin to the input? – joan 11 hours ago
  • yes it does. if i use a sipmple on/off switch code it turn on and off but when i try to intigrat it like in the code above it wont do anything. – Liron Aviv 9 hours ago 
  • Can you edit your question, and include the code that does work for you. – Chad G 9 hours ago
  • Does this answer your question? Can you use a 5V Relay Module with the Pi? – Milliways 9 hours ago
  • If it works the only conclusion is you have wired to the wrong GPIO. A clear photo may help. – joan 54 mins ago

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2 Answers


Ah, so have the relay always on problem, which is the Rpi relay newbies’ constant (well, 12 years) sorrow.

The very sad story began in the good old happy days, when we hobbyists played with only 5V Arduino and all logical levels are sort of 5V TTL, life was easy.

It is only when 3V3 Raspberry Pi came along, and later also 3V3 Arduino (Pro Mini 328 3V3 8MHz), life has become confusing, especially for the oldies/newbies who only know about Arduino/TTL 5V logic.

To understand why all (well, almost) newbies get confused, we need to look closely the following logical level chart, showing the root cause of newbie 3V/5V sorrows.

logical chart

Let us focus on the left most two columns, TTL and Arduino. In those were the days, my Arduino friends thought the imperial Arudino empire would live happily ever after, never imagined that some big guys like Rpi would soon appear. So the story goes than the Arduino engineers devised a new logical level standard/specification:

(Arduino) High level means at least 4.2V

Low level means at most 0.8V

The result is that most devices, say actuators, including relays, solenoids, buzzers, you name it, meet this spec, with (the latter Rpi guys scary) requirement that to do something using High level, you need to give 4.2V or higher (But how can 3V3 Rpi do that!).

Of course this makes the life of Rpi, born later, very miserable, because they are weak 3V3 guys, and their High level is usually 2.4V to at most 3.2V.

This is what I usually refer as

The (Arduino compatible only) Low Trig Relay is always on, because Rpi’s High is Not High Enough Problem.

Of course there are solutions and workarounds. You can search for a couple of my suggestions in this Rpi SS and EE SE forums. EG. Appendix C of this Q&A (1) How to properly use a relay module with JD-VCC from Arduino/Raspberry? EESE, Asked 10 months ago Active 3 months ago Viewed 7k times.

ShareEditDeleteFlaganswered 9 mins agotlfong013,63133 gold badges77 silver badges2222 bronze badgesAdd a comment2

Most relay modules are active LOW and you need to set the pin HIGH to turn the relay offShareEditFollowFlaganswered 12 hours agoBra1n89355 silver badges66 bronze badgesAdd a comment

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