Writing threads into code for 4 channel relay on pi

Question

Hi i am a super noob at all things PI, i have done a few simple projects. That said coding is really new to me. Ive read python for dummies and quite a few articles on the web so far, but my latest project im stuck.

Im trying to write a code to controll a 4 channel relay board with my pi. But i want my relays to turn off and on at different intervals independant of each other. So far i think i need to use threading commands, but im not sure how to write them? As an example of what im trying to accomplish.

Relay 1 on 3 seconds off 5 seconds repeating forever

Relay 2 on 5 minutes then off permanently

Relay 3 on permanently

Relay 4 on permanently

If anybody can point me in a direction where i can learn these code commands i will be super grateful!

Ive tried searching all over and cant find what i need. Im sure i just need to get better at searching though lol.

fmilburn · Answer

Hello John, I will try and get you started.... Using threads is a somewhat advanced topic. Here is another way to do it. Relay 3 and 4 are simple as you probably already know. Just turn them on and leave them that way with the GPIOZero library or whatever library you are using. Relay 1 and relay 2 are not that much more complicated. Use the time.monotonic method which will keep steady time in seconds . Don't use sleep. Start with something like this.... import time start = time.monotonic() onRelay1 = 3 offRelay1 = 5 stopRelay2 = start + 5*60 In your loop, continuously check to see if 5 minutes has passed using something like nowTime = time.montonic() compared to stopRelay2. For relay 1, check if onRelay1 seconds have passed if it is on, and offRelay1 if it is off. Frank

fmilburn · Answer

Cristina is writing in pseudo C code to show you structure and flow. My example to get you started is Python. time.monotonic() returns the time in seconds in an ever increasing way with no jumps backwards or forwards to account for leap years, daylight savings time, etc. If you do a search for something like "multiple blinking LED at different intervals" you will find C/C++ code that does a general case of what you are looking for. For example: https://forum.arduino.cc/index.php?topic=214664.0 Be sure and keep your relay names consistent - you have changed behavior between the first and second post. E.G. in first post relay 1 cycles on and off forever while in second post it is relay 3. So I am not sure your order but try something like this (Python pseudocode). import time 
start = time.monotonic() 
onRelay1 = 3
offRelay1 = 5
changeRelay1 = start + onRelay1
relay1On = True
stopRelay2 = start + 5*60

relay1(on) # cycle forever
relay2(on) # stay on 5 minutes
relay3(on) # turn it on and leave it on
relay4(on) # turn it on and leave it on

while(True):
 nowTime = time.monotonic()
 if (nowTime >= stopRelay2): # turns it off after 5 minutes
 relay2(off)
 if (nowTime >= changeRelay1): # cycle
 if (relay1On == True):
 relay1(off)
 relay1On = False
 changeRelay1 = nowTime + offRelay1
 else:
 relay1(on)
 relay1On = True
 changeRelay1 = nowTime + onRelay1
 # do other stuff as needed This is untested and it is almost a certainty that I have introduced syntax and other errors somewhere but this is the general idea in a straight forward but not very elegant manner. Try to get something running and if you are still having trouble post the Python script and describe the issue. Edit: saw a couple of errors in the pseudo code and tried to fix some of it

phoenixcomm · Answer

john2674 John you are going to have to set a timer.

you are not clear about the order of precedence. Do you know who is one first?

#include <stdio.h>

#include <time.h>

#define FOREVER if(;;);

# define ON 1

#define OFF 0

int relay1 = OFF;

int relay 2 OFF;

int relay 3 = OFF;

int relay 4 = OFF

time_t seconds;

relay1 = ON;

now1 =time(&seconds)l;

FOREVER{

now // Stores time seconds

if ( now1 =time(&seconds) == now + 3) { relay1=ON;}

else if ( now2 =time(&seconds) == now + 5) {

relay1=OFF;

now1 = 0;

now2 = 0;

relay1 = 0n;

now1 =time(&seconds)l; }

if ( now3 =time(&seconds) > ontime2 + 300) { relay1=OFF;}

relay3 = ON; //set and forget

relay4 = ON; //set and forget

}

shabaz · Answer

Hi John, The periodic counter/tick system recommended by Frank and Cris is a decent way to implement this. For what it's worth, here's some code just tried - it too uses a tick, in this case using some 'asyncio' library. This only works with very recent versions of Python though (Python 3.8, and perhaps Python 3.7). The beginning bit (lines 1-32) just sets up the framework for the ticks. Lines 33 onward handle the relay logic. There is a tick for 1 second, a tick for 10 seconds, and a tick for 60 seconds. I just used the 1 second and 60 second ticks. I'm a Python beginner (I don't like it much but slowly beginning to appreciate its usefulness) so I'm sure there are better ways to do it in Python, but this seemed to work. You'd need to replace or augment the lines containing print messages, to perform the actual relay switching. It outputs a dot every second, a D every 10 seconds, and an M every 60 seconds, and a relay-related message after 3, 8 seconds and 5 minutes. The program starts at line 67, because everything above it are functions. #!/usr/bin/python3.8
import asyncio

PERIOD = 1.0
FOREVER = True
counter=0

# these three variables become 1 when 1, 10 or 60 seconds expire
event1=0
event10=0
event60=0

# counters for relays. rel1ctr counts in seconds, rel2ctr counts in minutes
rel1ctr=0
rel2ctr=0

# tick routine. This updates the event variables
async def tick():
 await asyncio.sleep(PERIOD)
 global counter, event1, event10, event60
 counter=counter+1
 if (counter%10==0):
 event10=1
 if (counter%60==0):
 event60=1
 counter=0
 event1=1

# main function which checks the event variables and handles the relay states
async def mainprog():
 global event1, event10, event60
 global rel1ctr, rel2ctr
 await asyncio.sleep(0.1)

if (event1):
 event1=0
 print(".")
 rel1ctr=rel1ctr+1
 if (rel1ctr==3): # 3 seconds expired?
 print("relay 1 ON")
 if (rel1ctr>=8): # 3+5=8 seconds expired?
 print("relay 1 OFF")
 rel1ctr=0 # set back to zero so we can repeat the sequence for relay 1

if (event10):
 event10=0
 print("D")

if (event60):
 event60=0
 print("M")
 rel2ctr=rel2ctr+1
 if (rel2ctr>=5): # 5 minutes expired?
 print("relay 2 OFF")

print("set relay 1 off")
print("set relay 2 on")
print ("set relay 3 on")
print ("set relay 4 on")
while FOREVER:
 asyncio.run(tick())
 asyncio.run(mainprog())

john2674 · Answer

Ahh sorry for the slow replies. i have been researching all things related to time.monotonic all day. As i said i am extremely new to programming. as such i dont even know the right questions to ask yet. this project is my first attempt to make a code that performs a task of my own choosing. I have followed along on a few simple tutorials in the past, but i have much to learn!

as far as order of precedence.. I would say relay1 and relay 2 on first then relay 3 on 3 seconds then off 5 seconds. i would like relay 4 to come on close to the same time that relay 3 starts cycling and stay on for 5 minutes. then turn off.

The project this will be going into is an experimental outdoor biomass burner. relays 1,2 will turn on the combustion and circulating fans. relay 3 will control an auger to feed the burn chamber. and relay 4 will control the ignitor. in the future i plan to expand with high medium and low feed rates and safety proof of fire and high limit switches. but this first program is just for working out the timings and testing.

john2674 · Answer

setting up another pi so i can play with some codes. i think i will start with yours shabaz. i actually have an old laptop somewhere with ubuntu on it also, maybe ill dig it out and update and use it for practice.

fmilburn · Answer

Since the Pi runs an operating system you can have multiple Python scripts running on a single Pi at once — almost as if there were multiple computers. You could start and stop them from a bash shell or write a little script to do it.

shabaz · Answer

Hi Frank, Thanks! It is new to me too, not sure I like it or if I'm using it correctly, apparently it's the new way to do asynchronous stuff. Much of the documentation for it, perhaps because it is recent, expects a higher level of knowledge than I had.. For simple stuff JavaScript has a 'setTimer' capability to create a type of tick function. It isn't pretty either, but at the end of the day we're trying to replicate a simple old-school timer interrupt type capability, rather than require a decent app framework so it's not so bad I guess..

phoenixcomm · Answer

john2674 John when using C it is much easier to deal with it in a "REAL" IDE like Eclipse. You can deal with an Arduino or any other CPU with it as well. And BTW you get real error messages not the vagary with the Arduino IDE. Here are some of my posts:

Software Dev: Arduino sketch & eclipse on Linux -1.0

Software Dev: Arduino sketch & eclipse on Linux -2.0

Software Dev: Arduino sketch & eclipse on Linux - 3.0

Back to College: Setting up Eclipse Demystified