Adding the 4 channel relay board ks0212 to the MQTT universe

We just hacked a trotec dehumidifier for Herwigs Observatory. The idea was to additionally activate the dehumidifier when the difference between outside and inside humidity is above 10%. Normally there is a fan taking care of it but sometimes the differents gets to high. As there is already a raspberry pi running in the observatory for the weatherstation and the flightradar24 installation we just added the 4 channel relay board ks0212 from keyestudio. Not touching the 220V part we directly used the relay to “press” the TTL switch on the board for 0.5 seconds to turn on and off the dehumidifier. Here are the code snipped we used for this. The control is completely handled via MQTT.

Installing necessary programs and libraries

sudo apt install python python-pip python-dev
sudo pip install wiringpi paho-mqtt

For the sake of simplicity we used python and the GPIO library wiringpi. Therefore we first install the python development parts and them the python libraries for wiringpi and MQTT. As this is a dedicated hardware installation we don’t use virtualenv and directly install the library as root system wide.

The python program

import time
import wiringpi
import paho.mqtt.client as mqtt

def setup():
   wiringpi.wiringPiSetup()
   wiringpi.pinMode(3,  1)
   wiringpi.pinMode(7,  1)
   wiringpi.pinMode(22, 1)
   wiringpi.pinMode(25, 1)

def short(pin):
    switch_on(pin)
    time.sleep(0.5)
    switch_off(pin)

def switch_on(pin):
    wiringpi.digitalWrite(pin, 1)

def switch_off(pin):
    wiringpi.digitalWrite(pin, 0)

def on_connect(self, client, userdata, rc):
    mqclient.subscribe("sternwarte/relay/#")

def on_message(client, userdata, msg):
    m = msg.topic.split("/")
    pin = 0
    if m[-1] == "j3": 
        pin = 3
    if m[-1] == "j2": 
        pin = 7
    if m[-1] == "j4": 
        pin = 22
    if m[-1] == "j5": 
        pin = 25
    if pin != 0:
        if msg.payload == "on":
            switch_on(pin)
        if msg.payload == "off":
            switch_off(pin)
        if msg.payload == "press":
            short(pin)

if __name__ == "__main__":
    setup()
    mqclient = mqtt.Client(clean_session=True)
    mqclient.connect("192.168.2.5", 1883, 60)
    mqclient.on_connect = on_connect
    mqclient.on_message = on_message
    mqclient.loop_forever()

Again, a very simple python script, basically attaching to a (you need to change the code, there is no config) mqtt server and subscribes itself to a certain topic. Then it waits for messages and cuts off the last part of the topic to identify the relay. The naming convention is based on the relay name printed on the ks0212 pcb. As payload you can send “on“, “off” and “press“. “press” switches the relay on for half a second in order to simulate a button press as we need it for our dehumidifier.

Adding a systemd service

In order to keep the wantabe daemon up and running and also start it automatically at system start we add this service configuration file in “/lib/systemd/system/relayboard.service“:

#cat /lib/systemd/system/relayboard.service
[Unit]
Description=ks0212 Relay Board
After=multi-user.target

[Service]
Type=simple
ExecStart=/usr/bin/python /home/pi/ks0212.py
Restart=on-abort

[Install]
WantedBy=multi-user.target

Activating the service

The following lines activate the service:

sudo chmod 644 /lib/systemd/system/relayboard.service
sudo systemctl daemon-reload
sudo systemctl enable relayboard.service
sudo systemctl start relayboard.service

Checking the status can be done with:

sudo systemctl status relayboard.service

ks0212 Pinout

If you want to do some hacking with the ks0212 relay board on your own here is the pin mapping table. I used the very cool side https://pinout.xyz/pinout/wiringpi for getting the numbers:

Relay	WiringPi	BCM	GPIO	Link
J2	7	4	7	https://pinout.xyz/pinout/pin7_gpio4
J3	3	22	15	https://pinout.xyz/pinout/pin15_gpio22
J4	22	6	31	https://pinout.xyz/pinout/pin31_gpio6
J5	25	26	37	https://pinout.xyz/pinout/pin37_gpio26