Sunday, May 23, 2021

Arduino Relay Shield

This blogpost is about the Arduino relay shield from my 2021 May Mailbag.
It contains 4 relays and provides NO (Normally Open) / NC (Normally Closed) interface and leds showing the state of the relay.

It uses I/O pins 7,6,5 and 4 for relays 1,2,3 and 4. 
The screw terminals for the relay contacts. However the terminals from one of the relays is not very safe due to the position near the metal USB connector on the Arduino !

On there is this diagram showing the connections:

One bad design issue of this shield is that some screw terminal pins (J1) make contact with the metal USB connector of the Arduino. So it is needed to add some protect, at least 2 layers of insulation tape is recommended in articles i found on internet!
Working with main voltage (220V) on the screw terminals is dangerous and having a connection to the metal USB connector is asking for problems!
As i want to stay safe i decided to do it different and did put long pin connectors between the Arduino an this shield. This gives more space between the screw connectors and the metal USB as i expect this is really needed for safety. 

To test this shield i modified a simple Arduino blink example sketch.
The interval was extended to 4000 as for the relay a slower on and off switching time would be more appropriate.
Also the pin number was changed to check if the digital pins 4,5,6 and 7 are the correct one's. And indeed this are the channels. When switching the corresponding led blinks and it is possible to hear the clicking of the relay. (also the connection can be measured with ohm meter.)

  Blink without Delay

  Turns on and off a light emitting diode (LED) connected to a digital pin,
  without using the delay() function. This means that other code can run at the
  same time without being interrupted by the LED code.

  The circuit:
  - Use the onboard LED.
  - Note: Most Arduinos have an on-board LED you can control. On the UNO, MEGA
    and ZERO it is attached to digital pin 13, on MKR1000 on pin 6. LED_BUILTIN
    is set to the correct LED pin independent of which board is used.
    If you want to know what pin the on-board LED is connected to on your
    Arduino model, check the Technical Specs of your board at:

  created 2005
  by David A. Mellis
  modified 8 Feb 2010
  by Paul Stoffregen
  modified 11 Nov 2013
  by Scott Fitzgerald
  modified 9 Jan 2017
  by Arturo Guadalupi

  This example code is in the public domain.

// constants won't change. Used here to set a pin number:
const int ledPin =  7;// the number of the LED pin

// Variables will change:
int ledState = LOW;             // ledState used to set the LED

// Generally, you should use "unsigned long" for variables that hold time
// The value will quickly become too large for an int to store
unsigned long previousMillis = 0;        // will store last time LED was updated

// constants won't change:
const long interval = 4000;           // interval at which to blink (milliseconds)

void setup() {
  // set the digital pin as output:
  pinMode(ledPin, OUTPUT);

void loop() {
  // here is where you'd put code that needs to be running all the time.

  // check to see if it's time to blink the LED; that is, if the difference
  // between the current time and last time you blinked the LED is bigger than
  // the interval at which you want to blink the LED.
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    // save the last time you blinked the LED
    previousMillis = currentMillis;

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;

    // set the LED with the ledState of the variable:
    digitalWrite(ledPin, ledState);

Conclusion: The relay shield is interesting however some additional safety mods are really needed as one of the screw terminals is located to close to the metal USB connector on the Arduino!

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