Relay module with delay, HCW-M421, adjustable time, display, 6–30V

The HCW-M421 delay relay module is a compact board used to turn an external load on or off after an adjustable time, either on a single command or in repeated on-off cycles. It is useful in Arduino automation projects, where the microcontroller only needs to send a short trigger signal, while the rest of the timing is handled independently by the module, without using up the development board's resources.

How it works:

The module receives on the IN pin a High Level type signal, between 3 and 24 V, passed through an optocoupler that electrically isolates the input from the rest of the circuit. The microcontroller on the board detects the signal edge, starts an internal counter and controls the relay according to the operating mode chosen from P1.1, P1.2, P1.3, P2, P3.1, P3.2 and P4. Each mode uses the parameters OP (duration the relay stays active), CL (duration it stays inactive between cycles) and LOP (number of repeated cycles), set directly on the module using the STOP, SET, UP and DOWN buttons. In the test from this tutorial we use mode P1.1, where a single trigger pulse on IN activates the relay for exactly the set OP duration, after which it deactivates automatically, regardless of how long the received signal lasts.

What you need:

• 1 x HCW-M421 delay relay module, adjustable time, display, 6-30V
• 1 x Uno R3 development board with ESP8266 Wifi, Arduino compatible
• 1 x 9V power supply for the module
• 1 x 40 Dupont wires 10cm, Male-Female
• 1 x Breadboard 830 points MB-102

Wiring:

The HCW-M421 module is not powered from Arduino, it needs a separate 6 - 30 VDC power source on the screw terminal connector or 5 VDC through the MicroUSB port. The Arduino Uno is used only to send the trigger signal on the IN pin, connecting the Arduino ground to the module's G pin for a common reference. If you later connect a load powered from 220 VAC to the relay output, only work with that load unplugged, since the NO, COM and NC relay contacts can switch dangerous voltages to the touch.

The code:

The program below periodically sends a short pulse on pin D7, connected to the module's IN input, and displays on the serial monitor each pulse sent.

// HCW-M421 Timer Relay Trigger Demo
// Sends periodic HIGH pulses to the module IN pin to start one timing cycle
// Use together with the module set to mode P1.1 for a simple delay ON test
const int TRIGGER_PIN = 7;                     // Digital pin connected to module IN pin
const unsigned long PULSE_DURATION_MS = 200;   // Length of the trigger pulse in milliseconds
const unsigned long CYCLE_INTERVAL_MS = 10000; // Time between two trigger pulses in milliseconds
unsigned long lastTriggerTime = 0;
unsigned long pulseCount = 0;
void setup() {
  Serial.begin(9600);
  pinMode(TRIGGER_PIN, OUTPUT);
  digitalWrite(TRIGGER_PIN, LOW);
  Serial.println("HCW-M421 trigger demo started");
}
void loop() {
  unsigned long now = millis();
  if (now - lastTriggerTime >= CYCLE_INTERVAL_MS) {
    lastTriggerTime = now;
    pulseCount++;
    // Send a short HIGH pulse on the trigger pin
    digitalWrite(TRIGGER_PIN, HIGH);
    delay(PULSE_DURATION_MS);
    digitalWrite(TRIGGER_PIN, LOW);
    Serial.print("Trigger pulse #");
    Serial.print(pulseCount);
    Serial.println(" sent to HCW-M421 IN pin");
  }
}

How the code works:

At the beginning of the program, TRIGGER_PIN is defined as the digital pin connected to IN, PULSE_DURATION_MS as the duration of the sent pulse and CYCLE_INTERVAL_MS as the interval between two successive triggers. In the setup function, serial communication is initialized at 9600 baud, TRIGGER_PIN is configured as an output and a startup message is displayed on the serial monitor.

In the loop function, the current time is read with millis and compared to lastTriggerTime, using CYCLE_INTERVAL_MS to decide when a new pulse must be sent, without blocking the rest of the program with a long delay. When the interval has passed, pulseCount is incremented, the TRIGGER_PIN is set HIGH for the PULSE_DURATION_MS duration and then returns to LOW, and the serial monitor displays the number of the pulse sent to the HCW-M421 module.

Result:

Open the serial monitor at 9600 baud, after uploading the program. Every 10 seconds you should hear the click of the relay on the module, and a message with the number of the sent pulse should appear on the serial monitor.

HCW-M421 trigger demo started
Trigger pulse #1 sent to HCW-M421 IN pin
Trigger pulse #2 sent to HCW-M421 IN pin
Trigger pulse #3 sent to HCW-M421 IN pin
Trigger pulse #4 sent to HCW-M421 IN pin
Trigger pulse #5 sent to HCW-M421 IN pin
Trigger pulse #6 sent to HCW-M421 IN pin

Common issues:

No relay click is heard, even though pulse sent messages appear on the serial monitor. Check that the module is properly powered at 6 - 30 VDC or 5 VDC through MicroUSB, since without this separate power supply the module's processor does not function, regardless of the signal received on IN.

The relay activates only once and then no longer responds to the following pulses. Check the operating mode selected on the display, modes P1.2 and P1.3 handle a new signal appearing during OP differently from P1.1, and if mode P4 was accidentally left set, the relay directly follows the signal state, not a short pulse.

The module's display remains off even though the module appears to be powered. Check if C-P mode has been activated, in which the display turns off automatically after 5 minutes of inactivity, a normal situation which can be changed by holding STOP pressed for 2 seconds to return to O-d mode.

Pulses appear to be sent by Arduino, but the module does not react at all. Check the common ground connection between Arduino GND and the module's G pin, a High Level signal without a common ground with the module's power source is not correctly recognized by the optoisolated input.

The time measured by the relay does not match the expected one. Check the position of the decimal point displayed during setting, since the same number can represent seconds or minutes depending on the position of the point, as described in the OP and CL parameter setting mode.

Limitations and tips:

The module times intervals between 0.1 seconds and 999 minutes, with a resolution of 0.1 seconds on short intervals, sufficient for most home or laboratory automation applications, but not for millisecond-range timing. The relay contacts can switch DC or AC loads, but the current and voltage must stay within the limits indicated in the module's specifications, and the 220 VAC side must only be wired with the load unplugged. For more complex projects, the same triggering principle can be extended by using multiple HCW-M421 modules controlled by different digital pins of the same Arduino board, each with its own timing program.