MOSFET E-Switch module, 3–20V control, 5–36V output, with optocoupler

This tutorial uses the Arduino Uno R3 board to control a 12 V LED strip via PWM with the E-Switch MOSFET module with optoisolator. The module is useful in projects where a development board with 5 V outputs needs to switch on, switch off or dim a DC load that is larger than a pin can drive directly, for example LED strips, motors, pumps or solenoid valves.

How it works:

The board carries an F5305S P-channel MOSFET, mounted on the positive line of the load. The control signal on IN+ and IN- lights the LED inside the PC817 optocoupler, and its output side brings the MOSFET gate into conduction. As long as there is a control signal, the load is powered, and when the signal disappears, the MOSFET turns off. The optocoupler electrically separates the two circuits, which is why the Arduino ground must not be connected to the negative of the 12 V supply.

The control current is approximately 5 mA, a value that an Arduino pin can supply without problems. The 5 V from a pin of the Uno board falls within the control range of the module, 3 - 20 VDC.

With PWM, the average voltage applied to the load is the supply voltage multiplied by the duty cycle. The analogWrite function receives a value between 0 and 255, and the duty cycle is value / 255. For example, at the value 128 the duty cycle is 128 / 255, that is approximately 50 %, and a 12 V LED strip receives on average approximately 6 V. Pin D9 of the Uno board generates PWM of approximately 490 Hz, so the period is approximately 2 ms, a frequency suitable for this module.

Wiring:

The setup uses a 12 VDC mains power supply with at least 2 A, a 5.5 x 2.1 mm jack adapter with screw terminals and a single-colour 12 V LED strip. Connect everything with the power supply disconnected and power it on last. The two terminal blocks are marked + and -. On the + block, the positive of the supply goes to the screw at the tail of the arrow on the board, and the positive of the load leaves from the screw at the tip of the arrow. The - block is the common negative of the supply and the load. No level converter, pull-up resistors or separate power for the control side are required.

Board and libraries:

In the Arduino IDE, select at Tools > Board the Arduino Uno board from the Arduino AVR Boards package, which comes installed with the Arduino IDE. The program uses only the basic Arduino functions, so no library needs to be installed.

The code:

The program sets five brightness levels in turn, then performs a smooth increase and decrease of the brightness, printing to the serial monitor what is happening.

/*
  E-Switch MOSFET module with optoisolator (F5305S + PC817) - PWM brightness demo
  Board: Arduino Uno (Tools > Board > Arduino AVR Boards > Arduino Uno)
  Libraries: none, only the core Arduino functions are used.
  Wiring:
    Module IN+  -> Arduino Uno D9 (PWM, about 490 Hz)
    Module IN-  -> Arduino Uno GND
    Module + (screw 1) -> 12 VDC supply +
    Module - (screw 1) -> 12 VDC supply -
    Module + (screw 2) -> 12 V LED strip +12V
    Module - (screw 2) -> 12 V LED strip -
*/
const int MOSFET_PIN = 9;                          // PWM pin connected to IN+
const int LEVELS[] = {0, 64, 128, 192, 255};       // test brightness levels
const int NUM_LEVELS = sizeof(LEVELS) / sizeof(LEVELS[0]);
// Send a PWM value (0 - 255) to the module and print it
void setBrightness(int value) {
  analogWrite(MOSFET_PIN, value);                  // set the duty cycle
  Serial.print("Brightness level: ");
  Serial.print(value);
  Serial.print(" / 255 (");
  Serial.print(value * 100L / 255);                // convert to percent
  Serial.println(" %)");
}
void setup() {
  Serial.begin(9600);                              // start the serial monitor
  pinMode(MOSFET_PIN, OUTPUT);                     // control pin as output
  analogWrite(MOSFET_PIN, 0);                      // start with the load off
  Serial.println("E-Switch MOSFET module test (Arduino Uno, pin D9)");
}
void loop() {
  // Step through the fixed levels, two seconds each
  for (int i = 0; i < NUM_LEVELS; i++) {
    setBrightness(LEVELS[i]);
    delay(2000);
  }
  // Smooth fade up from 0 to 255
  Serial.println("Fade up...");
  for (int value = 0; value <= 255; value++) {
    analogWrite(MOSFET_PIN, value);
    delay(10);
  }
  // Smooth fade down from 255 to 0
  Serial.println("Fade down...");
  for (int value = 255; value >= 0; value--) {
    analogWrite(MOSFET_PIN, value);
    delay(10);
  }
  analogWrite(MOSFET_PIN, 0);                      // load off before repeating
  delay(1000);
}

How the code works:

The constant MOSFET_PIN has the value 9 and indicates the PWM pin to which IN+ is connected. The LEVELS array contains the five test values, 0, 64, 128, 192 and 255, and NUM_LEVELS is their count.

In the setup function, the serial communication is started at 9600 baud, the MOSFET_PIN pin is configured as an output and the value 0 is applied, so that the load starts switched off.

The setBrightness function receives a value between 0 and 255, sends it with analogWrite to the module and prints the value and the corresponding percentage.

In the loop function, a for loop goes through the LEVELS array and keeps each level for two seconds. Then two for loops increase the value from 0 to 255 and decrease it back to 0, with a 10 ms pause between steps, which gives a smooth fade-in and fade-out effect. After a pause of one second the sequence starts again.

Result:

Open the serial monitor at 9600 baud. The LED strip cycles through the five brightness levels, then fades smoothly on and off, and a text similar to the one below appears on the monitor.

E-Switch MOSFET module test (Arduino Uno, pin D9)
Brightness level: 0 / 255 (0 %)
Brightness level: 64 / 255 (25 %)
Brightness level: 128 / 255 (50 %)
Brightness level: 192 / 255 (75 %)
Brightness level: 255 / 255 (100 %)
Fade up...
Fade down...
Brightness level: 0 / 255 (0 %)
Brightness level: 64 / 255 (25 %)
Brightness level: 128 / 255 (50 %)

Common problems:

The load stays on all the time. The most frequent cause is connecting the supply and the load in the opposite direction to the arrow on the board, in which case the current bypasses the MOSFET. Reconnect the supply to the screw at the tail of the arrow and the load to the screw at its tip.

The load does not switch on at all. Check that IN+ is connected to D9 and IN- to GND, that the 12 V supply is powered and that the negative of the supply and of the load are on the - block. Also check that the control voltage is at least 3 V.

The brightness does not vary, the strip only lights fully. Check that the signal is on a PWM pin, marked with ~ on the Uno board, and that the code uses analogWrite, not digitalWrite.

The module heats up strongly. The load current exceeds 5 A without cooling. Reduce the load or fit a heatsink and a fan.

With motors or pumps, the Arduino board resets or behaves unstably. A motor produces voltage spikes when it switches off. Fit a protection diode in parallel with the load, with the cathode towards the positive, and use wires of an adequate gauge.

Limitations and tips:

The module switches only DC loads of 5 - 36 VDC and is not suitable for alternating voltage. The recommended control voltage is 3 - 20 VDC, and the control current is approximately 5 mA. Up to 5 A no cooling is needed, and with a heatsink and a fan it can reach 22 A, while checking the temperature of the MOSFET.

The optocoupler limits the usable PWM frequency, so the approximately 490 Hz from D9 is suitable, but very fast PWM is not recommended. With the same setup you can build a dimmer for the LED strip with a potentiometer, a speed controller for a DC motor or a switch for pumps and solenoid valves controlled by sensors.