Relay TRS-12VDC-SB-L15 Controlled by Arduino

As it is known, the microcontroller for obvious reasons can control the electromagnetic relay only through a special driver and most often this driver is the simplest transistor switch. In this article I will calculate the transistor switch for the relay TRS-12VDC-SB-L15 (12V).


For the correct choice of the transistor, it is desirable to consider the so-called strength factor equal to 1.5 (Ic = 12mA*1.5 = 18mA) and for such a load current is quite suitable transistor C1921 with a maximum collector current of 50mA which I have here are in stock.

► 1. The load of the transistor is a relay (TRS-12VDC-SB-L15) with a coil resistance of 970 ohms (Rc). Calculate the collector current of the transistor (Ic):

Ic = Vcc/Rc = 12/970 = 0.012A = 12mA (Vce(sat) => 0)

► 2. The DC current gain (β) of the transistor (C1921) is 30...300, so I take the minimum value β = 30 and calculate the base current (Ib):

Ib = S*(Ic/β) = 4*(0.012/30) = 0.0016A = 1.6mA (S = 4)

► 3. Calculating base resistor value (Rb):

Ib = Vr/Rb => Rb = Vr/Ib = (Vin-Vbe)/Ib = (5-0.7)/0.0016 = 2688Ω = 2.7kΩ

The saturation coefficient of the transistor (S) for such transistor switches is recommended to be taken within 3...5, therefore, in step 2 of my calculation I took S = 4. This ensures a sufficient level of saturation of the transistor to bring one to the open state.

// Relay testing (blink)

  int relayPin = 4;  // Pin D4

void setup () {

  pinMode (relayPin, OUTPUT);


void loop () {

  digitalWrite (relayPin, HIGH);
  delay (1000);
  digitalWrite (relayPin, LOW);
  delay (1000);

}  // Arduino IDE 1.8.4

To test this relay module, I used a standard sketch for blinking LED through the 13th pin with a time delay of 1 second, after changing the variable name for this pin and the number of this pin. Everything is very simple here and it does not need a special detailed explanation.


For visual control of the current state of the relay module (ON/OFF), I soldered parallel to the relay coil (12V) a LED with a resistor of 1 kilohm and this is not shown on the diagram (left).
Since 2005, Payoneer's cross-border payments platform has empowered millions of entrepreneurs across the world to expand beyond their borders and grow successful and sustainable businesses. Thousands of leading companies and marketplaces including Airbnb, Amazon, Google, Upwork and others rely on Payoneer's services to send funds to their merchants worldwide. Register NOW and get your Payoneer prepaid MasterCard!