• Relay: SRD-05VDC-SL-C
• NPN transistor (breadboard-friendly): PN2222A
• Flyback diode (either option works; pick one):
  • Small signal: 1N4148-1
  • Rectifier: 1N4007RLG
• LED: LED THT D5.0mm
• LED series resistor (through-hole): CFR-12JB-52-330R (330 Ω)
• Base resistor for transistor (through-hole): MF3W-680Ω±5%-QT73 (680 Ω)
• Base pulldown (optional, keeps relay OFF at reset): Through Hole Resistor 10 kΩ
• Controller: Arduino Uno R4 WiFi
• Breadboard + jumper wires

• According to the SRD datasheet, the 5 V coil draws about 70 mA (coil resistance ≈ 70 Ω, pull-in ≤ 75% VNOM, drop-out ≥ 10% VNOM). An Arduino GPIO cannot source that directly.
• Best practice: drive the coil with an NPN transistor and place a flyback diode across the coil. Knowledge rule: the diode current rating should be at or above the relay’s steady-state coil current (≥ 70 mA). Both 1N4148-1 and 1N4007RLG satisfy this.

• Power and ground:
  • Arduino 5V -> relay coil pin A2 (either coil pin; coil is non-polarized).
  • Arduino GND -> transistor emitter AND the relay’s LED return resistor’s ground.
• Transistor driver (example uses Arduino D8):
  • Arduino D8 -> MF3W-680Ω±5%-QT73 -> PN2222A base.
  • Optional: 10 kΩ from base to GND (keeps it off at boot).
  • PN2222A emitter -> GND.
  • PN2222A collector -> relay coil pin A1.
• Flyback diode across coil:
  • 1N4148-1 cathode to 5V (coil A2), anode to transistor collector (coil A1).
• Relay contacts to switch the LED from 5 V:
  • COM -> 5V.
  • NO -> LED anode -> CFR-12JB-52-330R -> GND.
  • When the relay energizes, NO closes to COM and the LED turns on. If you prefer LED ON by default, move from NO to NC.

Cpp
const int RELAY_CTRL = 8;
void setup() {
  pinMode(RELAY_CTRL, OUTPUT);
  digitalWrite(RELAY_CTRL, LOW); // keep OFF at power-up
}
void loop() {
  digitalWrite(RELAY_CTRL, HIGH); // energize relay -> LED ON
  delay(1000);
  digitalWrite(RELAY_CTRL, LOW);  // de-energize -> LED OFF
  delay(1000);
}

• The 680 Ω base resistor targets solid saturation for the ~70 mA coil current; 1 kΩ will also usually work for this relay, but 680 Ω gives more margin.
• Tie Arduino GND and the relay driver’s GND together. Do not power the coil from a GPIO pin—use 5V + transistor as shown.