Practical case: Lamp control with relay and push button

Materials

• 1 × 5 V DC relay, contacts for 230 V AC (for example, SPST or SPDT, ≥ 3 A).
• 1 × Normally open (NO) low-voltage push button.
• 1 × 1N4007 diode (or similar) for coil protection.
• 1 × 5 V DC power supply (adapter or lab bench, ≥ 200 mA).
• 1 × 230 V AC lamp (incandescent or LED bulb with base and lamp holder).
• 1 × 230 V AC mains cable with plug and suitable insulation.
• Several low-voltage (5 V) connecting wires.
• Several high-voltage (230 V AC) wires, with suitable insulation.
• 1 × Breadboard (only for the 5 V part if desired; the 230 V part is better with terminal blocks).
• 2 × Terminal blocks or connector strip to clearly separate the high-voltage mains side.

Wiring guide

WARNING: The 230 V AC part is dangerous. Make the connections with the power off, double-check them, and if you have no experience with mains electricity, only build the 5 V part and simulate the lamp with an LED and a resistor.

• Low-voltage power connections (5 V):
• Connect the positive terminal of the 5 V supply to the +5V node.
• Connect the negative terminal of the 5 V supply to the GND node.
• Relay coil and push button connection:
• Connect one terminal of the relay coil to the +5V node.
• Connect the other terminal of the relay coil to the VB node.
• Connect one terminal of the push button to the VB node.
• Connect the other terminal of the push button to the GND node.
• Protection (flyback) diode in parallel with the coil:
• Connect the cathode of diode [D1] (marked with a line on the body) to the +5V node.
• Connect the anode of diode [D1] to the VB node.
• Make sure the diode is in parallel with the coil and reversed with respect to the supply polarity (it must not conduct under normal conditions).
• Lamp and 230 V AC mains connections (power side):
• Identify on the relay the common contact [COM] and the normally open contact [NO].
• Connect one conductor of the 230 V AC mains (for example, phase L) to the relay’s [COM] contact.
• Connect the relay’s [NO] contact to one of the lamp holder terminals.
• Connect the other lamp holder terminal to the other conductor of the 230 V AC mains (for example, neutral N).
• Make sure the relay only interrupts one of the conductors (normally the phase) and that the connections are protected and insulated.

Schematic

                               Parte de control (5 V DC, bobina y pulsador)

             +5V
             |
             +-------------------------+
             |                         |
          [Bobina] Relé              [D1] 1N4007
             |                         |
             o VB node                 |
             |                         |
           [Pulsador]                  |
             |                         |
            GND------------------------+

   Parte de potencia (230 V AC, contactos y lámpara)

       Red 230V AC
        L (fase)            Relé (contactos)           Lámpara
          |                      |                      |
          +------------------[COM]                      |
                                 |                      |
                                 o VC node              |
                                 |                      |
                             [NO] contacto              |
                                 |                      |
                                 +------------------[Lámpara]
                                                        |
       Red 230V AC                                      |
        N (neutro)--------------------------------------+

Measurements and tests

• Before energizing (power off):
  • Check push button continuity with the multimeter in ohmmeter mode:
  • Place the multimeter probes on the two push button terminals.
  • Make sure it shows an open circuit (very high resistance) when not pressed and low resistance when pressed.
  • Check relay coil continuity:
  • Measure between the two coil terminals; you should see a resistance of a few hundred ohms (depends on the model).
  • Make sure there is NO direct continuity between the 5 V side and the relay’s power contact terminals.
• Measurements on the 5 V side (with only 5 V connected, no 230 V AC):
  • Measure the voltage across the coil (V_coil):
  • V_coil = voltage between the +5V node and the VB node.
  • Without pressing the button, V_coil must be ≈ 0 V.
  • While pressing the button, V_coil must be ≈ 5 V (or the supply’s nominal voltage).
  • Observe and listen to the relay:
  • While pressing the button, you should hear a clear “click”; when releasing, another “click” when it returns.
• Measurements on the 230 V AC side (WITH GREAT CAUTION):
  • Measure the voltage on the lamp (V_lamp):
  • V_lamp = voltage between the two lamp holder terminals.
  • With the relay at rest (push button not actuated), V_lamp must be ≈ 0 V.
  • While pressing the push button, V_lamp must be ≈ 230 V AC (or the local mains voltage).
  • Verify operation visually:
  • The lamp must light only while the push button is pressed and turn off when released.
• Optional: coil current measurement (I_coil):
  • I_coil = current flowing through the relay coil when the button is pressed.
  • To measure I_coil, set the multimeter to DC ammeter mode in series with the coil:
  • Disconnect the wire going from +5V to the coil.
  • Connect one multimeter probe to the +5V node and the other probe to the coil terminal that was connected to +5V.
  • Press the button and read the value; compare it with the relay’s typical value (see datasheet).

Common mistakes and how to avoid them

• Connecting diode [D1] backwards:
• If the diode is reversed (anode to +5 V and cathode to VB), it will short the 5 V supply when you press the button.
• Always check that the cathode (marked with a band) goes to +5V and the anode to VB.
• Confusing the relay coil with the contacts:
• The coil is usually indicated on the package with a coil symbol or specific pin numbers.
• Check the datasheet or drawing of the relay; do not connect 230 V to the coil.
• Using a relay not suitable for 230 V AC:
• Check on the relay case the maximum contact voltage and current (for example, “250 VAC 5 A”).
• If it is not clearly specified, do NOT use it for mains.
• Not properly isolating the 230 V AC side:
• Never mix mains cables with 5 V wires in the same area without physical separation.
• Use terminal blocks, heat-shrink tubing, and an enclosure or box to avoid accidental contact.

Safety

• Always work with the mains disconnected while making or modifying 230 V AC connections.
• Use a main switch or power strip with switch to be able to quickly cut off the supply.
• Never touch mains cables or lamp terminals when the circuit is energized.
• If you are not sure about the 230 V AC part, limit the experiment to the 5 V side and replace the lamp with an LED with resistor and a second relay or simply a light indicator.

Possible improvements

• Replace the push button with a two-position switch so the lamp can stay on without holding the button down.
• Add a transistor or microcontroller circuit to activate the relay from a logic signal (for example, Arduino).
• Replace the electromechanical relay with a solid state relay (SSR) to avoid mechanical noise and increase lifetime.
• Add a fuse in series with the lamp or the 230 V AC supply line for extra protection.
• Implement a latching (self-maintaining) system using the relay itself so that one short press turns on and another short press turns off (switch-like function with push button).

More Practical Cases at Prometeo.blog

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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