Practical case: Light alarm with relay and door sensor

Materials

• 1 × 12 V DC power supply (500 mA or more).
• 1 × 12 V DC relay, 1 changeover pole (SPDT: COM, NO, NC contacts).
• 1 × Magnetic door switch (reed type with magnet, normally closed or normally open contact; we will use normally closed/NC type).
• 1 × Protection diode for the relay coil (for example 1N4007).
• 1 × 12 V lamp or 12 V LED strip (max. current according to relay contact capacity; e.g. ≤ 1 A).
• 1 × Indicator LED (optional, for example standard 5 mm red LED).
• 1 × Resistor for the indicator LED (1 kΩ, 1/4 W).
• Wires and breadboard (if the relay is for PCB) or terminal block.
• 1 × Digital multimeter (for voltage and current).

Wiring guide

Treat this section as the official connection specification. The ASCII schematic in the following section implements EXACTLY these connections.

• Main power:
• Connect the positive terminal of the 12 V supply to the node labeled +12V.

• Connect the negative terminal of the 12 V supply to the node labeled GND.

• Relay coil:

• Connect one terminal of the relay coil to the +12V node.

• Connect the other terminal of the relay coil to the VB node.

• Magnetic door switch (sensor):

• Connect one of the terminals of the magnetic switch to the VB node.
• Connect the other terminal of the magnetic switch to the GND node.

• A normally closed (NC) switch is assumed: with the door closed, the contact is closed and connects VB node to GND.

• Protection diode (flyback diode):

• Connect the diode cathode (marked with a band, in the schematic it will appear as [D1] 1N4007) to the +12V node.
• Connect the diode anode to the VB node.

• The diode is in parallel with the relay coil, but reverse-biased with respect to the power supply (it MUST NOT conduct in normal operation).

• Relay power contacts (for the 12 V lamp/LED):

• Use the relay’s common (COM) and normally open (NO) contacts.
• Connect the relay COM contact to the +12V node.
• Connect the relay NO contact to the VL node.
• Connect the other terminal of the 12 V lamp/LED to the GND node.
• Connect the VL node to one of the terminals of the 12 V lamp/LED.

• The relay NC contact is not used in this practical case; leave it unconnected.

• Relay activation indicator LED (optional):

• Connect the indicator LED anode (positive side) to the +12V node through the resistor:
  • Connect the +12V node to one end of the resistor [R1] 1 kΩ.
  • Connect the other end of [R1] to the VI node.
• Connect the indicator LED cathode to the VB node.
• The VI node only serves as a junction between resistor [R1] and the indicator LED anode.

Schematic

            +12V
            |
            +--------------------+--------------------+
            |                    |                    |
          [R1] 1kΩ              [K]                  COM (relé)
            |                 Bobina                  |
      VI node o                 |                     |
            |                 VB node o---------------+
          [LED] Indicador        |                  NO (relé)
            |                 [D1] 1N4007             |
            |                  |                      |
           GND                 |                      o VL node
                               |                      |
                           Interruptor                [LAMP] 12V
                          magnético NC                |
                               |                      |
                              GND                    GND

Quick legend:
– [K] Bobina is the relay coil (between +12V and VB node).
– COM and NO are the relay power contacts.
– [LAMP] 12V is the 12 V lamp or load.
– [LED] Indicador is the small panel LED, with its resistor [R1] 1kΩ.
– [D1] 1N4007 is the coil protection diode.

Measurements and tests

• Basic voltage verification:

  • Measure the voltage between +12V and GND with the multimeter in DC voltmeter mode. It should read ~12 V.
  • With the door CLOSED (NC switch closed), measure between VB node and GND. It should read ~0 V (it is at ground through the switch).
  • With the door OPEN (switch open), measure between VB node and GND. It should read approximately 12 V (the coil is powered).

• Relay activation check:

  • With the multimeter in DC voltmeter mode, measure between COM and NO of the relay:
    • Door closed: COM-NO should show a voltage close to that of the load or be “open” (if you measure continuity, it should indicate open circuit).
    • Door open: COM-NO should show a very low resistance (almost a short circuit); in voltmeter mode, the voltage between COM and NO will be ≈ 0 V when the relay is activated.
  • Observe the 12 V lamp/LED: it should clearly turn on when the door opens and turn off when the door closes.

• Relay coil current measurement (I_coil):

  • I_coil (coil current) is the current that flows through the relay coil when it is activated.
  • To measure it, set the multimeter to DC ammeter mode and place it in series with the coil:
    • Disconnect the wire that goes from +12V to the coil.
    • Connect the positive terminal of the ammeter to +12V.
    • Connect the negative terminal of the ammeter to the coil terminal that previously went to +12V.
  • Open the door to activate the relay and read the current: compare it with the typical value from the datasheet (e.g., 40 mA). It should be in a similar range.

• Verification of voltage drop across the load (V_LAMP):

  • V_LAMP is the voltage that appears directly across the terminals of the 12 V lamp/LED.
  • With the multimeter in DC voltmeter mode, place the test leads on each end of the lamp.
  • With the door open (relay activated), it should read a value close to 12 V.
  • With the door closed (relay deactivated), V_LAMP should be ≈ 0 V.

• Dynamic functional tests:

  • Open and close the door repeatedly, checking:
    • That the relay operates without flickering or strange noises.
    • That the indicator LED (if installed) lights only when the relay is activated (door open).
    • That neither the relay nor the lamp heats excessively for prolonged use times (touch carefully after several minutes).

Common errors and how to avoid them

• Forgetting the coil protection diode:
• When the coil is disconnected, a voltage spike is generated that can damage other components.

• Make sure that [D1] 1N4007 is in parallel with the coil, with cathode to +12V and anode to VB node.

• Swapping the relay contacts (accidentally using NC instead of NO):

• Check the datasheet or the drawing on the relay housing to correctly identify COM, NO, and NC.

• If you connect the lamp to NC instead of NO, the light will be on at rest and off when the door is opened (inverted behavior).

• Using the wrong type of magnetic switch:

• This practical case assumes an NC switch (closed with the door closed).

• If your sensor is normally open (NO), the logic is inverted: review the measurements and adapt if necessary (you might want to use the relay’s NC contact to keep the desired behavior).

• Overloading the relay contacts:

• Check the maximum allowable current of the relay contacts (e.g. 3 A at 12 V DC).

• Make sure the lamp or load does not exceed that value; if it is higher, you must use a more robust relay or an intermediate stage (contactors, MOSFET, etc.).

• Incorrect connection of the indicator LED:

• Do not forget the series resistor [R1] 1 kΩ to limit LED current.
• Check polarity: anode toward the resistor (and then to +12V), cathode toward VB node.

Safety and good practices

• Always work with the supply disconnected while making or modifying connections.
• Even though you are only using 12 V, avoid short circuits: they can damage the supply or dangerously heat the wires.
• If you adapt this circuit to control mains voltages (230 V AC) through the relay:
• Maintain complete physical separation between the low-voltage (12 V) side and the mains side.
• Do not handle the mains side unless you have specific and certified training.
• Use junction boxes, terminal blocks, and insulation suitable for mains electricity.

Possible improvements and extensions

• Add a 12 V siren in parallel with the lamp to have both audible and visual alarms.
• Add a timer (for example, using an RC circuit with a transistor or a small microcontroller) so that the light turns off by itself X minutes after opening the door.
• Implement a manual deactivation switch that allows you to temporarily disable the alarm without dismantling the system.
• Integrate an additional sensor (for example, an ambient light sensor) so that the lamp only activates at night.

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).

Follow me:

Who we are