Practical case: Use of coil in electromechanical bell

Materials

• 1 × Regulated 9–12 V DC power supply (or good‑quality 9 V battery; preferably a pack of 8×AA 1.5 V).
• 1 × Doorbell coil or small electromagnet (typical DC resistance 20–60 Ω).
• 1 × General‑purpose NPN transistor (e.g., 2N2222 or BC337).
• 1 × Base resistor [R1] of 4.7 kΩ (for the transistor).
• 1 × Protection diode [D1] 1N4007 (or similar rectifier).
• 1 × Normally open push button.
• 1 × Metal plate or small bell (the doorbell “gong”).
• 1 × Metal piece or screw as a movable core / hammer (or flexible metal tongue).
• 1 × Protoboard (breadboard) or mounting base.
• Several × Breadboard wires (jumpers).
• 1 × Digital multimeter (to measure voltages and currents).
• 1 × Simple mechanical support (wood/cardboard) to fix the coil and bell (optional but recommended).

Wiring Guide

Treat this section as the official wiring specification. The schematic will follow these connections exactly.

• Power connections:
• Connect the positive terminal of the 9–12 V supply to the node labeled +V.

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

• Coil (inductor) connection:

• Connect one end of the doorbell coil to the +V node.

• Connect the other end of the coil to node VC (transistor collector).

• Protection diode across the coil:

• Connect the cathode of [D1] 1N4007 (end marked with a band) to node +V.
• Connect the anode of [D1] to node VC.

• That is, the diode is in parallel with the coil but oriented opposite to the normal polarity of the supply.

• NPN transistor (switching stage):

• Connect the collector of the NPN transistor to node VC.
• Connect the emitter of the NPN transistor to node GND.

• Connect the base of the NPN transistor to node VB.

• Base resistor and push button:

• Connect one end of resistor [R1] 4.7 kΩ to node +V.
• Connect the other end of [R1] to node VBTN.
• Connect one terminal of the push button to node VBTN.
• Connect the other terminal of the push button to node VB.

• Optionally connect a 100 kΩ pull‑down resistor [R2] between node VB and GND (if you have one; it helps keep the transistor fully off when not pressed).

  • One end of [R2] to node VB.
  • The other end of [R2] to node GND.

• Measurement node:

• Node VC will be the point where you measure collector voltage (V_CE) with respect to GND.
• Node VB will be the point where you measure base voltage (V_B) with respect to GND.
• You will measure coil current (I_COIL) by inserting the multimeter in series between +V and the coil during tests.

Schematic

                                                +V
                 |
                 +------------------------------+
                 |                              |
               [R1] 4.7kΩ                       |
                 |                              |
             VBTN node o                        |
                 |                              |
              Pulsador                          |
                 |                              |
             VB node o------------------------+ |
                 |                            | |
               [R2] 100kΩ (opc)               | |
                 |                            | |
                GND                           | |
                                              | |
                                              | |
                 +V                           | |
                 |                            | |
                 o----------------[COIL]------+ |
                 |                Bobina        |
                 |                    |         |
                 |                   VC node o--+
                 |                     |
                  +---------------------+
                 |                     |
                [D1] 1N4007          |
              cátodo arriba           |
                 |                     |
                 +---------------------+
                 |
                GND


Etapa transistor NPN:

           VC node o----C   Q1 NPN   E----o GND
                         \    |    /
                          \   |   /
                           \  |  /
                            \ | /
                             \|/
                              B
                              |
                         VB node
                            ---

Measurements and Tests

• Basic verification without transistor (coil only):

  • With the supply disconnected, measure the DC resistance of the coil with the multimeter in ohmmeter mode (typical value between 20–60 Ω).
  • Calculate the expected current I_COIL = V / R_COIL (for example, for 12 V and 40 Ω → I_COIL ≈ 0.3 A) to confirm that your supply can provide it.

• Supply voltage measurement:

  • Set the multimeter to DC voltmeter mode.
  • Measure the voltage between +V and GND to ensure that you actually have 9–12 V (red probe to +V, black probe to GND).

• Measuring V_B (base voltage, VB):

  • V_B means “voltage at the transistor base with respect to GND”.
  • Place the black multimeter probe on GND and the red one on node VB.
  • With the button not pressed, V_B should be close to 0 V (especially if you use [R2] as a pull‑down resistor).
  • With the button pressed, V_B should rise to about 0.7–1 V (typical of a conducting base‑emitter junction).

• Measuring V_CE (collector‑emitter voltage):

  • V_CE means “voltage between the collector and emitter of the transistor”.
  • Place the black probe on GND (emitter) and the red on node VC (collector).
  • With the button not pressed, V_CE ≈ supply voltage (+V), because the coil does not conduct and the transistor is cut off.
  • With the button pressed, V_CE should drop to a low value (≈ 0.2–0.4 V) if the transistor saturates correctly, indicating that most of the voltage is now across the coil.

• Measuring I_COIL (coil current):

  • I_COIL means “current flowing through the coil”.
  • Put the multimeter in DC ammeter mode (appropriate range, for example 1 A or 400 mA as expected).
  • Disconnect the wire going from +V to the coil and instead connect the multimeter in series:
    • Red multimeter probe to node +V.
    • Black multimeter probe to the end of the coil that previously went to +V.
  • Press the button briefly and read the current: it should be close to the calculated value (for example, 0.2–0.4 A). Do not hold it down for too long if the transistor gets hot.

• Mechanical operation tests:

  • Make sure the moving part (hammer or tongue) is well aligned: when the coil is energized, it should move toward the metal plate or bell.
  • Press the button repeatedly: each time you should hear a click or a small “ding”.
  • If there is no sound, try adjusting the distance between the coil core and the moving part (too far and it will not move; too close and it may stick).

Key Concepts About the Coil in This Doorbell

• Inductor as an electromagnet:
• The coil is an inductor: a set of wire turns that, when current flows, generates a magnetic field.

• This magnetic field attracts ferromagnetic metal parts (iron, steel), which is what moves the doorbell hammer.

• Self‑induction and protection diode:

• When you interrupt the coil current (by releasing the button), the magnetic field collapses.
• This rapid change in the field induces an opposite voltage (called “induced voltage” or “inductive spike”) that can be very high.

• Diode [D1], placed in reverse parallel across the coil, provides a safe path for this induced current and limits the overvoltage, protecting the transistor.

• Using the transistor as an electronic switch:

• The NPN transistor acts here as a switch controlled by base current.
• When the base receives current through [R1] (button pressed), the transistor conducts and allows current to flow through the coil.
• When the base is at 0 V (button released), the transistor switches off and the coil is de‑energized.

Common Mistakes and How to Avoid Them

• Forgetting the protection diode [D1]:
• Without [D1], the transistor can be damaged by coil voltage spikes.

• Symptoms: transistor getting very hot or failing suddenly.

• Reversed diode polarity:

• If you connect the diode the wrong way around (anode to +V, cathode to VC), the coil will not activate because the diode will cause a short circuit (with high current).

• Remember: diode band (cathode) toward +V, unmarked end (anode) toward VC.

• Transistor pins swapped:

• Different models (2N2222, BC337, etc.) have different pin layouts (E‑B‑C).

• Always check the datasheet to know which pin is collector, base, and emitter.

• AC doorbell coil used with DC without checking:

• Some commercial doorbells are designed for 230 V AC and not for low‑voltage DC.
• Always use a coil/electromagnet suitable for 9–12 V DC or a low‑voltage doorbell.

Possible Improvements and Extensions

• Automatic (repeating) doorbell:
• Add a mechanical contact that opens and closes due to the hammer’s own motion (classic doorbell style), so it makes a continuous “drrrrring” while you keep the button pressed.

• This can be implemented with a flexible tongue that touches a contact when at rest and releases it when attracted by the coil.

• Control from a microcontroller:

• Replace the push button with a digital output from an Arduino or similar.

• Keep the same power stage with transistor and diode so as not to overload the microcontroller output.

• Waveform measurement with an oscilloscope:

• If you have an oscilloscope, observe the voltage waveform at node VC when you cut the current.

• You will see how the diode clips the inductive spike and limits the voltage.

• Doorbell optimization:

• Try coils with different numbers of turns or different cores (iron screw, solid bolt, laminated core).
• Adjust the distance between the coil and the moving piece to maximize force and sound without it getting stuck.

With this practical project you will have seen, in a very tangible way, how an inductor works as an electromagnet, why it generates voltage spikes when disconnected, and how to protect the circuit with a diode, all applied to something very everyday: an electromechanical doorbell.

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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