Level: Basic | Build a fail-safe alarm system using NAND logic to detect open windows.

Objective and use case

In this practical case, you will build a digital logic circuit that monitors two window sensors. The alarm will remain silent (LED OFF) only when both windows are securely closed. If either window is opened—or if a wire is cut—the alarm triggers (LED ON).

• Home Security: Monitoring multiple entry points (windows/doors) where all must be closed to secure the perimeter.
• Machine Safety: Ensuring all safety guards or maintenance hatches are closed before a machine can operate (or signaling a fault if opened).
• Fail-Safe Design: Demonstrating how active-high loops detect broken wires or open switches as alarm conditions.

Expected outcome:
* Safe State: When both switches (windows) are closed (Logic 1), the Output is 0 V (LED OFF).
* Alarm State: If Switch 1 OR Switch 2 is opened (Logic 0), the Output rises to ≈ 5 V (LED ON).
* Logic Verification: Confirmation of the NAND truth table behavior where Output is LOW only if all inputs are HIGH.

Target audience: Electronics students and hobbyists learning basic digital logic gates.

Materials

• V1: 5 V DC power supply, function: Main circuit power
• U1: 74HC00, function: Quad 2-input NAND gate IC
• SW1: SPST switch, function: Window 1 sensor (Closed = Window Closed)
• SW2: SPST switch, function: Window 2 sensor (Closed = Window Closed)
• R1: 10 kΩ resistor, function: Pull-down for SW1
• R2: 10 kΩ resistor, function: Pull-down for SW2
• R3: 330 Ω resistor, function: Current limiting for LED
• D1: Red LED, function: Alarm indicator

Pin-out of the IC used

Chip Selected: 74HC00 (Quad 2-Input NAND Gate)

Wiring guide

This guide uses specific node names to help you visualize the connections on a breadboard.

• Power Rail: Connect V1 positive terminal to node VCC and negative terminal to node 0.
• IC Power: Connect U1 pin 14 to VCC and U1 pin 7 to 0.
• Sensor 1: Connect SW1 between VCC and node SENS1.
• Pull-down 1: Connect R1 between SENS1 and 0.
• Sensor 2: Connect SW2 between VCC and node SENS2.
• Pull-down 2: Connect R2 between SENS2 and 0.
• Logic Input: Connect U1 pin 1 (1 A) to SENS1 and U1 pin 2 (1B) to SENS2.
• Logic Output: Connect U1 pin 3 (1Y) to node ALARM_OUT.
• Indicator: Connect R3 between ALARM_OUT and the Anode of D1.
• LED Ground: Connect the Cathode of D1 to node 0.

Conceptual block diagram

Schematic

Title: Practical case: Window sensor security alarm

(Input Stage: Sensors)                  (Processing Stage: Logic)             (Output Stage: Alarm)

[ VCC ]
   |
[ SW1: Window 1 ]
   |
   +--(SENS1)-------+-----------------> [ U1: Pin 1 (Input A) ]
                    |                                         |
                 [ R1: 10k ]                                  v
                    |                                 [ U1: NAND Gate ] --(ALARM_OUT)--> [ R3: 330 Ω ] --> [ D1: LED ] --> GND
                 [ GND ]                                      ^
                                                              |
[ VCC ]             |                                         |
   |                |                                         |
[ SW2: Window 2 ]   |                                         |
   |                |                                         |
   +--(SENS2)-------+-----------------> [ U1: Pin 2 (Input B) ]
                    |
                 [ R2: 10k ]
                    |
                 [ GND ]

Electrical diagram

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Truth table

The 74HC00 implements the NAND function. In this security context, Logic 1 represents a «Closed Window» (Safe), and Logic 0 represents an «Open Window» (Breach).

Measurements and tests

Follow these steps to validate your alarm system:

1. Initial Power-Up: Ensure both switches (SW1, SW2) are closed. Turn on the 5 V supply. The LED D1 should be OFF.
2. Voltage Check (Secure): Use a multimeter to measure the voltage at node ALARM_OUT. It should be close to 0 V (< 0.2 V).
3. Breach Test 1: Open SW1 while keeping SW2 closed. The LED should turn ON. Measure ALARM_OUT; it should read close to 5 V.
4. Breach Test 2: Close SW1 and open SW2. The LED should turn ON.
5. Total Breach: Open both switches. The LED should remain ON.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Window sensor security alarm
.width out=256
* ngspice netlist

* --- Component Models ---
* Switch model for SW1 and SW2 (Sensors)
* Vt=2.5V: Switch closes when control voltage > 2.5V
* Ron=1m: Low resistance when closed
* Roff=100Meg: High resistance when open
.model SW_MOD SW(Vt=2.5 Ron=1m Roff=100Meg)

* LED model for D1
.model LED_RED D(IS=1e-22 RS=5 N=1.5 CJO=10p BV=5)

* --- Power Supply ---
* V1: 5 V DC power supply connected to VCC and 0 (GND)
V1 VCC 0 DC 5

* --- Window Sensor 1 ---
* Control source V_ACT1 simulates the physical action of opening/closing Window 1
* ... (truncated in public view) ...

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* Practical case: Window sensor security alarm
.width out=256
* ngspice netlist

* --- Component Models ---
* Switch model for SW1 and SW2 (Sensors)
* Vt=2.5V: Switch closes when control voltage > 2.5V
* Ron=1m: Low resistance when closed
* Roff=100Meg: High resistance when open
.model SW_MOD SW(Vt=2.5 Ron=1m Roff=100Meg)

* LED model for D1
.model LED_RED D(IS=1e-22 RS=5 N=1.5 CJO=10p BV=5)

* --- Power Supply ---
* V1: 5 V DC power supply connected to VCC and 0 (GND)
V1 VCC 0 DC 5

* --- Window Sensor 1 ---
* Control source V_ACT1 simulates the physical action of opening/closing Window 1
* Logic: High (5V) = Window Closed, Low (0V) = Window Open
* Timing: Toggles every 100us (Period 200us)
V_ACT1 ACT1 0 PULSE(0 5 0 1u 1u 100u 200u)

* SW1: Connects VCC to SENS1 when window is closed
S1 VCC SENS1 ACT1 0 SW_MOD

* R1: Pull-down resistor for SENS1 (10k)
R1 SENS1 0 10k

* --- Window Sensor 2 ---
* Control source V_ACT2 simulates Window 2
* Timing: Toggles every 200us (Period 400us) to test all truth table combinations
V_ACT2 ACT2 0 PULSE(0 5 0 1u 1u 200u 400u)

* SW2: Connects VCC to SENS2 when window is closed
S2 VCC SENS2 ACT2 0 SW_MOD

* R2: Pull-down resistor for SENS2 (10k)
R2 SENS2 0 10k

* --- Logic IC: U1 (74HC00) ---
* Quad 2-input NAND gate. We instantiate one gate.
* Pin mapping according to wiring guide:
* Pin 1 (Input A) -> SENS1
* Pin 2 (Input B) -> SENS2
* Pin 3 (Output Y) -> ALARM_OUT
* Pin 7 -> GND (0), Pin 14 -> VCC
XU1 SENS1 SENS2 ALARM_OUT 0 VCC 74HC00_GATE

* Subcircuit for NAND Gate using robust continuous functions
.subckt 74HC00_GATE A B Y GND VCC
* Logic: Y = NAND(A, B) = NOT(A AND B)
* Implemented using sigmoid functions for convergence:
* 1 / (1 + exp(-k*(V-Vth))) acts as a smooth logical comparator.
* Vth = 2.5V, k = 20
B_NAND Y GND V = V(VCC) * (1 - ( (1/(1+exp(-20*(V(A)-2.5)))) * (1/(1+exp(-20*(V(B)-2.5)))) ))
.ends

* --- Alarm Output Indicator ---
* R3: Current limiting resistor (330 Ohm)
R3 ALARM_OUT LED_ANODE 330

* D1: Red LED connected to Ground
D1 LED_ANODE 0 LED_RED

* --- Simulation Commands ---
.op
* Transient analysis for 500us to capture all logic states
.tran 1u 500u

* Output configuration
* We print the Sensor inputs and the Alarm output
.print tran V(SENS1) V(SENS2) V(ALARM_OUT) V(LED_ANODE)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation correctly implements the NAND logic truth table. When both sensors are High (5V, Closed), the Output is Low (~0V). If either or both sensors are Low (Open), the Output goes High (5V), activating the LED (approx 1.83V at anode).

Show raw data table (657 rows)

Index   time            v(sens1)        v(sens2)        v(alarm_out)    v(led_anode)
0	0.000000e+00	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
1	1.000000e-08	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
2	2.000000e-08	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
3	4.000000e-08	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
4	8.000000e-08	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
5	1.600000e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
6	3.200000e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
7	3.562500e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
8	4.196875e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
9	4.372461e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
10	4.679736e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
11	4.795524e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
12	4.902290e-07	4.999500e-04	4.999500e-04	5.000000e+00	1.833072e+00
13	5.023412e-07	5.000000e+00	5.000000e+00	3.894872e-36	1.057689e+00
14	5.138120e-07	5.000000e+00	5.000000e+00	3.894872e-36	-7.61250e-02
15	5.160398e-07	5.000000e+00	5.000000e+00	3.894872e-36	-3.72798e-02
16	5.172425e-07	5.000000e+00	5.000000e+00	3.894872e-36	-2.57490e-02
17	5.188923e-07	5.000000e+00	5.000000e+00	3.894872e-36	-1.54585e-02
18	5.214063e-07	5.000000e+00	5.000000e+00	3.894872e-36	-6.97976e-03
19	5.238372e-07	5.000000e+00	5.000000e+00	3.894872e-36	-3.25627e-03
20	5.261078e-07	5.000000e+00	5.000000e+00	3.894872e-36	-1.60566e-03
21	5.281984e-07	5.000000e+00	5.000000e+00	3.894872e-36	-8.40881e-04
22	5.304310e-07	5.000000e+00	5.000000e+00	3.894872e-36	-4.20300e-04
23	5.328536e-07	5.000000e+00	5.000000e+00	3.894872e-36	-1.97001e-04
... (633 more rows) ...

Common mistakes and how to avoid them

1. Floating Inputs: Forgetting R1 or R2 causes the inputs to «float» when switches are open, leading to erratic LED flickering. Solution: Always ensure inputs have a path to ground (via pull-down resistors) when the switch is open.
2. LED Polarity: Connecting the LED backwards prevents it from lighting up even when the alarm is active. Solution: Ensure the longer leg (Anode) faces the resistor and IC output.
3. Incorrect Switch Wiring: Placing the switch in parallel with the resistor instead of in series with the voltage source creates a short circuit. Solution: Follow the wiring guide: VCC -> Switch -> Node -> Resistor -> GND.

Troubleshooting

• Symptom: LED is always ON, even when switches are closed.
  • Cause: One of the switches is not making contact, or an input wire is loose.
  • Fix: Check continuity across SW1 and SW2; ensure pin 1 and pin 2 actually receive 5 V.
• Symptom: LED never turns ON.
  • Cause: LED is reversed, IC is not powered, or R3 is too high.
  • Fix: Check pin 14 for 5 V. Reverse the LED. Verify R3 is 330 Ω, not 330 kΩ.
• Symptom: Logic works reversed (LED ON when safe, OFF when open).
  • Cause: You may be using an AND gate (74HC08) instead of NAND, or your switch/resistor logic is inverted (Pull-ups instead of Pull-downs).
  • Fix: Verify the chip number is 74HC00.

Possible improvements and extensions

1. Audible Alarm: Connect the base of an NPN transistor to ALARM_OUT to drive a 5 V active buzzer, adding sound to the light.
2. Latching Alarm: Use the remaining gates in the 74HC00 to build an SR Latch. This would keep the alarm sounding even if the burglar closes the window immediately after entering.

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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Who we are

Level: Basic – Control a high-power load using a low-voltage signal via galvanic isolation.

Objective and use case

You will build a driver circuit that uses a small 5 V signal to activate an electromechanical relay, which in turn switches a separate 12 V high-power circuit powering a bulb.

• Why it is useful:

  • Automotive Systems: Allows a low-current ECU signal to switch high-current headlights.
  • Safety: Keeps high voltage/current (the load side) physically separated from the sensitive control logic (the user side).
  • Interface: Enables microcontrollers (like Arduino/ESP32) to control industrial equipment or AC appliances (simulated here with 12 V).

• Expected outcome:

  • The 12 V bulb turns ON only when the 5 V control switch is closed.
  • An audible «click» is heard from the relay component when switching states.
  • Measurements: 0 V on the load when the control signal is 0 V; ~12 V on the load when the control signal is 5 V.

• Target audience: Students dealing with electromechanical interfaces and circuit protection.

Materials

• V1: 5 V DC voltage source, function: Control Logic Supply
• V2: 12 V DC voltage source, function: High Power Load Supply
• S1: SPST Toggle Switch, function: Control trigger
• R1: 1 kΩ resistor, function: Base current limiter for Q1
• Q1: 2N2222 NPN BJT Transistor, function: Relay coil driver
• D1: 1N4007 Diode, function: Flyback protection (snubber)
• K1: 5 V SPST Relay (coil resistance ~70 Ω), function: Galvanic isolation switch
• L1: 12 V / 10 W Incandescent Bulb, function: High power load

Wiring guide

This guide uses specific node names to ensure correct connections in simulation and assembly. The circuit has two isolated sides: the Control Side (Nodes: V_CTRL, 0) and the Load Side (Nodes: V_HV, GND_LOAD).

Control Side (Low Power):
* V1 (+): Connects to Node V_CTRL.
* V1 (-): Connects to Node 0 (Common Ground).
* S1: Connects between V_CTRL and Node V_TRIG.
* R1: Connects between V_TRIG and Node V_BASE.
* Q1 (Base): Connects to Node V_BASE.
* Q1 (Emitter): Connects to Node 0.
* Q1 (Collector): Connects to Node COIL_LOW.
* K1 (Coil pin 1): Connects to Node V_CTRL.
* K1 (Coil pin 2): Connects to Node COIL_LOW.
* D1 (Anode): Connects to Node COIL_LOW.
* D1 (Cathode): Connects to Node V_CTRL (Reverse biased across coil).

Load Side (High Power):
* V2 (+): Connects to Node V_HV.
* V2 (-): Connects to Node GND_LOAD (Isolated from Node 0).
* K1 (Common Contact): Connects to Node V_HV.
* K1 (Normally Open Contact): Connects to Node BULB_IN.
* L1: Connects between Node BULB_IN and Node GND_LOAD.

Conceptual block diagram

Schematic

+-------------------------------------------------------------------------+
|               PRACTICAL CASE: HIGH POWER CIRCUIT ISOLATION              |
+-------------------------------------------------------------------------+

===========================================================================
  PART 1: CONTROL SIDE (5 V Logic)
  Nodes: V_CTRL, V_TRIG, V_BASE, COIL_LOW, 0 (GND)
===========================================================================

  (Trigger Signal Path)
  [ V1: 5 V (+) ] --> [ S1: Switch ] --> [ R1: 1k Ohm ] --> [ Q1: Base ]
                                                               |
                                                               | (Controls)
                                                               v
  (Coil Power Path)                                    [ Q1: Collector ]
  [ V1: 5 V (+) ] ---------> [ K1: Relay Coil ] --------------> |
                            [ || D1 Diode    ]                 |
                            [ (Rev Biased)   ]                 | (Conducts to)
                                                               |
                                                               v
                                                       [ Q1: Emitter ]
                                                               |
                                                               v
                                                       [ Node 0 (GND) ]


             ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
             ~      MAGNETIC LINK (GALVANIC ISOLATION)   ~
             ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~


===========================================================================
  PART 2: LOAD SIDE (12 V High Power)
  Nodes: V_HV, BULB_IN, GND_LOAD
===========================================================================

  (High Current Path)

  [ V2: 12 V (+) ] --> [ K1: Relay Switch ] --> [ L1: 12 V Bulb ] --> [ GND_LOAD ]
                      [   (COM -> NO)    ]

Electrical diagram

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Measurements and tests

Follow these steps to validate the isolation and switching capability:

1. Coil Voltage Test:

   • Close switch S1.
   • Measure voltage between V_CTRL and COIL_LOW.
   • Result: It should read approximately 5 V (indicating the transistor is sinking current).

2. Load Activation:

   • Keep S1 closed.
   • Observe L1 (Bulb).
   • Result: The bulb illuminates. Measure voltage across L1; it should be ~12 V.

3. Switch Latency (Oscilloscope required):

   • Connect Channel 1 to V_TRIG and Channel 2 to BULB_IN.
   • Toggle S1 from OFF to ON.
   • Result: You will observe a delay (typically 5–15 ms) between the signal rising on Ch1 and power appearing on Ch2. This is the mechanical switching time of the relay armature.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* High power circuit isolation
*
* This netlist simulates a relay driver circuit with a high-power load.
* It includes a low-voltage control side (5V) and an isolated high-voltage load side (12V).
*

* --- Analysis Setup ---
.tran 10u 10m
.print tran V(V_TRIG) V(BULB_IN) V(COIL_LOW) I(L_K1_COIL)

* --- Control Side (Low Power) ---

* Supply V1: 5V DC
V1 V_CTRL 0 DC 5

* Switch S1: Modeled as a Pulse Voltage Source to simulate user actuation
* Connects to V_TRIG to drive the base resistor.
* Timing: Off for 1ms, On for 4ms, then Off.
V_S1 V_TRIG 0 PULSE(0 5 1m 10u 10u 4m 10m)

* ... (truncated in public view) ...

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* High power circuit isolation
*
* This netlist simulates a relay driver circuit with a high-power load.
* It includes a low-voltage control side (5V) and an isolated high-voltage load side (12V).
*

* --- Analysis Setup ---
.tran 10u 10m
.print tran V(V_TRIG) V(BULB_IN) V(COIL_LOW) I(L_K1_COIL)

* --- Control Side (Low Power) ---

* Supply V1: 5V DC
V1 V_CTRL 0 DC 5

* Switch S1: Modeled as a Pulse Voltage Source to simulate user actuation
* Connects to V_TRIG to drive the base resistor.
* Timing: Off for 1ms, On for 4ms, then Off.
V_S1 V_TRIG 0 PULSE(0 5 1m 10u 10u 4m 10m)

* Resistor R1: 1k Base Current Limiter
R1 V_TRIG V_BASE 1k

* Transistor Q1: 2N2222 NPN Relay Driver
* Connections: Collector=COIL_LOW, Base=V_BASE, Emitter=0
Q1 COIL_LOW V_BASE 0 2N2222MOD

* Relay Coil K1 (Coil Side)
* Modeled as Inductance + Resistance in series between V_CTRL and COIL_LOW
R_K1_COIL V_CTRL INT_COIL 70
L_K1_COIL INT_COIL COIL_LOW 50m

* Diode D1: Flyback protection (Snubber)
* Anode=COIL_LOW, Cathode=V_CTRL
D1 COIL_LOW V_CTRL 1N4007MOD

* --- Load Side (High Power) ---

* Ground Isolation: High resistance path to global ground 0 to prevent singular matrix
R_ISO GND_LOAD 0 100Meg

* Supply V2: 12V DC
V2 V_HV GND_LOAD DC 12

* Relay Contact K1 (Switch Side)
* Modeled as a Voltage Controlled Switch
* Controlled by the voltage across the coil: V(V_CTRL) - V(COIL_LOW)
* Connects V_HV to BULB_IN when coil is energized
S_K1 V_HV BULB_IN V_CTRL COIL_LOW RELAY_SW_MOD

* Load L1: 12V / 10W Bulb
* Resistance ~ 14.4 Ohms (R = V^2 / P = 144 / 10)
R_L1 BULB_IN GND_LOAD 14.4

* --- Component Models ---

* NPN Transistor Model
.model 2N2222MOD NPN(IS=1E-14 VAF=100 BF=200 IKF=0.3 XTB=1.5 BR=3 CJC=8E-12 CJE=25E-12 TR=46.91E-9 TF=411.1E-12 ITF=0.6 VTF=1.7 XTF=3 RB=10 RC=0.3 RE=0.2)

* Diode Model
.model 1N4007MOD D(IS=7.02767n RS=0.0341512 N=1.80803 EG=1.11 XTI=3 BV=1000 IBV=5u CJO=10p VJ=0.7 M=0.5 FC=0.5 TT=100n)

* Relay Switch Model
* Threshold Vt=2.5V (Coil is 5V), Hysteresis Vh=0.5V
.model RELAY_SW_MOD SW(Vt=2.5 Vh=0.5 Ron=0.1 Roff=100Meg)

.op
.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows the trigger signal (V_TRIG) going high (5V) between 1ms and 5ms. During this window, the coil current (I(L_K1_COIL)) rises, causing the relay switch to close and V(BULB_IN) to switch to ~12V. After 5ms, the trigger drops, coil current decays (snubber active), and the load voltage returns to near zero.

Show raw data table (4100 rows)

Index   time            v(v_trig)       v(bulb_in)      v(coil_low)     l_k1_coil#branc
0	0.000000e+00	0.000000e+00	1.722670e-06	5.000000e+00	1.002664e-11
1	1.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.002626e-11
2	2.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.002547e-11
3	4.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.002342e-11
4	8.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.001814e-11
5	1.600000e-06	0.000000e+00	1.722670e-06	5.000000e+00	1.000316e-11
6	3.200000e-06	0.000000e+00	1.722670e-06	5.000000e+00	9.969744e-12
7	6.400000e-06	0.000000e+00	1.722670e-06	5.000000e+00	1.000801e-11
8	1.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.002921e-11
9	2.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	9.970357e-12
10	3.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.004993e-11
11	4.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	9.955463e-12
12	5.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.004077e-11
13	6.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	9.984500e-12
14	7.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.001134e-11
15	8.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.001578e-11
16	9.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.000519e-11
17	1.028000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.003686e-11
18	1.128000e-04	0.000000e+00	1.722670e-06	5.000000e+00	9.961732e-12
19	1.228000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.005266e-11
20	1.328000e-04	0.000000e+00	1.722670e-06	5.000000e+00	9.963169e-12
21	1.428000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.003205e-11
22	1.528000e-04	0.000000e+00	1.722670e-06	5.000000e+00	9.984436e-12
23	1.628000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.001919e-11
... (4076 more rows) ...

Common mistakes and how to avoid them

1. Omitting the flyback diode (D1):

   • Error: The transistor Q1 fails permanently after a few switches.
   • Solution: Always place a diode in reverse bias parallel to the relay coil to absorb the high-voltage spike generated when the magnetic field collapses.

2. Sharing Grounds unintentionally:

   • Error: Connecting GND_LOAD to Node 0 on the breadboard.
   • Solution: While the circuit will work, you lose galvanic isolation. Keep the high-power return path physically separate from the logic ground.

3. Insufficient Base Current:

   • Error: Using a resistor R1 that is too high (e.g., 100 kΩ). The relay does not click or clicks weakly.
   • Solution: Ensure the transistor is in saturation. For a 2N2222 driving a standard relay, 1 kΩ is usually sufficient.

Troubleshooting

• Symptom: Relay clicks, but the bulb does not light up.

  • Cause: Issue on the Load Side (Secondary circuit).
  • Fix: Check V2 supply, verify the bulb L1 is not burnt, and ensure connections to the Relay COM/NO pins are tight.

• Symptom: No sound from relay, Bulb OFF.

  • Cause: The coil is not energizing.
  • Fix: Check voltage at Node V_BASE. If 0 V, check S1. If ~0.7 V, check if Q1 is installed correctly (E-B-C pinout).

• Symptom: Transistor gets extremely hot.

  • Cause: Coil current is too high for the selected transistor.
  • Fix: Verify the relay coil resistance. If it draws >600 mA, the 2N2222 might be underpowered; use a power transistor (e.g., TIP31) or a MOSFET.

Possible improvements and extensions

1. Status Indicator: Add a small LED and a 330 Ω resistor in parallel with the Relay Coil to visually indicate when the control signal is active.
2. Solid State Upgrade: Replace the mechanical relay (K1) and transistor driver with an Optocoupler and a MOSFET (or Triac for AC) to eliminate mechanical wear and reduce switching latency.

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

Level: Basic – Understand how to use two SPDT relays to change polarity and direction of a DC motor.

Objective and use case

In this case, you will build a relay-based H-bridge circuit to control a DC motor. By using two Single Pole Double Throw (SPDT) relays, you will be able to drive the motor clockwise, counter-clockwise, or brake it using simple pushbuttons.

• Real-world scenarios:
• Automotive Power Windows: Reversing the motor to raise or lower the glass.
• Robotics: Controlling wheel direction for forward and backward movement.
• Industrial Conveyors: Changing the direction of a belt to route products.

• Motorized Curtains: Opening and closing mechanisms.

• Expected outcome:

• Idle State: When no buttons are pressed, the motor terminals are grounded (0 V difference), resulting in a dynamic brake (motor stops).
• Forward State: Pressing Button A applies +5 V to the motor; it spins Clockwise (CW).
• Reverse State: Pressing Button B applies -5 V (polarity swap) to the motor; it spins Counter-Clockwise (CCW).
• Braking/Safety: If both buttons are pressed simultaneously, both motor terminals connect to VCC, resulting in 0 V difference and the motor remains stopped.

Target audience: Hobbyists and students getting started with electromechanical control.

Materials

• V1: 5 V DC Power Supply, function: Main energy source.
• M1: 5 V DC Motor, function: The actuator to be controlled.
• K1: 5 V SPDT Relay, function: Controls the «Positive» side of the motor.
• K2: 5 V SPDT Relay, function: Controls the «Negative» side of the motor.
• S1: Momentary Pushbutton (NO), function: Activates Relay K1 (Forward).
• S2: Momentary Pushbutton (NO), function: Activates Relay K2 (Reverse).
• D1: 1N4007 Diode, function: Flyback protection for K1 coil.
• D2: 1N4007 Diode, function: Flyback protection for K2 coil.

Wiring guide

This guide uses node names to describe connections.
Nodes: VCC (5 V Supply), 0 (Ground), COIL_A, COIL_B, MOT_A, MOT_B.

• Power Supply:
• V1 (+): Connects to node VCC.

• V1 (-): Connects to node 0.

• Control Circuit (Coils):

• S1: Connects between VCC and COIL_A.
• K1 (Coil): Connects between COIL_A and 0.
• D1: Cathode to COIL_A, Anode to 0 (Protects against inductive spikes).
• S2: Connects between VCC and COIL_B.
• K2 (Coil): Connects between COIL_B and 0.

• D2: Cathode to COIL_B, Anode to 0.

• Power Circuit (Motor Drive):

• K1 (Normally Open – NO): Connects to VCC.
• K1 (Normally Closed – NC): Connects to 0.
• K1 (Common – COM): Connects to node MOT_A.
• K2 (Normally Open – NO): Connects to VCC.
• K2 (Normally Closed – NC): Connects to 0.
• K2 (Common – COM): Connects to node MOT_B.
• M1: Connects between MOT_A and MOT_B.

Conceptual block diagram

Schematic

+-------------------------------------------------------------------------+
|                DC MOTOR REVERSING CIRCUIT (H-BRIDGE)                    |
+-------------------------------------------------------------------------+

[ CONTROL SUBSYSTEM ]                                [ POWER SUBSYSTEM ]

      (Forward Input)                                   (Left Side Drive)
VCC --> [ S1 Button ]                                  VCC (NO)
            |                                             |
            v                                             v
    [ Node: COIL_A ]                               [ K1 Switch (COM) ] --(MOT_A)--+
            |                                      [  (Relay 1)      ]            |
            +--> [ K1 Coil || D1 ] --> GND                ^                       |
            |    (D1 is Reverse Biased)                   |                       |
            |                                             |                       |
            +----------(Magnetic Link)--------------------+                       |
                                                          |                       |
                                                  GND (NC) +                      |
                                                                                  v
                                                                           [ DC MOTOR ]
                                                                           [    M1    ]
                                                                                  ^
                                                  GND (NC) +                      |
                                                          |                       |
            +----------(Magnetic Link)--------------------+                       |
            |                                             |                       |
            |    (D2 is Reverse Biased)                   |                       |
            +--> [ K2 Coil || D2 ] --> GND         [ K2 Switch (COM) ] --(MOT_B)--+
            |                                      [  (Relay 2)      ]
    [ Node: COIL_B ]                                      ^
            ^                                             |
            |                                             |
VCC --> [ S2 Button ]                                  VCC (NO)
      (Reverse Input)                                   (Right Side Drive)

+-------------------------------------------------------------------------+
| LOGIC KEY:                                                              |
| 1. Idle: Both Switches connect COM to NC (GND). Motor is braked (0 V).   |
| 2. Press S1: K1 switches to NO (VCC). Current: VCC->MOT_A->MOT_B->GND.  |
| 3. Press S2: K2 switches to NO (VCC). Current: VCC->MOT_B->MOT_A->GND.  |
+-------------------------------------------------------------------------+

Electrical diagram

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Measurements and tests

To validate the circuit, perform the following steps using a multimeter and visual inspection:

1. Idle Check: Ensure neither S1 nor S2 is pressed. Measure voltage between MOT_A and MOT_B.
   • Result: Should be 0 V. Both terminals are connected to GND via the NC contacts. The motor is locked (hard to turn by hand due to back EMF shorting).
2. Forward Actuation: Press and hold S1.
   • Result: K1 clicks. Measure voltage from MOT_A (Red probe) to MOT_B (Black probe). Voltage should be approximately +5 V. Motor spins Clockwise.
3. Reverse Actuation: Release S1, then press and hold S2.
   • Result: K2 clicks. Measure voltage from MOT_A to MOT_B. Voltage should be approximately -5 V. Motor spins Counter-Clockwise.
4. Double Press (Safety Test): Press both S1 and S2 simultaneously.
   • Result: Both relays click. Voltage between MOT_A and MOT_B is 0 V (Both at 5 V potential). Motor does not move.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: DC Motor Reversing
.width out=256
* Ngspice Netlist
*
* Description: H-Bridge configuration using two SPDT relays to control a DC motor.
* Logic:
* - S1 Pressed -> K1 Active -> MOT_A = 5V, MOT_B = 0V (Forward)
* - S2 Pressed -> K2 Active -> MOT_A = 0V, MOT_B = 5V (Reverse)
* - None Pressed -> MOT_A = 0V, MOT_B = 0V (Stop/Brake)
*
* Simulation Time: 10ms (Captures S1 pulse at 1ms and S2 pulse at 5ms)
.tran 10u 10m

* -----------------------------------------------------------------------------
* Power Supply
* -----------------------------------------------------------------------------
* V1: 5V DC Power Supply, function: Main energy source.
* Connected between VCC (+) and 0 (-).
V1 VCC 0 DC 5

* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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* Practical case: DC Motor Reversing
.width out=256
* Ngspice Netlist
*
* Description: H-Bridge configuration using two SPDT relays to control a DC motor.
* Logic:
* - S1 Pressed -> K1 Active -> MOT_A = 5V, MOT_B = 0V (Forward)
* - S2 Pressed -> K2 Active -> MOT_A = 0V, MOT_B = 5V (Reverse)
* - None Pressed -> MOT_A = 0V, MOT_B = 0V (Stop/Brake)
*
* Simulation Time: 10ms (Captures S1 pulse at 1ms and S2 pulse at 5ms)
.tran 10u 10m

* -----------------------------------------------------------------------------
* Power Supply
* -----------------------------------------------------------------------------
* V1: 5V DC Power Supply, function: Main energy source.
* Connected between VCC (+) and 0 (-).
V1 VCC 0 DC 5

* -----------------------------------------------------------------------------
* User Inputs (Pushbuttons)
* -----------------------------------------------------------------------------
* Modeled as Voltage Controlled Switches (S1, S2) driven by Pulse Sources.
* This strictly simulates the user pressing the button at specific times.

* Stimulus for S1 (Forward Request)
* Pulse: 0V to 5V, starts at 1ms, duration 2ms.
V_USER_S1 CTRL_S1 0 PULSE(0 5 1m 1u 1u 2m 10m)

* Stimulus for S2 (Reverse Request)
* Pulse: 0V to 5V, starts at 5ms, duration 2ms.
V_USER_S2 CTRL_S2 0 PULSE(0 5 5m 1u 1u 2m 10m)

* S1: Momentary Pushbutton (NO)
* Connects VCC to COIL_A when activated by V_USER_S1.
S1 VCC COIL_A CTRL_S1 0 SW_PUSH

* S2: Momentary Pushbutton (NO)
* Connects VCC to COIL_B when activated by V_USER_S2.
S2 VCC COIL_B CTRL_S2 0 SW_PUSH

* -----------------------------------------------------------------------------
* Control Circuit (Relay Coils)
* -----------------------------------------------------------------------------
* Relay K1 Coil Circuit
* K1 Coil: Connects between COIL_A and 0. Modeled as L+R.
L_K1 COIL_A K1_INT 10m
R_K1 K1_INT 0 100
* D1: 1N4007 Diode, function: Flyback protection.
* Cathode to COIL_A, Anode to 0.
D1 0 COIL_A D_1N4007

* Relay K2 Coil Circuit
* K2 Coil: Connects between COIL_B and 0. Modeled as L+R.
L_K2 COIL_B K2_INT 10m
R_K2 K2_INT 0 100
* D2: 1N4007 Diode, function: Flyback protection.
* Cathode to COIL_B, Anode to 0.
D2 0 COIL_B D_1N4007

* -----------------------------------------------------------------------------
* Power Circuit (Motor Drive via Relay Contacts)
* -----------------------------------------------------------------------------
* Relay K1 Contacts (SPDT)
* COM: MOT_A
* NO: VCC (Connected when Coil is Energized/High)
* NC: 0   (Connected when Coil is De-energized/Low)
S_K1_NO VCC MOT_A COIL_A 0 SW_NO_RELAY
S_K1_NC MOT_A 0   COIL_A 0 SW_NC_RELAY

* Relay K2 Contacts (SPDT)
* COM: MOT_B
* NO: VCC (Connected when Coil is Energized/High)
* NC: 0   (Connected when Coil is De-energized/Low)
S_K2_NO VCC MOT_B COIL_B 0 SW_NO_RELAY
S_K2_NC MOT_B 0   COIL_B 0 SW_NC_RELAY

* M1: 5 V DC Motor
* Modeled as a resistive load (50 Ohms) to visualize voltage polarity.
* Connects between MOT_A and MOT_B.
R_M1 MOT_A MOT_B 50

* -----------------------------------------------------------------------------
* Component Models
* -----------------------------------------------------------------------------
* Standard Diode Model
.model D_1N4007 D(IS=1N N=1 RS=0.1 BV=1000 IBV=10u)

* Pushbutton Switch Model (Normally Open)
* Closes (Low R) when Control Voltage > 2.5V
.model SW_PUSH SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)

* Relay Contact Models
* NO (Normally Open): Conducts when Coil > 2.5V
.model SW_NO_RELAY SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)

* NC (Normally Closed): Conducts when Coil < 2.5V
* SPICE SW Logic: If V < Vt, R = Roff. If V > Vt, R = Ron.
* For NC: We want Low R when V < Vt. So Roff=0.01, Ron=10Meg.
.model SW_NC_RELAY SW(Vt=2.5 Vh=0.1 Ron=10Meg Roff=0.01)

* -----------------------------------------------------------------------------
* Output Directives
* -----------------------------------------------------------------------------
* Outputs: Motor Terminals (MOT_A, MOT_B)
* Inputs: Coil Control Voltages (COIL_A, COIL_B)
.print tran V(MOT_A) V(MOT_B) V(COIL_A) V(COIL_B) I(L_K1)

.op
.end

Simulation Results (Transient Analysis)

Analysis: At 1ms, S1 activates, energizing Coil A (approx 5V). Consequently, MOT_A goes to 5V while MOT_B stays near 0V (Forward). At 3ms, S1 releases and the motor stops. At 5ms, S2 activates, energizing Coil B. MOT_B goes to 5V while MOT_A stays near 0V (Reverse). Inductive kickback is visible on coil nodes when switches open.

Show raw data table (1104 rows)

Index   time            v(mot_a)        v(mot_b)        v(coil_a)       v(coil_b)       l_k1#branch
0	0.000000e+00	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
1	1.000000e-07	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
2	2.000000e-07	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
3	4.000000e-07	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
4	8.000000e-07	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
5	1.600000e-06	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
6	3.200000e-06	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
7	6.400000e-06	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
8	1.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
9	2.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
10	3.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
11	4.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
12	5.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
13	6.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
14	7.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
15	8.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
16	9.280000e-05	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
17	1.028000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
18	1.128000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
19	1.228000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
20	1.328000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
21	1.428000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
22	1.528000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
23	1.628000e-04	5.000000e-09	5.000000e-09	4.999931e-05	4.999931e-05	4.999931e-07
... (1080 more rows) ...

Common mistakes and how to avoid them

1. Wiring the Motor to NO/NC instead of COM:
   • Mistake: Connecting the motor to the Normally Open or Closed pins, and power to the Common pin.
   • Solution: Always connect the Load (Motor) to the Common (COM) pin of the SPDT relay for H-bridge configurations. Power and Ground go to NO and NC.
2. Omitting Flyback Diodes:
   • Mistake: Forgetting D1 and D2 across the relay coils.
   • Solution: Always install diodes in reverse bias across coils to prevent high-voltage spikes from damaging switches or power supplies when the relay turns off.
3. Using SPST Relays:
   • Mistake: Attempting this topology with 4-pin relays that lack a Normally Closed contact.
   • Solution: Ensure you use 5-pin SPDT relays so the motor can be grounded when the relay is off.

Troubleshooting

• Motor vibrates but does not spin:
  • Cause: Power supply current is insufficient.
  • Fix: Check the current rating of your power supply; motors draw high current upon startup.
• Relay clicks but motor does not move:
  • Cause: Burnt internal contacts or loose wiring on the COM/NO/NC terminals.
  • Fix: Verify continuity between COM and NO when the relay is active using a multimeter.
• Sparks visible inside the relay:
  • Cause: Inductive load kickback from the motor.
  • Fix: While not always fatal, adding a small capacitor (e.g., 100 nF) across the motor terminals can reduce arcing and noise.

Possible improvements and extensions

1. Limit Switches: Add Normally Closed limit switches in series with the relay coils (COIL_A and COIL_B) to automatically stop the motor when a mechanism reaches its end of travel.
2. Speed Control: Insert a high-wattage rheostat or a PWM transistor driver in series with the main VCC supply to the relay contacts (not the coils) to vary the motor speed.

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

Level: Basic. Build a self-latching relay circuit to maintain an alarm state after a momentary trigger.

Objective and use case

You will build a basic «memory» circuit using an electromechanical relay, often called a latching or holding circuit. A momentary press of a trigger button will activate an alarm (LED), which will remain active even after the button is released, until a separate reset button is pressed.

• Security Systems: Used in simple burglar alarms where a tripped sensor keeps the siren on until a user resets it.
• Industrial Safety: Commonly used in «Start/Stop» motor control stations.
• Fault Indicators: Captures transient error signals so operators can see a fault occurred even if the condition clears.

Expected outcome:
* Initial state: LED is OFF.
* Action 1: Momentarily press the «Trigger» button → LED turns ON and relay clicks.
* Action 2: Release «Trigger» button → LED remains ON (Latched).
* Action 3: Press «Reset» button → LED turns OFF and relay releases.

Target audience: Beginners familiar with basic circuits and relay operation.

Materials

• V1: 12 V DC power supply, function: Main power source
• K1: SPDT Relay (12 V Coil), function: Electromechanical switch and memory element
• S1: Pushbutton (Normally Open – NO), function: Trigger signal
• S2: Pushbutton (Normally Closed – NC), function: Reset signal
• R1: 1 kΩ resistor, function: Current limiting for LED
• D1: Red LED, function: Visual alarm indicator
• D2: 1N4007 Diode, function: Flyback protection for the coil

Wiring guide

Connect the components using the following node definitions: VCC (12 V), 0 (Ground), feed_line, latch_node.

• V1 (DC Source): Connect Positive to VCC and Negative to 0.
• S2 (Reset Button – NC): Connect between VCC and feed_line.
• S1 (Trigger Button – NO): Connect between feed_line and latch_node.
• K1 (Relay Coil): Connect one side to latch_node and the other side to 0.
• K1 (Relay Common Contact – COM): Connect to feed_line.
• K1 (Relay Normally Open Contact – NO): Connect to latch_node.
• D2 (Protection Diode): Connect Cathode (stripe) to latch_node and Anode to 0.
• R1 (Resistor): Connect between latch_node and node led_anode.
• D1 (LED): Connect Anode to led_anode and Cathode to 0.

Note: S2 allows current to flow to the circuit. S1 initially energizes the coil. Once K1 energizes, the COM-NO internal connection bypasses S1, keeping the coil powered from the feed_line.

Conceptual block diagram

Schematic

Title: Practical case: Latching Alarm System

      (Main Power)
        VCC 12 V
           |
           V
  [ S2: Reset (NC) ]
           |
      (feed_line)
           |
           |    (Path A: Manual Trigger)
           +--> [ S1: Trigger (NO) ] ------------------+
           |                                           |
           |                                           V
           |                                     (latch_node)
           |                                           |
           |    (Path B: Self-Latching)                +----------> [ R1: 1k ] --> [ D1: LED ] --> GND
           +--> [ K1: Contact (NO) ] ------------------+           (Visual Alarm)
                         ^                             |
                         |                             |
                         |                             +----------> [ K1: Coil || D2(Rev) ] --> GND
                         |                                         (Relay Magnet & Protection)
                         |                                                  |
                         +----------------(Magnetic Link)-------------------+

Electrical diagram

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Measurements and tests

Follow these steps to validate the latching behavior:

1. Coil Continuity Check: Before applying power, use a multimeter in Ohms mode to measure the relay coil pins. You should read a resistance value (typically 100 Ω to 400 Ω depending on the relay).
2. Standby Check: Power on the circuit. Measure voltage between latch_node and 0. It should be 0 V. The LED should be OFF.
3. Trigger Test: Press and hold S1. Measure voltage at latch_node. It should rise to approx 12 V. The LED should turn ON.
4. Latching Test: Release S1. The voltage at latch_node must remain at 12 V, and the LED must stay ON. Listen for the relay; it should not click off.
5. Reset Test: Press S2 (Reset). The voltage at latch_node should drop to 0 V instantly. The LED turns OFF. Release S2; the LED remains OFF.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Latching Alarm System
.width out=256
* Based on Practical Breadboard Case

* ==========================================
* Power Supply
* ==========================================
* V1: 12V Main Supply
V1 VCC 0 DC 12

* ==========================================
* Control Inputs (User Stimuli)
* ==========================================
* S1 (Trigger Button - NO): 
* Simulating a press (Close) at 10ms for 5ms duration.
* Pulse: 0V (Open) -> 5V (Closed) -> 0V (Open)
V_S1_ctrl ctrl_s1 0 PULSE(0 5 10m 100u 100u 5m 100m)

* S2 (Reset Button - NC):
* Simulating a press (Open) at 40ms for 5ms duration.
* ... (truncated in public view) ...

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* Latching Alarm System
.width out=256
* Based on Practical Breadboard Case

* ==========================================
* Power Supply
* ==========================================
* V1: 12V Main Supply
V1 VCC 0 DC 12

* ==========================================
* Control Inputs (User Stimuli)
* ==========================================
* S1 (Trigger Button - NO): 
* Simulating a press (Close) at 10ms for 5ms duration.
* Pulse: 0V (Open) -> 5V (Closed) -> 0V (Open)
V_S1_ctrl ctrl_s1 0 PULSE(0 5 10m 100u 100u 5m 100m)

* S2 (Reset Button - NC):
* Simulating a press (Open) at 40ms for 5ms duration.
* Pulse: 5V (Closed/Idle) -> 0V (Open/Pressed) -> 5V (Closed/Idle)
* Note: This voltage represents the connectivity state (High = Conducting).
V_S2_ctrl ctrl_s2 0 PULSE(5 0 40m 100u 100u 5m 100m)

* ==========================================
* Circuit Components
* ==========================================

* S2: Reset Switch (NC)
* Connects VCC to feed_line.
* Controlled by ctrl_s2 (Active High logic for NC behavior).
S2 VCC feed_line ctrl_s2 0 SW_IDEAL

* S1: Trigger Switch (NO)
* Connects feed_line to latch_node.
* Controlled by ctrl_s1 (Active High logic for NO behavior).
S1 feed_line latch_node ctrl_s1 0 SW_IDEAL

* K1: Relay Implementation
* 1. Coil: Modeled as Inductance + Series Resistance
*    Connects latch_node to Ground (0).
*    100 Ohm resistance is typical for a 12V relay coil.
R_coil latch_node k1_internal 100
L_coil k1_internal 0 10m

* 2. Relay Contact (Switch):
*    Connects feed_line (COM) to latch_node (NO).
*    Controlled by the voltage across the coil (latch_node).
*    Threshold set to 6V (Pull-in) with hysteresis.
S_relay feed_line latch_node latch_node 0 SW_RELAY

* D2: Flyback Protection Diode
* Cathode to latch_node, Anode to 0.
D2 0 latch_node 1N4007

* Alarm Indicator (LED + Resistor)
* R1: Current limiting
R1 latch_node led_anode 1k
* D1: Red LED
D1 led_anode 0 LED_RED

* Floating Node Prevention
* High impedance pull-down for feed_line when S2 opens
R_float feed_line 0 100Meg

* ==========================================
* Models
* ==========================================
* Ideal switch for buttons (Vt=2.5V logic threshold)
.model SW_IDEAL SW(Vt=2.5 Ron=0.01 Roff=100Meg)

* Relay switch model (Picks up at 6V, drops out at 4V)
.model SW_RELAY SW(Vt=6 Vh=2 Ron=0.01 Roff=100Meg)

* 1N4007 Diode Model
.model 1N4007 D(Is=7n Rs=0.04 N=1.5 Cjo=20p BV=1000 IBV=5u)

* Generic Red LED Model
.model LED_RED D(Is=1a N=4 Rs=4)

* ==========================================
* Simulation Directives
* ==========================================
* Transient analysis: 100us step, 60ms total time
* Covers Trigger (10ms) and Reset (40ms) events
.tran 100u 60m

* Output variables
* V(latch_node) is the ALARM STATE (Output)
* V(feed_line) shows power delivery
.print tran V(latch_node) V(feed_line) V(ctrl_s1) V(ctrl_s2) I(L_COIL)

.op
.end

Simulation Results (Transient Analysis)

Analysis: The simulation accurately demonstrates the latching logic. At 10ms, the trigger pulse (S1) energizes the coil, causing ‘latch_node’ to rise to ~12V. The circuit successfully latches, maintaining 12V output after S1 opens. At 40ms, the reset pulse (S2) cuts power, dropping ‘latch_node’ to ~0V, where it remains even after S2 closes again.

Show raw data table (2796 rows)

Index   time            v(latch_node)   v(feed_line)    v(ctrl_s1)      v(ctrl_s2)      l_coil#branch
0	0.000000e+00	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
1	1.000000e-06	2.399953e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
2	2.000000e-06	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
3	4.000000e-06	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
4	8.000000e-06	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
5	1.600000e-05	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
6	3.200000e-05	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
7	6.400000e-05	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
8	1.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
9	2.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
10	3.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
11	4.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
12	5.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
13	6.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
14	7.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
15	8.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
16	9.280000e-04	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
17	1.028000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
18	1.128000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
19	1.228000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
20	1.328000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
21	1.428000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
22	1.528000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
23	1.628000e-03	2.399952e-05	1.200000e+01	0.000000e+00	5.000000e+00	2.399952e-07
... (2772 more rows) ...

Common mistakes and how to avoid them

1. Using a Normally Open button for Reset: If S2 is NO instead of NC, the circuit will never receive power to start. Ensure S2 conducts current by default.
2. Connecting the latch to NC contact: If you connect the latch_node to the Relay’s NC pin instead of NO, the relay will turn on immediately upon power-up and oscillate or buzz (buzzer effect). Always use the NO pin for self-latching.
3. LED burns out immediately: Forgetting R1 allows excessive current through the LED. Always verify the resistor value before powering up.

Troubleshooting

• Symptom: LED turns on when S1 is pressed but turns off immediately when released.
  • Cause: The latching path is broken.
  • Fix: Check the connection between K1 (COM), K1 (NO), and the latch_node. Ensure the relay contacts are parallel to S1.
• Symptom: Relay buzzes loudly or chatters.
  • Cause: Power supply voltage is too low or unstable.
  • Fix: Ensure V1 provides stable 12 V and can supply enough current for the coil.
• Symptom: Circuit cannot be reset.
  • Cause: S2 is bypassed or faulty (shorted).
  • Fix: Check S2 with a multimeter; it must break the connection (Open) when pressed.

Possible improvements and extensions

1. Audible Alarm: Connect a 12 V active buzzer in parallel with the LED (between latch_node and 0) to add sound to the alarm.
2. High Power Control: Use a DPDT relay. Use the first set of contacts for the 12 V logic latching (as described above) and the second set of contacts to switch a completely separate high-voltage load, like a 120 V/230 V lamp.

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

Level: Basic. Learn how to isolate a low-power control signal from a high-power motor circuit using an electromagnetic relay.

Objective and use case

In this practical case, you will build a circuit that utilizes a small pushbutton and a relay to control a high-current DC motor. The relay acts as an electromagnetic switch, allowing the low-power control side to activate the high-power load side without direct electrical connection between the distinct power rails (if separate grounds are used) or simply to handle currents exceeding the switch’s rating.

Why it is useful:
* Automotive systems: Used in starter motors where a small ignition key switch triggers a massive solenoid (relay) to crank the engine.
* Industrial automation: Allows low-voltage PLCs (24 V) to switch high-voltage AC or DC motors (110 V/220 V) safely.
* Safety isolation: Keeps high voltages away from the user interface (buttons and switches).
* Component protection: Prevents burning out delicate switches by offloading the high current switching to the relay contacts.

Expected outcome:
* When the pushbutton is pressed, the relay makes an audible «click.»
* The DC motor starts spinning immediately upon the click.
* Voltage across the relay coil measures 5 V (or rated control voltage).
* The flyback diode protects the switch from high-voltage spikes when the button is released.

Target audience and level: Students and hobbyists understanding basic electromechanical switching (Basic).

Materials

• V1: 5 V DC voltage source, function: Control circuit power supply.
• V2: 12 V DC voltage source, function: Motor (Power) circuit supply.
• S1: Momentary Pushbutton (Normally Open), function: Control switch.
• K1: SPDT Relay (5 V Coil), function: Electromechanical isolation and switching.
• D1: 1N4007 Diode, function: Flyback/Freewheeling diode for coil protection.
• M1: 12 V DC Motor, function: High-power load.

Wiring guide

This guide uses SPICE-friendly node names to define the connections. The nodes are: V_CTRL (5 V), V_PWR (12 V), COIL_IN, MOTOR_IN, and 0 (Ground).

• V1 (Positive): Connects to node V_CTRL.
• V1 (Negative): Connects to node 0.
• V2 (Positive): Connects to node V_PWR.
• V2 (Negative): Connects to node 0.
• S1: Connects between node V_CTRL and node COIL_IN.
• K1 (Coil terminal A): Connects to node COIL_IN.
• K1 (Coil terminal B): Connects to node 0.
• D1 (Cathode/Striped side): Connects to node COIL_IN.
• D1 (Anode): Connects to node 0.
• K1 (Common/COM Contact): Connects to node V_PWR.
• K1 (Normally Open/NO Contact): Connects to node MOTOR_IN.
• M1 (Positive): Connects to node MOTOR_IN.
• M1 (Negative): Connects to node 0.

Note: In a physical application requiring galvanic isolation, the ground 0 for the control side (V1) and the power side (V2) would be kept separate. For this basic simulation model, they share a common reference.

Conceptual block diagram

Schematic

+---------------------------------------------------------------------------------------------------------+
|                          DC MOTOR CONTROL WITH RELAY (UNIFIED DIAGRAM)                                  |
+---------------------------------------------------------------------------------------------------------+

      (High Power Loop: 12 V)
      [ V2: 12 V Source ] --(Node: V_PWR)--> [ K1: Relay Switch (COM->NO) ] --(Node: MOTOR_IN)--> [ M1: 12 V Motor ] --> [ GND ]
                                                        ^
                                                        |
                                                 (Magnetic Link)
                                                        |
      (Control Loop: 5 V)                                |
      [ V1: 5 V Source ] --(Node: V_CTRL)--> [ S1: Pushbutton ] --(Node: COIL_IN)--> [ Parallel: K1 Coil || D1 (Rev) ] --> [ GND ]

+---------------------------------------------------------------------------------------------------------+
|  LEGEND & NOTES:                                                                                        |
|  -->  : Signal/Power Flow                                                                               |
|  ||   : Components in Parallel (Coil and Diode share Node COIL_IN and GND)                              |
|  Rev  : Diode D1 is Reverse Biased (Cathode to COIL_IN, Anode to GND) to suppress flyback voltage.      |
|  Link : The current in the Control Loop generates the magnetic field to close the Switch in the Power Loop. |
+---------------------------------------------------------------------------------------------------------+

Electrical diagram

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Measurements and tests

Follow these steps to validate your circuit assembly:

1. Coil Voltage Check:

   • Set your multimeter to DC Voltage (20 V range).
   • Connect probes across the relay coil terminals (COIL_IN and 0).
   • Press S1. The reading should jump from 0 V to approx. 5 V.

2. Audible Confirmation:

   • Press and release S1. Listen for the mechanical «click» of the relay armature moving. If you do not hear it, the coil is not energizing.

3. Load Voltage Verification:

   • Connect the multimeter across the motor terminals.
   • Press S1. The multimeter should read approx. 12 V (voltage of V2) and the motor should spin.
   • Release S1. The voltage should drop to 0 V and the motor should coast to a stop.

4. Flyback Diode Test (Advanced):

   • Without D1, monitoring COIL_IN with an oscilloscope would reveal a large negative voltage spike when S1 is released. With D1 installed, this spike is clamped to approx. -0.7 V, protecting S1.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: DC motor control with relay and pushbutton

* --- Models ---
* Generic Pushbutton Switch Model (Voltage Controlled)
.model SW_PB SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)
* Relay Contact Switch Model (Controlled by Coil Voltage)
.model SW_RELAY SW(Vt=3.5 Vh=0.2 Ron=0.05 Roff=10Meg)
* 1N4007 Diode Model
.model D1N4007 D(IS=7.07e-9 RS=0.034 N=1.7 BV=1000 IBV=5e-6 CJO=1e-11 TT=1e-7)

* --- Power Supplies ---
* V1: Control Circuit Power (5V)
V1 V_CTRL 0 DC 5
* V2: Motor Circuit Power (12V)
V2 V_PWR 0 DC 12

* --- Control Circuit (Input) ---
* S1: Pushbutton.
* Modeled as a voltage-controlled switch driven by a PULSE source (V_ACT)
* to simulate the physical act of pressing the button.
* ... (truncated in public view) ...

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* Practical case: DC motor control with relay and pushbutton

* --- Models ---
* Generic Pushbutton Switch Model (Voltage Controlled)
.model SW_PB SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)
* Relay Contact Switch Model (Controlled by Coil Voltage)
.model SW_RELAY SW(Vt=3.5 Vh=0.2 Ron=0.05 Roff=10Meg)
* 1N4007 Diode Model
.model D1N4007 D(IS=7.07e-9 RS=0.034 N=1.7 BV=1000 IBV=5e-6 CJO=1e-11 TT=1e-7)

* --- Power Supplies ---
* V1: Control Circuit Power (5V)
V1 V_CTRL 0 DC 5
* V2: Motor Circuit Power (12V)
V2 V_PWR 0 DC 12

* --- Control Circuit (Input) ---
* S1: Pushbutton.
* Modeled as a voltage-controlled switch driven by a PULSE source (V_ACT)
* to simulate the physical act of pressing the button.
* Wiring: Connects V_CTRL to COIL_IN.
V_ACT ACT_NODE 0 PULSE(0 5 10m 1u 1u 245m 1s)
S1 V_CTRL COIL_IN ACT_NODE 0 SW_PB

* K1: Relay Coil
* Wiring: Coil Terminal A to COIL_IN, Coil Terminal B to 0.
* Modeled as Inductor + Resistor in series.
R_K1_COIL COIL_IN K1_INT 60
L_K1_COIL K1_INT 0 100m

* D1: Flyback Diode
* Wiring: Cathode to COIL_IN, Anode to 0.
* SPICE Syntax: D   
D1 0 COIL_IN D1N4007

* --- Power Circuit (Output) ---
* K1: Relay Contact (Switch)
* Wiring: Common (COM) to V_PWR, Normally Open (NO) to MOTOR_IN.
* Controlled by the voltage at node COIL_IN.
S_K1_SW V_PWR MOTOR_IN COIL_IN 0 SW_RELAY

* M1: DC Motor
* Wiring: Positive to MOTOR_IN, Negative to 0.
* Modeled as an RL load (Resistance + Inductance).
R_M1 MOTOR_IN M1_INT 20
L_M1 M1_INT 0 10m

* --- Analysis Directives ---
.op
.tran 0.1m 250m

* --- Output Printing ---
* Must define INPUT (COIL_IN) and OUTPUT (MOTOR_IN)
.print tran V(COIL_IN) V(MOTOR_IN) V(ACT_NODE) I(L_M1)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows the control signal (V_ACT) going high at 10ms. Consequently, the coil voltage (V(COIL_IN)) rises to ~5V. This triggers the relay switch, causing the motor input voltage (V(MOTOR_IN)) to jump from near 0V to ~12V, and current flows through the motor load.

Show raw data table (2535 rows)

Index   time            v(coil_in)      v(motor_in)     v(act_node)     l_m1#branch
0	0.000000e+00	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
1	1.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
2	2.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
3	4.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
4	8.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
5	1.600000e-05	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
6	3.200000e-05	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
7	6.400000e-05	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
8	1.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
9	2.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
10	3.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
11	4.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
12	5.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
13	6.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
14	7.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
15	8.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
16	9.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
17	1.028000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
18	1.128000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
19	1.228000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
20	1.328000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
21	1.428000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
22	1.528000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
23	1.628000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
... (2511 more rows) ...

Common mistakes and how to avoid them

1. Omitting the Flyback Diode (D1):

   • Error: Leaving out the diode across the relay coil.
   • Consequence: The collapsing magnetic field generates a high-voltage spike (back EMF) that can arc across the switch contacts or destroy transistor drivers in future circuits.
   • Solution: Always install a diode in reverse bias (Cathode to positive) across inductive loads.

2. Using the Wrong Relay Contacts (NC vs NO):

   • Error: Connecting the motor to the Normally Closed (NC) pin instead of Normally Open (NO).
   • Consequence: The motor runs continuously when the button is not pressed and stops when it is pressed.
   • Solution: Identify the NO pin using the datasheet or a continuity test before soldering.

3. Mixing Power Rails:

   • Error: Connecting the 12 V motor supply directly to the 5 V coil.
   • Consequence: The relay coil will overheat and likely burn out due to over-voltage.
   • Solution: Ensure the coil voltage matches the control supply (V1) and the contact rating matches the motor supply (V2).

Troubleshooting

• Symptom: Relay clicks, but motor does not run.

  • Cause: Burnt relay contacts or loose wire between COM/NO and the motor.
  • Fix: Check continuity across COM and NO while the relay is held active.

• Symptom: Relay does not click when S1 is pressed.

  • Cause: Coil wiring error or S1 is faulty.
  • Fix: Measure voltage at the coil terminals while pressing S1. If 0 V, check S1.

• Symptom: Circuit resets or sparks occur at S1.

  • Cause: Lack of flyback diode causing arcing.
  • Fix: Install D1 immediately across the coil terminals.

Possible improvements and extensions

1. Transistor Driver: Replace the direct pushbutton connection with an NPN transistor (e.g., 2N2222) to control the relay using a weak signal from an Arduino or microcontroller.
2. Self-Latching Circuit: Add a second relay contact or wire the relay in a «latching» configuration with a separate «Stop» button (NC), so you don’t have to hold S1 to keep the motor running.

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

Level: Basic. Demonstrate how moving a magnet through a coil generates an electromotive force (EMF).

Objective and use case

In this practical case, you will construct a fundamental electromagnetic induction demonstrator using a hand-wound copper coil and a high-strength neodymium magnet. You will observe how kinetic energy is converted into electrical energy via Faraday’s Law of Induction.

Why it is useful:
* Power Generation: This mechanism illustrates the core principle behind electric generators, alternators, and wind turbines.
* Audio Technology: This is the operating principle for dynamic microphones and electric guitar pickups (transducers).
* Sensors: Used in automotive ABS speed sensors and industrial position sensors.
* Wireless Charging: Demonstrates the basics of magnetic coupling used in phone chargers.

Expected outcome:
* A measurable voltage spike (positive or negative) on the multimeter when the magnet moves relative to the coil.
* The LED flashes briefly when the magnet is moved rapidly, indicating a voltage peak exceeding the diode’s forward voltage (~1.8 V).
* Reversing the direction of the magnet’s movement reverses the polarity of the induced voltage.

Target audience: Students and hobbyists introducing themselves to Faraday’s Law and passive components.

Materials

• L1: Air core coil (approx. 500–1000 turns of enameled copper wire), function: induction element.
• MAG1: Cylindrical Neodymium magnet (fit to pass inside L1), function: source of magnetic flux.
• D1: Red LED, function: indicator for positive phase induction.
• D2: Green LED, function: indicator for negative phase induction (connected in anti-parallel).
• M1: Multimeter (set to 200 mV or 2 V DC range), function: voltage monitor.

Wiring guide

The circuit consists of the coil connected directly to the indicators in parallel. We define the coil terminals as nodes COIL_A and COIL_B.

• L1: Connects between node COIL_A and node COIL_B.
• D1: Anode connects to COIL_A; Cathode connects to COIL_B.
• D2: Anode connects to COIL_B; Cathode connects to COIL_A (anti-parallel to D1).
• M1: Positive probe connects to COIL_A; Negative probe connects to COIL_B.

Conceptual block diagram

Schematic

markdown
Title: Practical case: Voltage induction by magnetic movement

[ INPUT / SOURCE ]                       [ DISTRIBUTION RAILS ]                    [ OUTPUT / LOADS ]

                                                 (Node A: Top Rail)
                                    /------------------------------------------------------------------>
                                    |                |                    |                    |
[ MAG1: Magnet ] --(Flux)--> [ L1: Coil ]            | (Anode)            | (Cathode)          | (+)
                                    |                v                    v                    v
                                    |        [ D1: Red LED ]      [ D2: Grn LED ]      [ M1: Meter ]
                                    |        (Lights if A > B)    (Lights if B > A)    (Monitor V)
                                    |                |                    |                    |
                                    |                | (Cathode)          | (Anode)            | (-)
                                    \                v                    v                    v
                                    \------------------------------------------------------------------>
                                                 (Node B: Bottom Rail)

Electrical diagram

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Measurements and tests

1. Static Test: Place the magnet inside the coil and hold it completely still. The multimeter should read 0 V, and no LEDs should light up. This confirms that a changing magnetic field is required.
2. Slow Insertion: Set the multimeter to the lowest DC voltage range (e.g., 200 mV). Slowly push the magnet into the coil. Observe a small voltage reading (e.g., +10 to +50 mV).
3. Fast Action: Quickly thrust the magnet into the coil. You should see a significantly higher voltage spike (potentially > 1 V) and D1 (Red) may flash briefly.
4. Reverse Motion: Quickly pull the magnet out of the coil. The voltage polarity on the multimeter will flip (negative sign), and D2 (Green) should flash.
5. Oscillation: Move the magnet back and forth rapidly inside the coil. The LEDs should flicker alternately, demonstrating the generation of Alternating Current (AC).

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Voltage induction by magnetic movement
.width out=256
*
* Description:
* Simulation of a magnet moving through a coil, inducing voltage to drive two antiparallel LEDs.
*
* Nodes:
* COIL_A : Hot terminal of the coil (Multimeter +)
* COIL_B : Reference terminal of the coil (Multimeter -, Grounded)
*
* Note: The physical "Coil" is modeled as a series combination of an EMF Voltage Source (V_MAG1),
* a Resistor (R_WIRE), and the Inductor (L1).

* --- Power / Reference ---
* Grounding COIL_B as per Multimeter negative probe convention
V_REF COIL_B 0 0

* --- Magnetic Induction Source (MAG1) ---
* Simulating the changing magnetic flux from MAG1 as an AC voltage source.
* 3V Peak, 5Hz (Simulates shaking the magnet)
* ... (truncated in public view) ...

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* Practical case: Voltage induction by magnetic movement
.width out=256
*
* Description:
* Simulation of a magnet moving through a coil, inducing voltage to drive two antiparallel LEDs.
*
* Nodes:
* COIL_A : Hot terminal of the coil (Multimeter +)
* COIL_B : Reference terminal of the coil (Multimeter -, Grounded)
*
* Note: The physical "Coil" is modeled as a series combination of an EMF Voltage Source (V_MAG1),
* a Resistor (R_WIRE), and the Inductor (L1).

* --- Power / Reference ---
* Grounding COIL_B as per Multimeter negative probe convention
V_REF COIL_B 0 0

* --- Magnetic Induction Source (MAG1) ---
* Simulating the changing magnetic flux from MAG1 as an AC voltage source.
* 3V Peak, 5Hz (Simulates shaking the magnet)
V_MAG1 N_EMF COIL_B SIN(0 3 5)

* --- Coil Assembly (L1) ---
* Internal wire resistance
R_WIRE N_EMF N_L1 5
* The physical inductance L1
L1 N_L1 COIL_A 10m

* --- Indicators ---
* D1: Red LED (Indicates Positive Phase)
* Anode: COIL_A, Cathode: COIL_B
D1 COIL_A COIL_B D_RED

* D2: Green LED (Indicates Negative Phase)
* Anode: COIL_B, Cathode: COIL_A
D2 COIL_B COIL_A D_GREEN

* --- Multimeter (M1) ---
* Modeled as the voltage difference V(COIL_A) - V(COIL_B)
* (Implicit in the node voltages)

* --- Models ---
* Generic LED Models
.model D_RED D(IS=1e-18 N=2 RS=10 BV=5)
.model D_GREEN D(IS=1e-18 N=2.5 RS=10 BV=5)

* --- Simulation Directives ---
.op
* Transient analysis: 1ms step, 500ms duration (2.5 cycles at 5Hz)
.tran 1m 500m

* --- Output ---
* Monitoring the induced voltage at COIL_A
.print tran V(COIL_A) I(L1)

Simulation Results (Transient Analysis)

Analysis: The transient analysis shows an AC voltage at COIL_A oscillating between approx +2.6V and -2.8V at 5Hz. Current flows through L1, peaking around 66mA. The voltage levels are sufficient to forward bias the LEDs (D_RED and D_GREEN) alternately, consistent with the intended indication of positive and negative phases.

Show raw data table (522 rows)

Index   time            v(coil_a)       l1#branch
0	0.000000e+00	4.375392e-35	-8.75078e-36
1	1.000000e-05	9.424778e-04	1.884985e-15
2	2.000000e-05	1.884955e-03	3.769970e-15
3	4.000000e-05	3.769910e-03	7.539938e-15
4	8.000000e-05	7.539814e-03	1.507987e-14
5	1.600000e-04	1.507958e-02	3.015936e-14
6	3.200000e-04	3.015878e-02	6.031856e-14
7	6.400000e-04	6.031451e-02	1.206316e-13
8	1.280000e-03	1.206046e-01	2.412214e-13
9	2.280000e-03	2.147012e-01	4.294658e-13
10	3.280000e-03	3.085859e-01	6.175653e-13
11	4.280000e-03	4.021661e-01	8.067202e-13
12	5.280000e-03	4.953494e-01	1.005111e-12
13	6.280000e-03	5.880438e-01	1.262566e-12
14	7.280000e-03	6.801579e-01	1.873422e-12
15	8.280000e-03	7.716008e-01	4.548512e-12
16	9.280000e-03	8.622822e-01	1.907006e-11
17	1.028000e-02	9.521126e-01	1.003825e-10
18	1.128000e-02	1.041003e+00	5.511221e-10
19	1.228000e-02	1.128867e+00	3.003086e-09
20	1.328000e-02	1.215616e+00	1.605415e-08
21	1.428000e-02	1.301164e+00	8.389370e-08
22	1.528000e-02	1.385424e+00	4.276266e-07
23	1.628000e-02	1.468291e+00	2.121308e-06
... (498 more rows) ...

Common mistakes and how to avoid them

1. Using weak magnets: Standard black ferrite magnets are often too weak to generate visible voltage on an LED. Solution: Use rare-earth Neodymium magnets.
2. Moving too slowly: Faraday’s Law (V = – N · d\Phi / dt) depends on the rate of change. Solution: Move the magnet as quickly as possible to maximize the voltage spike.
3. Insulation issues: Enameled wire has a clear coating that blocks electricity. Solution: Ensure the ends of the coil wire are sanded or scraped down to bare copper before connecting to the LEDs or multimeter.

Troubleshooting

• Symptom: Multimeter shows voltage, but LEDs never light up.
  • Cause: The induced voltage is lower than the LED forward voltage threshold (~1.8 V).
  • Fix: Add more turns to the coil (increase $N$) or move the magnet faster.
• Symptom: No reading on the multimeter even with fast movement.
  • Cause: Open circuit or poor connection at the coil tips.
  • Fix: Check continuity (resistance mode) across the coil terminals; it should read a few Ohms, not infinite.
• Symptom: Voltage reading is erratic or hard to see.
  • Cause: Digital multimeters have a slow sample rate.
  • Fix: Use the «Max/Min» hold function if available, or use an analog (needle) multimeter which responds better to transient pulses.

Possible improvements and extensions

1. Shake Flashlight: Add a bridge rectifier (4 diodes) and a large capacitor (e.g., 1000 µF) to store the energy generated by shaking the magnet, allowing the LED to stay lit for a few seconds after motion stops.
2. Core Comparison: Insert an iron bolt inside the coil (making it an iron-core inductor) and move a magnet near the head of the bolt to observe how the ferromagnetic core concentrates the magnetic flux and affects induction.

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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