Practical case: Simple RC timer

Materials

• 1 × 5 V DC supply (USB power bank or bench supply)
• 1 × Breadboard
• 8 × Jumper wires
• 1 × R1 = 220 kΩ (timing resistor)
• 1 × C1 = 10 µF electrolytic capacitor (≥10 V rating)
• 1 × SW1 momentary pushbutton (to reset/discharge)
• 1 × Q1 = 2N7000 NMOS transistor
• 1 × D1 = Red LED
• 1 × R2 = 1 kΩ (LED series resistor)
• 1 × Multimeter (DMM)
• 1 × Oscilloscope (optional)

Wiring guide

• Power rails:
• Connect the supply +5 V to the breadboard’s positive rail and GND to the ground rail.
• RC timing node (abbreviation V_C):
• R1 from +5 V rail to the timing node (V_C).
• C1 from V_C to GND (observe polarity: C1’s negative lead to GND).
• SW1 from V_C to GND (when pressed, it quickly discharges C1 to reset the timer).
• MOSFET switch and LED:
• R2 from +5 V rail to D1 anode.
• D1 cathode to Q1 drain.
• Q1 source to GND.
• Q1 gate to V_C (the RC timing node).
• Abbreviations used on the schematic:
• V_C: Voltage at the RC timing node (measure relative to GND).
• V_D: Voltage at the LED cathode / MOSFET drain node (measure relative to GND).
• 2N7000 pinout note:
• Looking at the flat face with leads downward, typical pin order is G–D–S (left to right). Verify with your part’s datasheet.

Schematic

                 +5 V
                  │
                ┌┴┐            R1 = 100 Ω (limitación de carga)
                │ │
                │ │
                └┬┘
                  │
                [ S1 ]         S1 = Pulsador NO (cargar)
                  │
                  ├───────────────● V_C
                  │               │
                  │              ┌┴┐
                  │              │ │   C1 = 100 µF 16 V
                  │              │ │   (electrolítico)
                  │              └┬┘
                  │               │
                  │               │
                  │              GND


                  └───────────────┬─────────────────────────────
                                  │
                                 ┌┴┐
                                 │ │   R2 = 330 Ω (LED)
                                 │ │
                                 └┬┘
                                  │
                                  │      ● V_LED+
                                  │     ┌┴┐
                                  │     │ │   LED1 rojo (ánodo arriba)
                                  │     │ │
                                  │     └┬┘
                                  │      ● V_LED−
                                  │       │
                                  │       │
                                  │      GND

Measurements and tests

• Power-up behavior:
  • Press and release SW1 to reset, then observe: the LED turns on after a short delay that depends on R1 and C1 (approximately when V_C passes Q1’s gate threshold).
• V_C (timing node):
  • Place the DMM black probe on GND and the red probe on the dot labeled V_C.
  • Expect an exponential rise: V_C(t) ≈ 5 V × (1 − e^(−t/τ)), with τ = R1 × C1.
  • With an oscilloscope, connect the probe tip to ● V_C and ground clip to GND to see the charging curve.
• V_D (LED/drain node):
  • Place the DMM black probe on GND and the red probe on the dot labeled V_D.
  • Before Q1 turns on, V_D ≈ 5 V (no current through LED). When Q1 turns on, V_D drops near 0 V and the LED lights.
• Delay time estimation:
  • The LED turns on when V_C ≈ V_GS(th) of the 2N7000 (often ~2–3 V). Rough estimate:
    • t_on ≈ −τ × ln(1 − V_GS(th)/5 V)
    • Example with τ = 2.2 s and V_GS(th) = 2.5 V ⇒ t_on ≈ 1.6 s.
• Adjustments:
  • Increase delay: increase R1 or C1.
  • Brighter LED: decrease R2 (e.g., to 680 Ω), but keep LED current reasonable.

Common mistakes

• LED polarity reversed (anode must face +5 V through R2; cathode toward Q1).
• Electrolytic capacitor reversed (C1 negative to GND).
• Wrong MOSFET pinout (ensure gate to V_C, drain to LED cathode, source to GND).
• Missing ground reference during measurements (always reference V_C and V_D to GND).
• Not providing a discharge/reset path: without SW1, you must wait for C1 to leak down before retriggering.

Safety and good practices

• Work at 5 V; avoid higher voltages with small LEDs and breadboards.
• Start with longer delays (larger R1 or C1) to make behavior easy to observe.
• If the LED stays dimly lit, your Q1 may not be fully turning on; verify V_C and consider a slightly lower R2 or a 2N7000 with lower threshold.

Improvements and variations

• Add a second RC and NMOS to create both delay-on and delay-off channels.
• Replace Q1 with a logic-level NPN BJT plus a base resistor; the RC then drives the base.
• Add a Schmitt trigger (74HC14) between V_C and the MOSFET gate for a cleaner, more defined switching threshold.

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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