Practical case: Smoothing brightness variations in a mini

Materials

• 1 × 3 V to 5 V source (for example, 2 AA cells in series or a regulated lab power supply).
• 1 × White LED (or any color).
• 1 × [R1] Current-limiting resistor for the LED:
• 220 Ω for 5 V
• 100 Ω for 3 V (if you use only 2×AA, for example).
• 1 × [C1] 470 µF to 1000 µF electrolytic capacitor, 10 V or more (polarized).
• 1 × Simple switch (SPST) or normally open pushbutton.
• 1 × Breadboard.
• 4–6 × Jumper wires (male-male).
• 1 × Digital multimeter (to measure voltages and, optionally, current).

In the text I will use 5 V and R1 = 220 Ω as an example. If you use 3 V, just adjust the value of R1 as indicated.

Wiring guide

• Connect the positive terminal of the 5 V supply to the first contact of the switch.
• Connect the second contact of the switch to the left end of [R1] 220 Ω.
• Connect the right end of [R1] 220 Ω to the VA node, which will be the common point of the mini lamp.
• Connect the anode (long lead) of the LED to node VA.
• Connect the cathode (short lead) of the LED to GND (negative of the supply).
• Connect the positive terminal of [C1] 1000 µF to node VA.
• Connect the negative terminal of [C1] 1000 µF to GND.
• Connect the negative of the supply (0 V) to the GND rail of the breadboard.

This VA node is where you will see the effect of the capacitor: there the resistor coming from the supply, the LED, and the capacitor in parallel with the LED are all connected.

Schematic

                    +5V
                    |
                 Interruptor
                    |
                 [R1] 220Ω
                    |
             o VA node (lámpara)
             /              \
        [D1] LED           [C1] 1000µF
        (ánodo)             (+)
          |                  |
          +------------------+
          |                  |
         GND                GND

Measurements and tests

• Basic operation check:

  • Close the switch: the LED should light up with normal brightness.
  • Open the switch: the LED should turn off smoothly, not instantly; it should take a fraction of a second.
  • Repeat several times: observe whether small flickers or very brief cuts that you would have without the capacitor disappear.

• Measuring the voltage on the LED (V_LED):

  • V_LED means “voltage between the anode and the cathode of the LED”.
  • Place the black probe of the multimeter on GND and the red probe on the LED anode (VA node).
  • Set the multimeter to DC voltmeter mode (a 20 V range is sufficient).
  • Close the switch: V_LED should be close to the typical LED voltage (≈2 V for red, ≈3 V for white).
  • Open the switch and observe V_LED: it should fall progressively (for example, from 3 V to 0 V in 0.2–0.5 s), not in an instant jump.

• Measuring the discharge time (t_discharge):

  • t_discharge is the time it takes for node VA (and therefore the LED) to drop from the initial value to almost 0 V after opening the switch.
  • To measure it “by eye”, count mentally from when you open the switch until the LED looks almost off.
  • To measure it more precisely, observe V_LED with the multimeter and time how long it takes to drop, for example, from 3 V to 1 V.
  • Try changing the value of [C1] (for example, 470 µF, 1000 µF, 2200 µF) and compare how t_discharge becomes longer or shorter.

• Measuring the LED current (I_LED) — optional:

  • I_LED means “current flowing through the LED”.
  • Disconnect the LED anode from node VA.
  • Set the multimeter to DC ammeter mode (starting from the highest range available).
  • Connect the red probe of the multimeter to node VA and the black probe to the LED anode.
  • Close the switch and note I_LED when the LED is stably on.
  • Open the switch and observe how I_LED gradually decreases as the capacitor discharges.

• Tests of smoothing against micro power cuts:

  • With the switch closed, move the switch slightly to force micro power cuts (or gently tap the battery holder if it is a bit loose).
  • Without the capacitor (C1 disconnected), you should notice more evident flickering.
  • With the capacitor (C1 connected), this flicker is reduced: the LED stays on more steadily, because the capacitor “fills in” short gaps in the power supply.

Simple explanation of how it works

• The LED by itself responds almost instantly to any voltage change: if the power drops, its brightness drops abruptly.
• Capacitor [C1] behaves like a small charge reservoir:
  • When the switch is closed, it charges to a voltage close to that of node VA (almost the same as the LED).
  • When the switch is opened, the source disappears, but the capacitor remains charged and begins to discharge through the LED.
• During discharge:
  • The voltage at VA does not drop to zero immediately; it decreases progressively.
  • The LED receives current from the capacitor and you see a smooth turn-off.
• The larger C1 is (in µF):
  • The longer it takes to discharge.
  • The slower the brightness change → it better smooths variations.

In very simple formulas, the typical discharge time (time constant) is:

[
\tau = R_{equivalente} \cdot C
]

Where ( R_{equivalente} ) is the resistance through which the capacitor discharges (in this case, the combination of R1 and the LED seen as a dynamic resistance, but at a basic level you can think “more resistance → slower discharge”).

Common mistakes

• Reversing the polarity of the electrolytic capacitor:
  • The terminal marked with “–” must always go to GND.
  • The “+” terminal must go to node VA (positive side).
  • If you connect it backwards, it can heat up, get damaged and even burst (risk).
• Not using a current-limiting resistor (R1):
  • Never connect the LED directly to 5 V with a large capacitor in parallel.
  • Without R1, the LED can burn out when charging/discharging the capacitor.
• Using too low a voltage rating for the capacitor:
  • Check that the working voltage of the capacitor (for example, 10 V) is higher than the voltage of your source (5 V).
• Expecting an “eternal turn-off” with a small capacitor:
  • With 10–47 µF the effect will be very short, almost imperceptible.
  • From 470–1000 µF the turn-off is clearly visible.

Safety and good practices

• Always disconnect the supply when you are going to change connections on the breadboard.
• Do not use capacitors that are visibly bulging, damaged or leaking.
• If an electrolytic capacitor gets very hot or emits a strange smell, turn off the supply and check:
  • Polarity.
  • Sufficient voltage rating.
  • Absence of short circuits.
• Do not use unnecessarily high voltages: for this experiment, 3–5 V is more than enough.

Possible improvements and variations

• Vary the value of C1:
  • Try 100 µF, 470 µF, 1000 µF, 2200 µF and compare the duration of the smooth turn-off.
• Add another LED in parallel:
  • Connect a second LED (with its own resistor) to node VA and observe how both turn off in a similar way.
• Use a pushbutton switch:
  • Hold it down to turn on the LED; when you release it, observe the “fade out” effect (gradual turn-off).
• Measure the voltage on the capacitor (at VA) over time:
  • You can record values every 0.1 s and then draw a small discharge curve, seeing that it is not a straight line but a smooth curve.

With this setup you will have experienced a very typical use of capacitors: smoothing voltage and current variations in a simple lighting circuit.

More Practical Cases at Prometeo.blog

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

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