Exercises: Electronic components

On a circuit board, five electronic components numbered from 1 to 5 are observed:

1. A small cylinder with colored bands.

2. A cylinder with two leads and a side stripe indicating the negative pole.

3. A transparent component with two leads that emits light when current passes through it.

4. A component with three leads and a black plastic body.

5. A black rectangular package with two parallel rows of metal pins.

Identify each element and briefly explain what function it performs within a circuit.

Explain what function a resistor serves and what function an LED serves in a circuit, and explain why an LED is almost never connected alone, but rather with a resistor in series.

The figure shows the symbols of several electronic components. Name the three marked with ? and state what function each serves in a circuit.

Match each component to its function: battery, switch, buzzer, motor and light-dependent resistor (LDR). Functions: supply energy to the circuit, allow or block current flow, convert electricity into sound, convert electricity into motion, and change its resistance depending on light.

Explain what polarity is in a battery and in an LED. What happens if an LED is connected backwards in a circuit?

Draw in your notebook (or describe here) the symbol for a battery, a switch and a bulb, and set up with them the simplest circuit that lights the bulb only when the switch is closed.

You can do it on paper and send a photo with your answer.

Explain what a short circuit is and why it is dangerous to connect the two terminals of a battery directly with a wire.

Name three electronic components in your home (you can think of toys, remote controls or household appliances), state which appliance they are in and what function you think they serve.

Order these elements from lowest to highest opposition to current flow and justify the order: a copper wire, a $1\ \text{k}\Omega$ resistor and an open switch.

A capacitor can store electrical charge and release it gradually afterwards. Explain, using the example of a camera flash or a light that dims slowly, what this property is used for.

A transistor is said to work as a current-controlled switch. Explain this idea using the example of a circuit that turns on an LED when it gets dark: a light sensor (LDR) controls the base of the transistor.

A red LED and a blue LED do not operate at the same voltage. Explain why this matters when designing the circuit and what can happen if both are powered using the same resistor calculated for only one of them.

Explain the difference between a push button (only makes contact while pressed) and a switch (remains in position), and give a suitable use for each in the classroom or at home.

An LDR has high resistance when there is little light and low resistance when there is a lot of light. Reason how to use it so that a circuit "knows" whether it is day or night, indicating what would happen to the current in each case.

The buzzer in a circuit setup sounds very faint. Give two possible explanations related to the components (battery almost flat, series resistor too high, reversed polarity) and how you would check each one.

Explain what a potentiometer is and how it allows you to adjust the volume of a loudspeaker or the brightness of a lamp without changing any circuit components.

To make a car's interior light stay on for a few seconds after closing the door (opening the switch contact), a person connects a $2200\ \mu\text{F}$ capacitor directly in parallel with a $12\ \text{V}$, $6\ \text{W}$ light bulb. When testing it, they find that the bulb turns off almost instantaneously.

1) Justify by calculating the bulb's resistance and the time constant $\tau = R \cdot C$ why their circuit fails.

2) Propose and describe a modification to the circuit using an NPN transistor and an additional resistor, technically justifying how this new configuration does allow prolonging the illumination for several seconds with that same capacitor.

You have a $9\ \text{V}$ battery and two identical LEDs whose technical specifications are: forward voltage $V_F = 2,2\ \text{V}$ and nominal operating current $I_F = 20\ \text{mA}$. You want to connect them in series using a single current-limiting resistor so that they operate at their nominal current. In the workshop, you have standard commercial resistors of $150\ \Omega$, $220\ \Omega$, $330\ \Omega$, and $470\ \Omega$.

1) Justify using Ohm's law what exact theoretical resistance is required and which of the available values you should choose to ensure maximum brightness without exceeding the safe current limit.

2) If someone mistakenly connects the two LEDs in series with the $150\ \Omega$ resistor, calculate the current that will flow and predict the thermal and operational consequences for the diodes.

You are assembling a circuit for a precision temperature sensor that requires a resistor whose actual value is strictly between $540\ \Omega$ and $600\ \Omega$; if the value falls outside this range, the sensor reading becomes uncalibrated. In the component drawer, you find a resistor with the color bands green - blue - brown - gold.

1) Interpret the color code indicating the nominal value of the resistor and its tolerance percentage.

2) Calculate the actual range of values (minimum and maximum) that this resistor can have in practice.

3) Reasonably justify whether it is technically safe to use this component in the circuit or if you should replace it with a higher-precision resistor (with a brown tolerance band, of $1\ \%$), arguing your decision.

The digital output of a school microcontroller delivers $5\ \text{V}$ with a safety current limit of only $20\ \text{mA}$. A student connects a small $5\ \text{V}$ direct current motor that draws $250\ \text{mA}$ at full load directly to that pin. Upon activation, the microcontroller freezes and turns off immediately.

1) Technically justify why it is not viable to power the motor directly from the digital output.

2) Design the necessary switching stage using an NPN transistor as a switch (indicating how to connect the base, collector, and emitter with respect to the digital pin, the motor, and the power supply).

3) Explain what function the base resistor serves and predict what electrical disaster would occur to the transistor when turning off the motor if a flyback diode is not placed in reverse parallel across its terminals.

You want to design timed staircase lighting at $12\ \text{V}$. When briefly pressing either of the two normally open push buttons (one on the ground floor and another on the first floor), a $12\ \text{V}$ lamp must turn on and stay lit for a few seconds after releasing the push button.

An inexperienced installer proposes connecting the two push buttons in series with each other and placing a large-capacity capacitor directly in parallel with the lamp.

1) Refute the two serious flaws in the installer's proposal (in relation to the turn-on logic of the push buttons and the discharge time of the capacitor through the filament).

2) Design and describe in detail the correct circuit using the two push buttons, an NPN transistor, the capacitor, a biasing resistor, and the lamp, justifying the function and connection of each element.

In a workshop practical, a team builds a direct current power supply by connecting a transformer and a diode bridge rectifier, obtaining at the output a pulsating DC voltage with a peak value of $18\ \text{V}$. To filter the ripple, they connect an electrolytic capacitor whose casing reads: $1000\ \mu\text{F}\ /\ 16\ \text{V}$, connecting the short lead (marked with a gray longitudinal stripe) to the positive terminal of the rectified output. A few seconds after turning on the power supply, the capacitor begins to hiss, heats up sharply, and bulges at the top.

Identify and justify in detail the two serious mistakes made in the assembly, explaining what physical or electrochemical phenomenon occurs inside the component in each case, and specify what minimum voltage and connection characteristics the new capacitor should have to operate completely safely.

To protect a $12\ \text{V}$ electronic device against accidental reverse polarity connection when connecting it to a battery, two alternative solutions based on diodes are proposed:

• Option A: Place a standard silicon diode in series in the positive supply line.

• Option B: Place an antiparallel diode (in reverse parallel with the load) right after a fast-acting fuse installed in the positive line.

1) Analyze how the diode acts in each of the two options, both in normal operation and in case of mistakenly reversing the battery terminals.

2) Justify which of the two solutions is more energy-efficient during normal operation (taking into account the $0,7\ \text{V}$ voltage drop of silicon) and explain what happens to the circuit and the fuse in option B in the event of reverse polarity.

El sistema de tracción de un coche teledirigido utiliza el circuito en puente en H mostrado en la figura, formado por cuatro transistores ($Q_1$, $Q_2$, $Q_3$ y $Q_4$), para controlar el sentido de giro de un motor de corriente continua desde una batería de $6\text{ V}$.

During testing, the car moves forward normally when the receiver activates the bases of $Q_1$ and $Q_4$ at the same time. However, when you press reverse (which should activate $Q_2$ and $Q_3$), the motor does not move and the battery starts to heat up immediately. When you inspect the circuit in that situation, you find that $Q_3$ gets very hot, while $Q_2$ and $Q_4$ stay cold.

1. a) Explain the working principle of the H-bridge from the diagram, deducing the direction in which current flows through the motor in each direction of travel (forward and reverse).

1. b) Formulate a justified hypothesis about which specific transistor has failed (becoming permanently short-circuited between collector and emitter) so that forward motion works normally, but reverse causes a direct short circuit across the battery.