Exercises: Electronic components

The figure shows the symbols of six basic electronic components with their names, except for one that is covered.

The covered component is a resistor whose value depends on the light it receives; look at the arrows pointing towards the symbol. Name it, explain what happens to its value in bright light, and give an example of a device that uses it.

Explain the purpose of a fixed resistor in a circuit and reason what happens to the brightness of an LED if, keeping the same battery, you replace the resistor with one of a higher value.

Resistors do not have their value printed with numbers, but with colored bands. Explain what the color code is, why it is used instead of numbers, and what the first three bands indicate.

Topic challenge. Match each electronic component with its function in a circuit and justify one of your choices.

Components:

1. a) Capacitor

1. b) Diode

1. c) Relay

1. d) Transistor

Functions (in random order):

1. Switch controlled by a very small current that can also amplify it.

2. Lets current pass in one direction only and blocks it in the opposite direction.

3. Switch operated by an electromagnet, that is, by another current independent of the one it switches.

4. Stores electric charge and returns it to the circuit later.

What you must do:

Write the pairs in the format a) → number, b) → number, c) → number and d) → number. Then choose one of them and explain in your own words why that function corresponds to that component, giving an example of everyday use.

You will be assessed on:

• All four pairs being correct.

• The justification of the chosen pair being clear and well reasoned.

• Including an example of a real application.

Hint. Pay attention to the key words: "one direction only", "stores", "electromagnet" and "small current". The relay and the transistor are similar, but the relay uses an electromagnet and the transistor is a semiconductor component with no moving parts.

Explain what a potentiometer is and name two everyday devices where it is used, stating what is adjusted in each case.

Hint. Think about what happens to the electrical resistance when you turn or slide the control of the component.

The LDR and the NTC are variable resistors, but automatic ones: their value is not changed by hand, but depends on a magnitude of the environment.

1. a) Explain which physical magnitude changes the value of the LDR and which magnitude changes the value of the NTC.

1. b) State how the resistance of each one varies when that magnitude increases.

1. c) Give an example of everyday use for each of them.

The LED, like every diode, has polarity. Explain what happens if you connect it the wrong way round in a circuit, how to tell its positive leg from its negative one, and why it is a good idea to check this before assembling.

State whether these statements are true or false and justify each answer: (a) the diode conducts current in both directions; (b) the capacitor stores energy in the form of charge; (c) the relay allows a low-power circuit to control a high-power one; (d) the transistor is only used to amplify sound.

Explain the colour code method with a worked example of your own: a resistor with brown, black, and red bands. The first two bands are digits (brown $= 1$, black $= 0$) and the third is the multiplier (red $= \cdot 100$). Calculate its value in ohms and express it in kilohms.

Calculate the value of a resistor with red, red and brown bands (red $= 2$, brown multiplier $= \cdot 10$), explaining each step, and then state which bands a $560\ \Omega$ resistor would have using the table: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9.

The figure shows an actual resistor with its four colour bands.

The bands are red, violet, orange, and silver. Read its value using the code: the first two bands are digits (red $= 2$, violet $= 7$), the third is the multiplier (orange $= \cdot 1.000$), and the fourth is the tolerance (silver $= \pm 10\ \%$). Calculate the nominal value in ohms and kilohms, and the actual range within which the resistor's value can lie.

Reverse the code process: write the four bands (two digits, multiplier, and gold tolerance of $\pm 5\ \%$) for a $4{,}7\ \text{k}\Omega$ resistor, explaining how you break down the value $47 \cdot 100\ \Omega$.

A $100\ \Omega$ resistor has a gold tolerance of $\pm 5\ \%$. Calculate the actual range of values it can have, explain what tolerance means for the manufacturer, and reason whether this margin matters much in a torch LED or in the clock of a precision device.

The figure shows a transistor working as a switch: a small current arrives through one pin and controls a much larger one that powers an LED.

The pin hidden by "?" is the control pin, the one that decides whether or not the transistor lets current flow between the other two (C, collector, and E, emitter).

1. a) Name that control pin.

1. b) Explain the tap analogy: in a tap, a small turn of the handle controls a large flow of water. Indicate which part of the transistor would correspond to the handle, which to the flow of water, and what happens in the LED.

1. c) Explain why the transistor is said to work as a switch.

Explain the charging and discharging of a capacitor using the analogy of a tank filling and emptying, and reason qualitatively why it is used to create time delays: what happens to the brightness of an LED connected to a discharging capacitor.

Car headlights consume a lot of current and are turned on using a small switch on the dashboard. Explain how a relay solves this problem (the switch activates a coil that closes another high-power circuit), which two circuits are kept separated, and what safety advantage this separation provides.

In the workshop drawer there are $220\ \Omega$, $470\ \Omega$, $1\ \text{k}\Omega$, $4{,}7\ \text{k}\Omega$, and $10\ \text{k}\Omega$ resistors. Choose one for each case and justify it: (a) protecting a $20\ \text{mA}$ LED with a $6\ \text{V}$ battery; (b) a voltage divider with an LDR that requires values similar to those of the LDR itself in medium light.

In the workshop, you need a resistor for a precision circuit that requires a value between $4500\ \Omega$ and $4900\ \Omega$. In a drawer, you find a resistor with the following colour bands: yellow, violet, red, and gold.

1. a) Determine its nominal value and calculate the actual range of resistance (minimum and maximum values) it can have due to its tolerance (gold: $\pm 5\ \%$).
2. b) Justify reasonably whether you can use it completely safely in the circuit or if there is a risk that it falls outside the required range.
3. c) When measured with the digital multimeter, the screen reads $4{,}48\ \text{k}\Omega$. Is the resistor defective or is it within the tolerance guaranteed by the manufacturer? Reason your answer with the necessary calculations.

A student wants to regulate the brightness of a red LED ($V_{\text{LED}} = 2\ \text{V}$, $I_{\text{max}} = 20\ \text{mA}$) connected to a $9\ \text{V}$ battery, and decides to connect only the battery, the LED, and a $0$ to $1\ \text{k}\Omega$ potentiometer in series.

1. a) Predict what will happen to the LED if the potentiometer knob is turned to the minimum resistance limit ($0\ \Omega$). Justify it by calculating the current that would flow according to Ohm's law.
2. b) Redesign the circuit by adding a fixed protective resistor in series. Calculate the minimum ohmic value this resistor must have so that the current never exceeds the $20\ \text{mA}$ permissible for the LED.
3. c) With that calculated fixed resistor, determine what the minimum current (dimmest brightness) will be when the potentiometer is set to its maximum value ($1\ \text{k}\Omega$).

You want to build a frost warning device for a greenhouse that turns on an alarm LED when the temperature drops to $0\ ^\circ\text{C}$. You have a $9\ \text{V}$ battery, an NPN transistor (which begins to conduct when the base-emitter voltage $V_{\text{BE}}$ reaches $0{,}7\ \text{V}$), a fixed resistor $R_1$, an LED with its protective resistor, and an NTC thermistor whose resistance is $2\ \text{k}\Omega$ at $25\ ^\circ\text{C}$ and $10\ \text{k}\Omega$ at $0\ ^\circ\text{C}$.

1. a) For the transistor to turn on when the temperature drops, should you place the NTC in the upper branch of the voltage divider (connected to $+9\ \text{V}$) or in the lower branch (connected to ground/$0\ \text{V}$)? Justify your answer by analysing how the base voltage varies as the sensor cools down.
2. b) If the NTC is mounted in the correct position together with a fixed resistor $R_1 = 1\ \text{k}\Omega$, calculate the base voltage at both $25\ ^\circ\text{C}$ and $0\ ^\circ\text{C}$. Reasonably predict whether the LED will be on or off at each of these two temperatures.

A school robotics team must choose the appropriate component to act as a switch in three different parts of a prototype:

1. Turn on and off a high-power $230\ \text{V}$ direct current motor using a $5\ \text{V}$ digital control signal output by a microcontroller.

2. Blink an LED strip at a very high frequency ($500\ \text{Hz}$) using low-voltage control pulses, without producing any acoustic noise or mechanical wear.

3. Install an emergency stop push-button that a human operator can press to physically cut off the main power supply in case of danger.

For each case, select the best option among a manual switch, an electromechanical relay, and an NPN transistor. Justify each choice by analysing advantages and disadvantages regarding switching speed, mechanical wear, and galvanic isolation (control-power separation).

A classmate claims: "A red LED rated at $2\ \text{V}$ and $20\ \text{mA}$ connected to a $9\ \text{V}$ battery does not need a current-limiting resistor because, according to Ohm's law, the LED will only draw the $20\ \text{mA}$ it needs and will not burn out."

1. a) Refute their claim by explaining what actually happens to the internal resistance of a forward-biased diode, and predict what will happen to the LED if it is connected directly to the battery without limiting the current.
2. b) Calculate the exact value of the resistor you must connect in series with the LED so that it operates at its nominal operating point ($2\ \text{V}$ and $20\ \text{mA}$).
3. c) Calculate the power in watts dissipated by this resistor during normal operation, and justify whether it would be appropriate to use a standard $1/4\ \text{W}$ ($0{,}25\ \text{W}$) resistor or if a $1/2\ \text{W}$ ($0{,}5\ \text{W}$) one would be required.

The flash of a camera's flash lamp requires an instantaneous power of $1200\ \text{W}$ during a pulse lasting just $1\ \text{ms}$ ($0{,}001\ \text{s}$). However, the camera is powered by two batteries that can only continuously deliver a maximum power of $3\ \text{W}$.

1. a) Calculate the energy in joules ($E = P \cdot t$) consumed during a flash, and demonstrate why it is physically impossible to power the flash by connecting it directly to the batteries.
2. b) Explain how a capacitor solves this problem by analyzing the charging and discharging process: why does the camera need to wait a few seconds before allowing a new photo to be taken?
3. c) If the original capacitor is replaced with another one having twice the capacitance ($2C$), predict and reasonably justify what will happen to the recharge time between photos and to the energy available for the flash.

In a project, you must design the automatic lighting control for a model of a garage door:

• Upon briefly pressing a wall switch ($5\ \text{V}$ control signal), a high-power light bulb ($230\ \text{V}$) must turn on immediately.

• After releasing the push button, the light bulb must remain on for $10$ seconds before turning off automatically.

• For safety reasons, the low-voltage control circuit ($5\ \text{V}$) must be completely galvanically isolated from the $230\ \text{V}$ power circuit.

You have: a fixed resistor, a capacitor, an NPN transistor, an electromechanical relay, and a diode.

1. a) Design the solution by selecting four of these components, and explain the function each one serves to achieve timing and safe control.
2. b) Technically justify why it is essential to use the relay as the final actuator for the light bulb and why it would not be appropriate to replace it with the transistor.