Exercises: Simple electronic circuits

Explain what a breadboard is, why it does not require soldering, and how its holes are connected internally: what difference there is between the side power rails (long horizontal strips) and the short vertical terminal strips in the central area.

The figure shows a breadboard with its power rails at the top and its five-hole contact strips, separated by a central channel. Two areas are marked: one in green and one with a question mark.

The green area marks a strip of five holes connected internally in a vertical line. The orange area with ? covers the last two holes of an upper strip and the first two holes of the lower strip in the same column, and crosses the central channel of the board.

Answer:

1. a) Reason whether the holes in the orange area are connected to each other or separated across the channel.

1. b) Explain why that central channel exists.

1. c) Explain how components should be placed relative to the channel so as not to connect pins that must not touch each other.

An LED is never connected directly to a battery; a resistor is always placed in series with it. Explain what function this protection resistor performs in the circuit and what happens to the LED and the battery if it is wired without one.

Hint. Think about what happens to the current intensity when the LED, which offers hardly any resistance to the flow of current, is connected directly to the battery terminals.

Put the steps for assembling an LED with its resistor on a breadboard in order, and explain the reason for the second step: (a) place the LED respecting its polarity; (b) calculate the value of the resistor; (c) connect the resistor in series; (d) power with the battery and check.

Explain the difference between a circuit diagram (drawing with symbols) and a photo of an actual layout, and reason why the same diagram can be built in many different ways on a breadboard without changing the circuit.

Explain in your own words what the LDR does in a "night light" circuit: how its resistance changes as it gets dark and why this change can drive a transistor that turns on the LED.

Circuit simulators let you build a circuit on screen before touching a single physical component. Give two advantages of using a simulator compared with real assembly and one limitation of the simulator compared with real assembly. Then, explain why it is still a good idea to simulate first.

Explain what it means for a relay to be an "automatic switch": what operates it instead of a finger, and describe an everyday example where a relay allows a small signal to turn on a high-power device.

An LED requires $2\ \text{V}$ and $20\ \text{mA}$ and is to be connected to a $9\ \text{V}$ battery. Calculate the protective resistor step by step: the resistor must drop the excess voltage, $9 - 2 = 7\ \text{V}$, and the same current flows through it as through the LED. Apply Ohm's law and discuss which standard resistor value you would choose ($330\ \Omega$ or $390\ \Omega$).

The figure shows an LED circuit with its protective resistor, the value of which is missing.

The battery is $6\ \text{V}$ and the LED requires $2\ \text{V}$ and $20\ \text{mA}$. Calculate the value the resistor should have, explain what voltage it drops and what current passes through it, and choose between the standard values $180\ \Omega$ and $220\ \Omega$, reasoning which protects better.

Calculate the protective resistor for a flashlight with a $4{,}5\ \text{V}$ battery and a $1{,}8\ \text{V}$, $15\ \text{mA}$ LED. Show the three steps (voltage across the resistor, application of Ohm's law, value in ohms).

You have built a circuit with a $6\ \text{V}$ battery, an LED and a resistor in series. The LED has an operating voltage of $2\ \text{V}$, a recommended current of $20\ \text{mA}$ and a maximum allowable current of $30\ \text{mA}$. You have calculated a protection resistor of $200\ \Omega$, but you decide to try other values.

Reason using Ohm's law, calculating the current in each case (remember that the voltage across the resistor is $6\ \text{V}-2\ \text{V}=4\ \text{V}$):

1. a) What happens to the current and to the brightness of the LED if you use a resistor with twice the value?

1. b) What happens if you use a resistor with half the value? Is the maximum current of the LED exceeded?

1. c) Which of the two would you try first? Justify your answer.

The figure shows a darkness detector: when light is lacking, the LED turns on.

The covered component in the divider is the one that changes its value with light. Identify it, explain what happens to its resistance at dusk (it increases significantly), and how this change causes enough current to reach the base of the transistor to turn on the LED.

Topic challenge. A car's courtesy light does not switch off instantly when you close the door: it takes a few seconds to go out. In the circuit that controls it there is a capacitor connected to the base of a transistor that acts as a switch for the lamp.

1. a) Explain qualitatively what happens to the capacitor while the door is open and what happens at the moment the door is closed and the direct power supply is cut off.

1. b) Explain how this behavior of the capacitor makes the lamp stay on for a few seconds and then switch off.

1. c) State what would change (the time it takes to switch off) if you used a capacitor with a larger capacitance, and justify your answer.

You will be assessed on: describing the charging and discharging of the capacitor, relating the discharge to the current that continues to reach the transistor, and justifying the effect of a larger capacitance.

Describe how you would transfer the circuit diagram of an LED with a resistor to a breadboard step by step: which rail the positive battery terminal connects to, which strips you place each pin of the LED and resistor into, and how you check that no pins share a strip by mistake.

Interpret this circuit diagram: a battery, a switch, an NTC, and a buzzer in series. Predict what happens when the switch is closed on a hot day and on a cold day (the NTC changes its resistance with temperature), and explain what domestic application you can think of for this setup.

To illuminate a scale model, you have a $9\ \text{V}$ battery and two identical LED diodes ($V_F = 2\ \text{V}$, $I_F = 20\ \text{mA}$). A classmate proposes connecting them in parallel using an independent resistor for each branch, but you argue that placing them in series with a single commercial resistor ($220\ \Omega$, $270\ \Omega$, or $330\ \Omega$) is a better option. Design the series circuit by choosing the appropriate commercial resistor to protect the LEDs without exceeding their maximum current. Then, calculate the total power supplied by the battery in both setups (series and parallel) and mathematically justify which of the two designs provides greater battery life.

In the workshop, you have assembled an indicator by connecting a red LED ($2\ \text{V}$, $20\ \text{mA}$) to a $12\ \text{V}$ power supply using a $470\ \Omega$, $0{,}125\ \text{W}$ ($1/8\ \text{W}$) resistor. After a few minutes, you notice a burning smell and the resistor is burning hot to the touch. Demonstrate through calculations whether the chosen resistor is suitable for dissipating the thermal power generated in the setup, explain the physical cause of the overheating, and determine the minimum commercial characteristics (ohmic value and power rating) that the new resistor must have to operate completely safely.

You have assembled a twilight switch with an NPN transistor whose base voltage divider consists of an LDR (connected to $+9\ \text{V}$) and a fixed resistor of $10\ \text{k}\Omega$ connected to ground. The circuit activates a light when the base voltage drops below $0{,}7\ \text{V}$. When testing it in a hallway, you observe that the light only turns on in total darkness (the LDR reaches $120\ \text{k}\Omega$), but you need it to activate already at dusk/dim light (when the LDR is around $40\ \text{k}\Omega$). Justify with calculations why the current circuit does not turn on in dim light and design a modification by replacing the fixed resistor with a potentiometer: calculate what exact resistance value the potentiometer must have to achieve switching on right at $40\ \text{k}\Omega$ of the LDR.

A classmate has assembled a circuit on a breadboard with a $9\ \text{V}$ battery, a $470\ \Omega$ resistor, and an LED, but the LED does not turn on. To troubleshoot the fault without disconnecting components, you use a multimeter and obtain the following measurements:

• Voltage across the battery terminals: $9{,}1\ \text{V}$.

• Voltage across the ends of the resistor: $0\ \text{V}$.

• Voltage between the anode and cathode of the LED: $9{,}1\ \text{V}$.

Interpret what these readings indicate about the current flowing through the circuit, logically deduce which component or connection fault is responsible for the failure, and explain why a beginner might mistakenly conclude that the resistor is broken in an open circuit.

In a timer circuit for a staircase light, the discharge of a $100\ \mu\text{F}$ capacitor through a resistor connected to the base of an NPN transistor is used to keep a relay energized for a few seconds after pressing a normally open push button.

1. a) When testing the circuit, the light turns on when pressed, but turns off immediately as soon as the push button is released, without any delay. Propose two different physical faults in the components that would cause this exact symptom, justifying what happens to the current or electrical charge in each case.
2. b) If the circuit worked correctly but the time the light stayed on proved insufficient to climb the stairs, justify which two independent modifications you could make to the circuit to double the delay time.

You want to design an anti-theft alarm for a school backpack using a $9\ \text{V}$ battery, a buzzer, and a lever microswitch with three terminals: Common (C), Normally Open (NO), and Normally Closed (NC). When fully closed, the zipper physically presses the switch lever.

1. a) Design the circuit by indicating the exact connections between the battery terminals, the buzzer, and the switch terminals (C, NO, NC) so that the buzzer sounds only when the zipper is opened.
2. b) A classmate suggests using the opposite option: adding a transistor and keeping a closed circuit while the zipper is closed. Justify why your direct design with the proper contact is infinitely more efficient at extending the backpack's battery life.

You have simulated a voltage divider circuit on a computer with an ideal $9\ \text{V}$ battery and two identical $10\ \text{k}\Omega$ resistors, obtaining exactly $4{,}50\ \text{V}$ at the midpoint. However, when you assemble the real circuit on a breadboard and measure with a digital multimeter, you get $4{,}18\ \text{V}$ at the midpoint and $8{,}60\ \text{V}$ across the battery terminals.

1. a) Justify the reason for the voltage difference between the real battery and the simulator's battery, and calculate the theoretical value the midpoint should have with those $8{,}60\ \text{V}$ if the resistors were identical.
2. b) Given that the actual measurement ($4{,}18\ \text{V}$) does not match exactly half of $8{,}60\ \text{V}$, determine what real characteristic of carbon film resistors explains this deviation and deduce which of the two resistors (the one connected to positive or the one connected to ground) has a higher actual ohmic value.

In the workshop, you have a control circuit with an NPN transistor that activates a relay when the base voltage $V_B \ge 0{,}7\ \text{V}$. The base is connected to the midpoint of a voltage divider formed by a fixed resistor $R = 10\ \text{k}\Omega$ and an LDR connected to a $4{,}5\ \text{V}$ power supply. You measure the resistance of the LDR: in full light it is $1\ \text{k}\Omega$ and in darkness it is $100\ \text{k}\Omega$.

1. a) Calculate the base voltage $V_B$ under light and dark conditions if the LDR is connected between the base and ground (fixed resistor connected to $+4{,}5\ \text{V}$). Reasonably determine whether the circuit works as a light or dark detector.
2. b) If you need to convert the circuit so that it works in the exact opposite way (to turn on an alarm if someone turns on the light in a dark room), deduce what physical change you must make to the components and numerically demonstrate why it will now activate with light.

Unit challenge. Design a smart streetlight for your neighborhood! Your mission is to create an automatic dusk-to-dawn circuit that saves energy by turning on only when natural light is lacking.

1. Real data: Check the AEMET website for sunset time and average solar radiation in your province this month.

2. Design and simulation: Go to Tinkercad Circuits and assemble a circuit with a $9\text{ V}$ battery, a photoresistor (LDR), a $10\text{ k}\Omega$ potentiometer to calibrate sensitivity, an NPN transistor as a switch, and an LED with its $220\,\Omega$ resistor.

3. Measurement: Simulate dusk by varying the light level on the LDR. Use the virtual multimeter to measure the base voltage ($V_B$) and the LED current (for example, $18,5\text{ mA}$) in daylight and in dim light.

4. Final product: Submit a brief report with a screenshot of the circuit, a table with the measurements ($V$ and $I$), and a justified proposal for adjusting the potentiometer according to the AEMET data.

Assessment criteria:

• Correct assembly and operation of the circuit (proper biasing of the transistor and LDR) in Tinkercad.

• Rigorous recording of quantities ($V$ and $I$) with correct units and decimal comma.

• Technical justification of the circuit adjustment linked to real climatic data.