Chapter 4 · Book pp. 57–72

Basic Electronics Projects

ตะลุยโครงงานอิเล็กทรอนิกส์เบื้องต้น

Now you put the parts to work. Each project is first built and simulated in Tinkercad, then built for real on a breadboard and checked with a multimeter. The projects start simple and use only a few parts, then get a little harder each time.

📱 Demo videos

The photos of the real circuits include the book’s QR codes. Scan one with your phone to watch that circuit working.

4.1 Steps for building a project ขั้นตอนการต่อวงจร

Every project uses the same base: a breadboard and a power supply — here a 3 × AA battery holder with an on/off switch — plus the ZX-BUX mini board, which connects the battery to the breadboard.

**Fig 4-1**The parts for the DC supply: a 3 × AA battery holder with switch and wires, the ZX-BUX power board, and a breadboard

**Fig 4-2**The ZX-BUX power board: a screw terminal for the battery wires (+ and −), and two pins that plug into the breadboard — red for supply +, black for supply −

**Fig 4-3**Using the ZX-BUX with the battery holder: (01) red wire to +, black wire to −; (02) tighten the screws with a small screwdriver; (03) plug it into the breadboard’s + rail (red) and − rail (blue or black)

🛠️ Lab 4.1 · Prepare the power supply

Build the circuit on the breadboard.
Connect the wires from the 3 × AA holder to the ZX-BUX terminals, + and −, and tighten the screws firmly.
Plug the ZX-BUX into the breadboard’s power rails. Match the printed + to the red line and − to the black/blue line.
With the switch OFF (or before wiring the circuit), fit the batteries the right way round. Switch on, set the multimeter to DC volts, and measure red on + and black on − of the ZX-BUX. You should read about 4.5 V. Then switch off again.

4.2 Tools you need เครื่องมือที่ต้องใช้

A multimeter — needle or digital — that can measure DC voltage, DC current and resistance.
A DC supply: here, 3 × AA cells in a holder.
Pliers or side cutters to trim part legs before putting them on the breadboard.

Next: Projects 1 and 2 →

Project 1 · Simple time-delay circuit with one transistor

P1.1 How it works

Normally, current flows from the battery through the 1 kΩ resistor to the LED, so the LED is on.
Press S1: 4.5 V charges the two 10 µF capacitors (in parallel) until they reach about 4.5 V. Some current flows through the 100 kΩ resistor into the base of Q1, which turns on. Current now flows through Q1 from collector to emitter instead of through the LED, so the LED goes off.
Release S1: the capacitors slowly discharge through the 100 kΩ resistor into Q1’s base, so the LED stays off. When the capacitor voltage drops so far that the base is below about 0.7 V, Q1 stops conducting, current flows through the LED again, and it lights.
How long the LED stays off depends on the capacitance and the base resistor.

**Fig P1-1**A simple time-delay circuit with one transistor (S1, R1 100k, R2 1k, C1 and C2 10 µF 16 V, Q1 BC547, LED1, 4.5 V). You can test it in Tinkercad

P1.2 Building it in Tinkercad and on a real breadboard

Watch the transistor’s legs — the small letters on its pins help you avoid mistakes — and remember the pushbutton must be wired diagonally.

Click Start Simulation: the LED lights. Press the switch and it goes off at once; let go, and after a while it comes back on.

**Fig P1-2**Project 1 on a Tinkercad breadboard. Its power rails are the other way round from the real breadboard — here − is at the top and + at the bottom — so take care

**Fig P1-3**Project 1 built with real parts. The ZX-BUX connects the battery to the breadboard: − to the blue line, + to the red line. Scan the QR code for the demo video

When you build it for real, take extra care that the capacitors and the LED are the right way round. An electrolytic capacitor has a white stripe on its side marking the negative leg.

P1.3 What you learned

You can increase capacitance by putting capacitors in parallel. Try it with only one capacitor: the LED stays off for less time; with two, for longer.
A transistor stops conducting when its base is below about 0.7 V, and starts conducting when the base reaches about 0.7 V.

Project 2 · Time-delay circuit, version 2

P2.1 How it works

At the start, the LED is off.
Press S1: current from the battery charges C1 until it reaches the supply voltage. A little current flows through the 100 kΩ resistor into Q1’s base, so Q1 conducts and the LED lights.
Release S1: C1 keeps feeding current into the base, so the LED stays on for a while. It also discharges through the 500 kΩ resistor, so the LED finally goes off.
The LED stays on for the same length of time after every release.

**Fig P2-1**Project 2: a one-transistor time-delay circuit, version 2 (S1, R1 100k, R2 500k, R3 1k, C1 10 µF 16 V, Q1 BC547, LED1). You can test it in Tinkercad

P2.2 Building it in Tinkercad and on a real breadboard

It looks like Project 1, but this time the LED is off when the simulation starts. Press the switch and it lights at once; let go and it stays on for a moment, then goes off.

**Fig P2-2**Project 2 in Tinkercad, ready to simulate

**Fig P2-3**Project 2 built with real parts. Check the real circuit against the simulation. Scan the QR code for the demo video

Uses: anything that should keep running for a while after you let go — a garden light or an extractor fan, for example.

Next: Project 3 →

Project 3 · Simple LED dimmer

P3.1 How it works

The 4.5 V supply is three 1.5 V cells in series. Current flows through the variable resistor VR1, out of its middle leg (leg 2), through R1 to the base of Q1.
Turn VR1 to its highest resistance: very little current flows, the middle leg’s voltage is low, and below about 0.7 V both LEDs stay off.
Turn the resistance down: more current flows into the base, Q1 conducts more, and the LEDs get brighter — the brightness follows the base current.
You could use just one LED. Two are used to show that each red LED drops about 2 V; two in series need at least about 4 V, which is why the supply is 4.5 V.

**Fig P3-1**A simple LED dimmer drawn in Tinkercad (VR1 100k, R1 10k, R2 220, LED1, LED2, Q1 BC547, 4.5 V)

P3.2 Building it in Tinkercad and on a real breadboard

Mind the LED legs — the flat edge marks the cathode. The potentiometer may sit a little differently in Tinkercad and on a real board; what matters is where its middle leg goes.

**Fig P3-2**Project 3 on Tinkercad, ready to simulate

**Fig P3-3**Project 3 built with real parts. Scan the QR code for the demo video

On the real board, watch the potentiometer’s legs. If you swap the two outer legs, the action is reversed: turning left gives the brightest light instead of the dimmest, and turning right the dimmest.

P3.3 What you learned

You can control the current through Q1 by controlling its base current with a variable resistor. With no base current, the transistor does not conduct. As the base current rises and the base reaches about 0.7 V, Q1 conducts, the LEDs start to glow, and they are brightest when the transistor is fully on (saturated).

Next: Projects 4 and 5 →

Project 4 · Automatic light

P4.1 How it works

R1 and the light sensor LDR1 form a voltage divider.
In bright light the LDR’s resistance is low, so the base of Q1 is below 0.7 V. Q1 is off and the LEDs stay off.
In the dark the LDR’s resistance rises, the base voltage rises to about 0.7 V, Q1 conducts, and the LEDs light.

**Fig P4-1**An automatic light circuit (R1 100k, LDR1, R2 220, LED1, LED2, Q1 BC547, 4.5 V)

This is a simple light sensor. The LDR turns a change in light into an electrical signal you can use — to drive an amplifier or a comparator, for example. A real design would add more parts to make it reliable under the conditions it has to work in.

P4.2 Building it in Tinkercad and on a real breadboard

In Tinkercad the LDR is called a Photoresistor — it is the same part.

**Fig P4-2**Project 4 in Tinkercad, ready to simulate

**Fig P4-3**Project 4 built with real parts — very close to the Tinkercad layout. Scan the QR code for the demo video

The circuit lights the LEDs in the dark and turns them off in the light. It is only an idea: for real use you may need a way to adjust it, and a relay instead of LEDs to switch a mains lamp.

Project 5 · Heat alarm

P5.1 How it works

The key part is the temperature sensor: an NTC thermistor, whose resistance goes down as the temperature goes up.
RTH1 and the variable resistor VR1 form a voltage divider. VR1 sets the point at which the LEDs come on.
We do not care about the exact temperature — only that when it is warmer than the room (about 25 °C) the LEDs light.

**Fig P5-1**A heat alarm circuit (RTH1 NTC, VR1 100k, R2 220, LED1, LED2, Q1 BC547, 4.5 V)

P5.2 Building it

⚠️ Not simulatable

Tinkercad has no thermistor, so Fig P5-2 uses a ceramic capacitor as a stand-in because it looks similar. Use the layout only as a wiring guide for the real breadboard.

**Fig P5-2**Project 5 laid out in Tinkercad. It cannot be simulated — the blue ceramic capacitor stands in for the NTC thermistor

**Fig P5-3**The heat alarm built with real parts, with a variable resistor to adjust it. Scan the QR code for the demo video

To test it, find a source of heat: touch a hot soldering iron (switched off) near the thermistor, or hold the thermistor in your fingers so your body heat warms it until the circuit switches on.

Next: Project 6 →

Project 6 · Simple water-level meter

P6.1 How it works

The sensors are electrode poles. One carries the 4.5 V supply and is dipped in the water tank.
As the water rises, it bridges the supply electrode to the electrode for that level, and current flows into the base of that level’s transistor (Q1–Q3). The transistor conducts and its LED lights.
LED1 shows the lowest level, LED2 the middle level, and LED3 the highest.
Full tank: all three LEDs on. When the water drops below an electrode, that LED goes off; below all of them, every LED is off.

**Fig P6-1**A simple 3-level water meter (R1–R3 220, LED1–LED3, Q1–Q3 BC547, 4.5 V, with the electrode wires at the bottom)

P6.2 Building and testing it

Fig P6-2 shows how to wire it and Fig P6-3 shows a real version. This circuit cannot be simulated in Tinkercad — build and test it with real parts.

**Fig P6-2**How to wire and test the water-level meter: lowest, middle and highest LEDs, with the electrodes for each level and the +Vcc electrode in the tank

**Fig P6-3**The real circuit testing the water level in a glass: the reference (supply) electrode, and the sensing electrodes for the highest, middle and lowest levels. Scan the QR code for the demo video

🛠️ Lab 4.2 · Test the water levels

Dip the electrodes for each level into a glass of water together with the supply electrode, one at a time. Check that the matching LED lights each time, and that all three light when every electrode is in the water.

Next: review →

The six projects

Project	Input	LED comes on when…	Simulate?
1 · Time delay	Pushbutton + 2 × 10 µF	On at start; off while pressed and for a while after	✅
2 · Time delay v2	Pushbutton + 10 µF + 500 kΩ	Pressed, and for a while after release	✅
3 · Dimmer	100 kΩ potentiometer	Brightness follows the knob	✅
4 · Automatic light	LDR (photoresistor)	It is dark	✅
5 · Heat alarm	NTC thermistor + VR	It is warmer than the set point	❌ no thermistor
6 · Water level	Electrodes in water	Water touches that level’s electrode	❌ real parts only

🛠️ Activity · Experiment with the projects

These follow the book’s “what you learned” notes.

Project 1: simulate with both capacitors, then delete one. Time how long the LED stays off each time.
Project 1: add a multimeter (Voltage) between Q1’s base and ground. Note the voltage at the moment the LED comes back on. Is it close to 0.7 V?
Project 2: change R2 from 500 kΩ to 100 kΩ. Does the LED stay on longer or shorter after you let go?
Project 3: remove one LED and add a 220 Ω resistor in its place. Does the dimmer still work? Why might two LEDs need 4.5 V?
Project 4: during the simulation, click the photoresistor and slide the light level. Find the point where the LEDs switch on.

Check yourself

The book’s projects end with “what you learned” notes but no questions, so these are my own, based on its pages.

1. What does the ZX-BUX mini board do?

2. Before you build a project, how do you check the battery pack is working?

3. At about what base voltage does a BC547 transistor start to conduct?

4. In Project 1, how can you make the LED stay off for longer?

5. In Project 2, what happens when you press and release the switch?

6. Why does Project 3 need a 4.5 V supply to light two red LEDs in series?

7. In the automatic light (Project 4), when do the LEDs come on?

8. Why can’t Project 5 (heat alarm) be simulated in Tinkercad?

9. In Project 6, the water reaches the middle electrode. Which LEDs are on?