ClassX Video Lessons Youtube Channel The Engineering Mindset Transistor capacitor circuit design guide
Transistors, capacitors, LEDs, and resistors are essential components in creating a simple yet fascinating circuit board decoration that automatically turns lights on and off in a pattern. This article will guide you through understanding how this circuit works and how you can build your own. You can even download the circuit board design to try it yourself.
The circuit is based on a concept known as an astable multivibrator, or flip-flop. This type of circuit alternately turns LEDs on and off, and you can adjust the speed of this process by modifying the components used.
Transistors act as electronic switches. They prevent current from flowing until a small voltage is applied to the base pin. For instance, an LED connected across a transistor will not light up until the base pin receives about 0.7 volts, allowing current to flow through the circuit.
Capacitors store and release electrons. When connected to a battery, they charge up, and when the battery is removed, they discharge, powering the circuit. By connecting a capacitor to a resistor, you can control the charging and discharging time, affecting how long the LED stays on.
LEDs emit light when electrons pass through them. They are sensitive to current, so a resistor is used to protect them and control their brightness. Resistors limit the current, ensuring the LEDs receive the right amount of power.
The schematic for a simple flip-flop circuit can be translated into a physical circuit using a breadboard. When powered, the circuit will cause the LEDs to flash in sequence.
Here’s how it works: When power is supplied, current flows through a resistor into the base pin of a transistor, turning it on. This allows current to flow through another resistor and LED, charging a capacitor. Once the capacitor reaches 0.7 volts, it activates another transistor, turning it on and repeating the cycle, causing the LEDs to alternately light up.
In this project, we use three sets of LEDs. To accommodate this, we modify the circuit to include an additional row. Capacitors connect to different transistors, creating a loop that keeps the LEDs flashing. A 3V lithium cell, like the CR2450, is used as the power supply due to its compact size and high energy capacity.
A switch is added to turn the circuit on and off. Multiple LEDs are connected in parallel to ensure they all receive the same voltage. With a 3V supply, we use LEDs with low forward voltages, such as red and yellow, and a resistor to manage the current.
For this circuit, we use NPN transistors, such as the BC547, which are suitable for the voltages and currents involved. The flashing speed of the LEDs is determined by the capacitor and resistor values. A 10-kilohm resistor and a 100 microfarad capacitor work well, but you can experiment with different values to achieve the desired effect.
After testing the design on a breadboard, the next step is to design the PCB using software like Altium Designer. This involves arranging components, connecting them, and defining the board shape. Once designed, the PCB can be ordered from a manufacturer like JLCPCB.
After receiving the PCB, solder the components onto the board. Once assembled, connect the power supply and switch on the circuit to see the LEDs flash in sequence, creating a decorative display.
Building this circuit is a great way to learn about electronics and create something visually appealing. Feel free to explore more circuit designs and share your creations!
Gather the necessary components, including transistors, capacitors, LEDs, and resistors, and construct a basic flip-flop circuit on a breadboard. This hands-on activity will help you understand how the astable multivibrator works by seeing the LEDs flash in sequence.
Modify the resistor and capacitor values in your circuit to observe how these changes affect the flashing speed of the LEDs. Document your findings and explain how the timing of the circuit is influenced by these components.
Use PCB design software like Altium Designer to create a layout for your flip-flop circuit. Focus on arranging the components efficiently and ensuring all connections are correct. This will give you insight into the process of transitioning from a breadboard to a printed circuit board.
Conduct research on different types of transistors, such as NPN and PNP, and their applications in electronic circuits. Prepare a short presentation to share your findings with the class, highlighting the role of transistors in your circuit design.
Work in groups to design a new circuit that incorporates additional LEDs or other components. Set specific goals, such as creating a more complex light pattern or integrating a sensor. Present your design and demonstrate its functionality to the class.
Here’s a sanitized version of the provided YouTube transcript:
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Transistors, capacitors, LEDs, and resistors are all components used in this simple festive circuit board decoration to automatically turn the lights on and off in a pattern. In this video, I will show you how the circuit works and how to build your own. You can even download my circuit board design to build it yourself; links for that are in the video description below.
This circuit is based on something called an astable multivibrator or a flip-flop. A flip-flop circuit alternately turns the LEDs on and off. We can change the speed of this process by adjusting the components.
We will need some transistors, which act as electronic switches. They prevent current from passing through until a small amount of power is applied to the base pin. You can see here that the LED circuit is connected across a transistor, but it doesn’t turn on until we apply a voltage and current to the base pin, allowing current to flow through the main circuit and turning the LED on. The transistor requires around 0.7 volts at the base pin to activate.
We also need capacitors, which charge up and store electrons when a battery is applied, then release the electrons to power the circuit when the battery is removed. When I disconnect the power supply, the LED turns off instantly, but when I connect a capacitor to the circuit, the capacitor powers the LED when the power supply is removed. We can slow down the charging and discharging time of the capacitor by connecting it to a resistor. Resistors limit the current, which in turn limits how many electrons can flow through the wire and into or out of the capacitor over time.
Next, we use LEDs, which produce light when electrons pass through them. These are sensitive to current and can be damaged if too many electrons flow through. Therefore, we need to connect a resistor to protect them, which can also control how brightly the LEDs shine.
The circuit schematic for a simple flip-flop looks like this. We can convert this into a physical circuit board using a breadboard, and I highly recommend trying to build this yourself to understand how it works. When we connect a power supply, we see the circuit board begin to flash the LEDs.
When we connect the power supply, current flows through resistor 3 and into the base pin of transistor 1, turning it on. This allows current to flow through resistor 1 and LED 1, causing electrons to be pulled into capacitor 1 through resistor 2. When the voltage of capacitor 1 reaches 0.7 volts, it activates the base pin of transistor 2, turning it on. This allows current to flow through resistor 4 and LED 2, pulling electrons into capacitor 2 through resistor 3. This process causes transistor 1 to turn off. When the voltage across capacitor 2 reaches 0.7 volts, it turns transistor 1 on and transistor 2 off, repeating the cycle and causing the LEDs to alternately turn on and off.
In our decoration, we have three different sets of LEDs, so we will modify this circuit to include another row. Capacitor 1 connects to transistor 2, capacitor 2 connects to transistor 3, and capacitor 3 connects back to transistor 1. For the power supply, we can’t use standard batteries because they are too large; instead, we will use a 3V cell, specifically the lithium CR2450, which has a large energy capacity.
We will also need a switch to turn the circuit board on and off. The decoration requires multiple LEDs to turn on simultaneously. We have five LEDs per transistor, and since we only have a 3V supply, we can only use LEDs with low forward voltages, such as red and yellow. We will connect these in parallel to ensure they all receive the same voltage. If they were in series, the voltage drop would be too much. The forward voltage of the LED is around 1.8 volts, so we need a resistor to manage the current.
We can place a resistor before each LED or use a single resistor to limit the total current. Since this is a simple circuit, we will use a shared resistor. The voltage to the LED varies, affecting the current and brightness. We want to aim for around 10 milliamps per LED, which totals 50 milliamps for five LEDs. With a 3V battery, subtracting the 1.8V of the LED leaves us with 1.2V to remove. Dividing 1.2V by the total current of 0.05 amps gives us 24 ohms, so we need a 24-ohm resistor. I will use a 22-ohm resistor, which will work fine for this application. The yellow LED will require a slightly higher resistor, but the 22-ohm will suffice for this project.
For the transistors, we have two types: NPN and PNP. For this application, we will require an NPN transistor. Most standard small transistors will work fine, but I will use the BC547, which suits our voltages and currents well. We have covered how transistors work in detail in a previous video; links are down below for that.
The capacitor and resistor will determine how fast the LEDs flash. We could calculate this for an approximate answer, but instead, we will plug in components and see what happens. Using a 10-kilohm resistor and a 100 microfarad capacitor, we see the circuit works well. Changing to a 1,000 microfarad capacitor makes the circuit very slow, while a 10 microfarad capacitor makes it very fast. The larger the capacitor, the slower the flashing will be. The resistor also affects the charging time; a 100 microfarad capacitor with a 10-kilohm resistor flashes quickly, while a 47-kilohm resistor makes it very slow. A 1-kilohm resistor results in very fast flashing. I will use a 22-kilohm resistor and a 100 microfarad capacitor, as this combination works well and looks nice.
I’ve tested my design on a breadboard, and it is working well. Now we need to design the PCB. For this, we will use Altium Designer, who have kindly sponsored this video. All viewers can get a free trial of the software; I’ll leave a link in the video description below, so do check that out.
I’ll give a quick walkthrough of this part. We start a new project and add our components. There is an inbuilt feature for this, but I’ll use an add-on that simplifies importing from a supplier’s website. Many components are the same, so we can duplicate them and arrange them. We then connect the components together: the LEDs, capacitor, resistors, power supply, transistors, and switch. Your schematic should look something like this. You’ll want to configure the annotations, import the components to the PCB, and define the board shape. I’ll leave the star shape file in the video description for you to download.
Next, we place the components on the board in the desired order until it looks something like this. It’s important to place the LEDs in the correct order: group one, group two, and group three. We will use the auto route feature to connect everything together, but we must inspect the board and make any necessary modifications. After that, we create the polygon and finally export the files. That’s the board designed; now we just need to order it and build it.
To order the PCB, we head to jlcpcb.com, who have also sponsored this video. They offer exceptional value, with five circuit boards starting from just $2. I’ll leave a link in the video description below, so please check that out. Don’t forget, you can download my PCB file using the link in the video description as well.
We upload our Gerber files and check the preview. It looks good, so we can change the color of the board if we want, but I’ll leave it green and head to checkout. I enter my shipping address, choose a postage option, and then pay. A few days later, my circuit board arrives in the post, and they look very good.
Now that our circuit board has arrived, we can start soldering the components to the board. It doesn’t take long, and we should end up with something like this. When I connect the power supply and switch the circuit on, we see the lights flashing in sequence. We can then display our decorative circuit board and admire our work.
What do you think? Let me know in the comment section below. You can build even more circuit boards using the videos on screen now. Don’t forget to follow us on Facebook, LinkedIn, Instagram, Twitter, TikTok, and theengineeringmindset.com.
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This version maintains the technical content while removing any informal language or unnecessary filler.
Transistor – A semiconductor device used to amplify or switch electronic signals and electrical power. – The engineer used a transistor to amplify the weak audio signal in the speaker system.
Capacitor – An electronic component that stores and releases electrical energy in a circuit. – The capacitor in the circuit helps to smooth out voltage fluctuations.
Circuit – A complete and closed path through which electric current can flow. – The technician tested the circuit to ensure that the current was flowing correctly.
LED – A light-emitting diode, a semiconductor device that emits light when an electric current passes through it. – The LED indicator on the device shows that it is powered on.
Resistor – An electrical component that limits or regulates the flow of electrical current in an electronic circuit. – By adding a resistor, the engineer was able to control the current flowing through the circuit.
Voltage – The difference in electric potential between two points in a circuit, which causes current to flow. – The voltage across the battery terminals was measured to ensure it met the required specifications.
Current – The flow of electric charge in a circuit, typically measured in amperes. – The current flowing through the wire was too high, causing the fuse to blow.
Power – The rate at which electrical energy is transferred by an electric circuit, typically measured in watts. – The power supply unit provides the necessary power to run the computer components.
Design – The process of planning and creating a system or component to meet specific requirements. – The students were tasked with the design of a new, energy-efficient circuit board.
Electronics – The branch of physics and technology concerned with the design and application of circuits and devices using transistors, microchips, and other components. – The electronics lab was equipped with the latest tools for building and testing circuits.
