A Basic LED & Resistor Circuit – B1P1
Let’s learn how to build a very simple circuit that powers an LED. LED stands for Light Emitting Diode, and what this means is that it is simply a light source. When current flows through an LED, it will cause the LED to glow.
Generally speaking, the higher the current flowing through an LED, the brighter the LED will glow. Current is measured in Amps and the LEDs contained in the kit can handle a maximum current of 20mA. This means that we need a way to limit the current in the circuit and this can be done using a resistor. We will be using a 330 Ohm resistor for now.
In order to understand how the circuit works, we will start drawing the circuit diagram or schematic. A circuit diagram is a visual representation of the circuit using meaningful symbols and it gives you an idea of what the connections look like, which in turn gives you an idea of how the current flows through the circuit.
The symbols for a battery, resistor and LED are shown above. One terminal of the LED is longer than the other, and this is intentional. The longer terminal is the positive terminal, also called the anode, while the other one is the negative terminal or cathode.
Connecting the circuit is extremely simple, you simply connect a wire from the battery to the resistor, from the resistor to the LED and then from the LED back to the battery. This provides a path for the current to flow. If the LED is connected in the opposite orientation, it will not glow and can even damage it. The booklet contains the circuit diagrams for all the projects that we will be working on.
We can build the circuit by twisting the components together but things quickly get messy if you want to add more components which is why we will be using a breadboard.
The image above shows you what a typical breadboard looks like along with its internal structure. The metal strips provide some of the connections for us which greatly simplifies the circuit building process. We can also use jumper wires to bridge the necessary connections.
To connect the battery box, we can use the wire termination PCB that’s been provided in the kit. Simply insert it into the breadboard and use a screwdriver to fasten the wires as needed. Keep the red wire close to the red power strip while the black wire close to the blue power strip.
The booklet contains the breadboard layout which can be used as a reference to build the circuit. We already have the battery box connected, so let’s place the resistor and LED as shown. The longer LED terminal is represented by a slight bend in the breadboard layout. The blue lines indicate jumper wires that need to be placed, whereas the green highlights serve to indicate that something is connected along those metal strips. Let’s start by inserting a wire from the positive rail to the resistor, which is internally connected to the LED anode, and another one from the LED cathode to the negative rail. You can switch ON the battery box, to watch the LED glow.
Let’s take a closer look at the circuit. We know that the battery box gives us 3V and that the resistor has a value of 330 Ohms. To determine the current flowing through the circuit, we need to look at something called Ohms Law.
This is what the equation for Ohms Law looks like and it gives us the relationship between the voltage, resistance and current. By using the numbers we have, we can determine that the current will be 3/330 or approximately 9mA which is well below the LED limit of 20mA. Let’s measure the current to see if this is actually the case.
Since there is only one loop in this circuit, the same amount of current is going to flow between the three components. We can simply break the circuit, and insert a current measuring device called an Amp meter or ammeter. You can also use a device called a multimeter to take the measurements.
Let’s start measuring the current by breaking the circuit and inserting the probes in series. The current is about 3mA which is much less than the 9mA we thought was flowing. This is because when current flows through an LED is causes a voltage drop to appear across it and this voltage drop depends on different factors including the colour of the LED.
If we measure the voltage across the LED, you will see that it is about 2V.
This means that the voltage across the resistor is just 1V and Ohms law tells us that the current should be 3mA which is what we measured. The value of the resistor determines the current and we can change this to change the current, which will, in turn, change the LED brightness.
All the resistors in the kit are sorted as per their values, but the simplest way to determine the value is by using a multi-meter. Resistors like the ones in the kit also contain colour bands which are used to indicate their value. The booklet contains a colour code chart which can be used as a reference.
Resistors commonly have 4 or 5 bands. If it has 4 bands then the first two indicate the digits, the third indicates the multiplier while the last one indicates the tolerance. If it has 5 bands, then the first 3 indicate the digits followed by the multiplier and tolerance. If we look at the first resistor, it starts with an orange band, and by using the 1st digit table, we can see that orange corresponds to the number 3. The second band is also orange which gives us the number 3 again. The 3rd band is brown which gives us the value 10 while the 4th band is gold, which gives us a value of 5%.
The end result is a 330 Ohm resistor with a tolerance of 5%. The 5% tolerance states that the actual value of the resistor could be anywhere between 95% to 105% of the indicated value. For a 330 Ohm resistor, this would be anywhere between 313.5 to 346.5 Ohms.
That’s it for this post. We will look at variable resistors in the next one.