Lets make a LED shine

A light emitting diode (short LED) is an electronic part with two legs which emits light when current flows through it. It is a light source with a long life time (meaning it takes a long time till a LED stops working) and is very energy efficient (meaning not much energy is transformed to heat instead of light).

Nowadays you can get LEDs which light up in any color.

It also happens that a LED is also a diode, meaning that current can only flow into one direction: If a LED is put into an electronic circuit the other way around as usual, it not only does not light up, also no current flows at all. Usually this is just a fun fact, but it can be exploited in some cases:

For example, you could put a green and a red LED in parallel (just one the other way around as the other). This way a LED would always shine, no matter which way the battery is connected. If the battery is put in the one way, the red LED would light up. If the battery would be put in the other way, the green LED would light up.

The challenge of connecting a LED

To light up an LED, it takes more that just connecting it to a battery: A LED requires a specific voltage difference between its two legs. At this specific voltage a specific current flows throws through the LED, lighting up the LED. If the LED does not get enough voltage, the LED might not light up. If the LED gets too much voltage, it might break.

So, lets say we have an LED and in the datasheet which belongs to the LED we find the following values:

Forward voltage typical:  2.0 V
Forward voltage max:      2.5 V
Steady current:            30 mA

The battery we would like to connect the LED to provides 1.5 V. That is not enough, but luckily we have two of them.

So how do we match the pieces?

Voltage and current when electronic parts are connected in serial

As we found out in our last blog post, voltage is always measured between two points of a circuit.

What we can do is connect both batteries in serial, just like this.

As the voltage from point A to point B is 1.5 V and the voltage from point B to point C is 1.5 V, the voltage from point A to point C must be 3 V.

Voltage over multiple parts connected in serial is added together.

Now this is too much voltage for the LED, which can only take 2.0 V – 2.5 V (see above). So we do the same as with the battery. We add a part in serial, so that the LED and that part together take 3 V: The LED 2.25 V, and that other part 0.75 V.

The part we will use is a resistor, a part which, as we learned in the last blog post, turns electrical energy into heat. All which is still not clear is which resistor we need.

The next puzzle part is that the current which flows through the electric circuit loop is the same everywhere: If 1 A flows through the LED, 1 A will also flow through the resistor and the batteries.

We know the resistor will work on a voltage of 0.75 V and now we also know that we want 30 mA current will flow through the resistor because this is the current which also shall flow through the LED (see datasheet). The resistor value we need can now be easily calculated with the formula from our last blog post. We just need to reform the formula:

    U in Volt  =  I in Ampere  *  R in Ohm
<=> U / I = R
<=> 0.75 V / 30 mA = R
<=> 0.75 V / 0.03 A = 25 Ohm

Now we know we need 25 Ohm, or the next higher available value (which would leave a bit less voltage for the LED, which is acceptable).

Putting theory into practice

To build the electronic circuit in real-life we need:

  • A breadboard
  • Jumper cables
  • A LED
  • Battery case for 2 1.5 V batteries or 1 9 V block battery
  • The resistor which fits to the LED and the batteries (calculate resistor value as above)

Connect everything using the breadboard. If you never used a breadboard before: Here is a great tutorial on how these great things work.

If you did everything correctly, you will see a light shining. If not, try putting in the LED the other way around (read above for the explanation).

What’s next?

In this blog post, I explained only the bits and pieces needed to calculate the resistor value needed in this specific example.

In the next blog post I will explain the rules on calculating electronic parts connected in series and parallel more.

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