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How to light several LEDs

In a previous article Chris Gosling showed us how to light an LED. In this article, Chris goes on to show us what to do if we want to light more than one.

In the previous article we built a circuit that looked something like this:

How to light several LEDs

Okay, so it wasn't exactly like that. In the previous article we used a 9V battery whereas in the circuit above we've use two 1.5V cells in series to give us a 3V supply. This means that we have a different resistor value. We've also added a switch so we can turn the circuit on and off without having to disconnect the battery.

So, how about a couple more?

If you've built, or at least understand, what's involved in lighting up a single LED then you have already mastered all the hard stuff and we're not going to rehash everything we covered in that first article. However we will be using it so if you are cheating and reading ahead you may find the following difficult to understand. We advise that you go back and read that first article.

LEDs in Parallel

Perhaps the most obvious thing to do if you want to light two LEDs is to build two circuits however it should be fairly obvious that you could use the same battery and switch and end up with this:

How to light several LEDs

This is the simplest way to do it but it does have a drawback however in that it will use up your batteries quicker. That is because each LED will draw its own 20 mA (or so) of current from the supply.

It is important to note that each LED has it's own resistor. You might be thinking that since they can share the battery and switch that perhaps they could share the resistor too. However that would be like throwing some food to a pack of dogs and expecting them to share it fairly. Having a single resistor means that the LEDs would fight for their share of the power. Having multiple resistors ensures that it's evenly distributed.

LEDs in Series

An alternative is to do it like this:

How to light several LEDs

Note that:

  1. By wiring the LEDs in series, we make the same 20 mA from the battery light up more than one LED, thus saving power from our batteries and making them last longer. This is the IF value from the formula that we used to calculate the value of the resistor. Clearly if the LEDs are all using the same current, they must be of the same type.

  2. When calculating the value for the resistor we need to add the voltages required by the LEDs (the VF values from the calculations in the previous article) to give us a total value for use in our formula.

  3. Your total VF through the chain of LEDs must be smaller than the voltage supplied by the power supply. That's why the circuit above has switched to using a 9V battery. The two AA cells shown in the previous circuits can't supply the 4V needed by the LEDs. If you put the values into our formula you'll see that 4V worth of LEDs using a 3V supply would need a resistor with a negative value (and they don’t make those).

  4. It may occur to you, or somebody may suggest, that if your total VF value is equal to the battery (e.g. if you used three LEDs that need 2V and a 6V supply) then you don't need a resistor, however this is a bad idea. The resistor is not just there to make up the difference between the supply voltage and that needed by the LEDs. The resistor also protects the LEDs from drawing too much current. No resistor = no protection = an increased chance of destroying the LEDs.

Lots-O-Lights (Series/Parallel)

Note that you can also mix these techniques such that you end up with a number of chains of LEDs in parallel:

How to light several LEDs

Series versus Parallel

Perhaps the hardest thing to get to understand about lighting multiple LEDs is when you should use a series circuit and when you should use a parallel circuit. The fact is that you can usually use either one however there will be advantages and disadvantage depending on the given scenario. As an example, let's take an example where we want to light 4 LEDs:

In SeriesIn Parallel
All LEDs must be the same type.LEDs can be different types.
1 resistor needed4 resistors needed
VF will be about 8V so we need a supply of at least 9V.VF will be about 2V so we could use a 3V supply i.e. 2x AA batteries.
The LEDS share the current from the battery so the battery lasts longer.Each LED draws its own current from the battery so the battery gets used up more quickly.
Generally needs less wire to make the circuit.Generally needs more wite to make the circuit.
If an LED dies, the others in the chain stop working.Failed LEDs have no effect on the others.

As you can see, there are various pros and cons and you'll need to decide which approach is most suited to the project that you are working on.

In future articles we'll go on to explain how to make LEDs flash on and off independently as well as in synchronised patterns. This will necessitate the introduction of a few more electronics components so we strongly recommend that in the meantime, you have a go at a project or two using the techniques that we've already explained.

We finish this article with a picture of Chris Gosling's Billboard which beautifully demonstrated how an already splendid piece of terrain can benefit from a little illumination. We look forward to seeing YOUR results on the forum or in our gallery.

How to light several LEDs

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