Fun with Arduino 05 Connect Multiple LEDs with a Relay or a FET

In the previous video Fun with Arduino 04 we switched just one LED on and off as an example, but we actually want to switch the street lights on our layout. Suppose these are 20 white LEDs, which we also like to burn quite bright so they each draw a rather high current? No way we can connect those to our Arduino directly … we need something in between.

Read on below the video …

 

A couple of easy to use modules that are capable of switching higher currents are available, tailored for use with the Arduino. One is the relay module which can be had with 1, 2, 4 or 8 relays on a board. The other is a power FET module. (A FET is a special kind of transistor (Field Effect Transistor) that is often used in switching applications). Scroll all the way down for links where these can be bought.

relay_fet

When to choose a relay, when a FET? Both can be used, but there are some differences:

  • The relay module can switch AC up to 250V 10A. It’s a mechanical switch, it makes an audible click.
  • Arduino FET modules mostly are specified for DC up to 24V 5A. It’s an electronic switch, it makes no noise. They can operate at very high frequencies, which make them suitable as a dimmer for a LED strips (Pulse Width Modulation).

How to connect multiple LEDs? If we use a 5V DC power supply and if our LEDs have a 3V voltage drop, we can not connect them in series, we need to place them in parallel.

diodes-parallel

The way shown at the left generally is not a good idea. Although it might work if you try, due to tolerances chances are that one LED will draw more current than the others and they will burn with different brightness. Also, the resistor gets the combined current of all LEDs together, which can become too much for a standard 1/4 W resistor, it may get hot, you’ll probably have to use a higher wattage resistor. All this is avoided if we use one resistor per LED as shown on the right, this is the way to go.

diodes-parallel-12v-good-2

If we use a 12V power supply we can place 3 LEDs in series with one resistor. This is often seen on 12V LED strips.

The image below shows how the Arduino, the relay or FET and the LEDs can be wired (click to see the  enlarged version):

led_relay_fet

Links where the items in this article can be bought:

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