Simple LED Circuits

In this post we will learn how to build a few interesting LED circuits, and also learn how to connect LEDs correctly in a circuit.

LED stands for Light Emitting Diode, which is actually a semiconductor diode which has the property of emitting light, when current is passed through it in the right direction, or when the LED is forward biased.

An LED has two terminals for connecting to an electrical circuit. Since, an LED is basically a diode, its terminals have polarity in the form of anode and cathode.

The anode terminal is supposed to be connected with the positive supply, and the cathode to a negative supply.

Mostly, the maximum voltage an LED can tolerate is 3.5 V, however 3.3 V is the optimal value that is recommended for most standard LEDs.

LED Resistor

Although an LED is a diode, it's highly sensitive to current, and cannot tolerate anything exceeding its specified range.

In order to ensure that an LED is safe from an over-current situation, a calculated resistor is normally added in series with one of the terminals of the LED. This resistor may be connected in series with either the cathode terminal or the anode terminal of the LED.

This current limiting resistor can be simply calculated using the following formula:

R = Input Supply - LED Voltage rating / LED max current.

For example let's say an LED has a forward voltage rating of 3.3 V, and max current limit of 20 mA (0.02 Amps), then assuming the input supply is 6 V, the value of the series limiting resistor can be calculated as follows:

R = 6 - 3.3 / 0.02 = 135 ohms, the nearest safe available value being 150 ohms.

How to Connect an LED

Connecting an LED to a supply DC, for getting an optimal illumination, is very easy. A simple connection diagram can be seen in the following image, which is applicable to all LEDs. The current limiting resistor must be calculated as explained in the above paragraphs.

Here, the shorter terminal which is the cathode goes to the negative supply input, while the longer terminal which is the anode pin of the LED is connected with the positive input of the DC supply through a limiting resistor.

Application Circuits

LEDs are fascinating devices since these are able to produce powerful light output in different colors, as desired by the user for a given application.

LEDs can be used for making plenty of eye catching ornamental or indicator circuits for many useful purposes.

Without further ado, let's take a look at a few interesting LED application circuits as presented in the following paragraphs.

Smallest LED Flasher

Flashing LED indicator look very attractive but the design can be more interesting if the circuit uses the least number of parts. The following circuit shows how a single LED can be configured with a single transistor to create a reliable flashing LED indicator.

For more information about this circuit you can refer this article.

Random LED Flashing Light for Christmas Tree

The best use of LED devices is their ability to decorate anything as desired by the user. The following circuit shows how a single IC 4060 could be used for building a multiple LED flasher circuit as shown in the following diagram. All the connected LED strings will flash and twinkle at different random rate depending on the adjustment of the P1 pot or the value of the capacitor C1. It is used for decorating a Christmas tree or applied for making a twinkling LED necklace around an idol.

Full description of the design is provided in this article.

LED Rotating Light

If you are interested to build a rotating police or ambulance light effect without actually using a rotating mechanism for the lamp, then the following circuit can help you.

The LED used in this circuit is a 1 watt LED which will generate a slow alternating bright illumination and fading, producing a rotating LED light effect.

More information about the circuit can be found in this article.

LED Back light Nameplate

The following image shows an example of how an attractive back light illuminated LED name plate circuit can be built using just 4 LEDs attached horizontally at the 4 corners of the nameplate, internally.

More information on this and the full building procedure can be read in this article

LED Cube Circuit

To build a LED cube light you will need a plastic cube, a bunch of LEDs, and a cascaded delay circuit. For each LED a two transistor delay ON circuit is used, and many of these delay ON circuits are cascaded with one another, depending upon the number of LEDs, to form a long delay chain, looped from end to end. When power is applied, the LEDs begin switching ON one by one until all the LEDs installed on the cube are lit up. After all the LEDs are lit up, the reverse happens, and the LED turn off one by one, and the cycle keeps repeating.

The details of the circuit can be found in this article

Water Level Indicator

LEDs can be also used for indicating the level of water in a water tank. For this we need a handful of LEDs some transistors, and resistors. The complete design can be seen in the following diagram.

When the water bridges the contacts (A to D), between the transistor base resistors and the positive supply, the respective LEDs illuminate in succession, indicating the rising level of the water.

More on this can be learned from this article.

Simple Continuity Test

Just a couple of transistors along with an LED is all that may be needed to make a simple continuity tester circuit. This circuit can be used for testing the continuity of transformers, wire bundles, or any electrical system having complex wiring system.

In this continuity tester configuration, when one end of the wire bundle is touched with one hand and the other hand held over the positive supply, then touching the 1M resistor end with the other end of the wire indicates the continuity of the wire bundle

More on this can be learned through this article.

In the above article we discussed a few interesting LED circuits, but this might be just the tip of the iceberg, since there are innumerable more circuits that can be designed using LEDs for getting fascinating lighting effects or for useful indication purposes.

If you are interested to investigate more LED circuits, you can refer to the following link:

DIY LED Projects