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When the housewife helps and the two primary school goers assist, electrical engineering can get indigenous.....

The recipe for LED chain is like this!!

You will need

1. LEDs, obviously!!
2. Power Supply, to supply the current to allow the LED to glow
3. Resistors, to limit the excess current that the Power Supply may indulge in supplying if not restricted!
4. Switches, again obvious to its literal meaning, to toggle between the ON/OFF states
5. Wires, a whole lot of them
6. Accessories will include soldering iron and solder wires, tapes, screwdrivers, etc.

Design steps

You need first to decide how many LED's you require in a chain and how many chains will be there.

For me, there were 5 chains as I got that numbers of differently coloured LEDs and decided each chain to have 10 LEDs each. Also, our decision was to connect the LED's in parallel, as though series connection would have been a bit simpler, but the philosophy would be that if a single LED gets disconnected, the entire chain must not stop working, much the same line of thought as applied to domestic housewiring.

Now, if each LED is allowed to consume ~ 4 mA of current, then each chain will drain 40 mA current from the source and when all the 5 are connected, it will amount to 200 mA current net. The DC power supply current rating must be well beyond that. 24 V, 2 A power supplies are available in the market and it was chosen.

Now, the scheme will be like this

V1 is the power source and Lx are the chains. The scheme of a chain is like the following

Each chain consists of 10 LEDs (DLx1-DLx10) connected in parallel, the entire assembly being series connected to a resistor Rx1 and a switch SWx1 and the terminals marked for +ve (+x) and -ve (-x) polarities. Now,  one thing must be kept in mind. LED, unlike normal bulbs are polar devices, i.e., it has a +ve and a -ve terminal. These markings are engineered while fabrication and please be careful to connect all +ve leads with one wire and the -ve leads with another. But don't worry, nothing will blow spontaneously if connected incorrectly, but some will remain OFF and some will glow with excessive brightness leading to its failure, if connected in the wrong way.

Now, coming to the choice of Rx1, it is a bit tricky than simply choosing a single resistor from the market. Actually, these comes in various standard ratings and you have to choose among them. For this, we need to know the supply voltage, the drop across the LED and the current that will flow through it. The drop across the LED is ~ 0.7 V (as this is a diode and their nature is to have tentatively this amount of voltage across them while conducting) and the power supply chosen has output rating of 24 V. So, the remaining (24 - 0.7) V  = 23.3 V is to be dropped across the resistor. Each chain is allowed 40 mA current. This will correspond to a resistance of (23.3/40) kΩ = 582.5 Ω. Its power dissipation will be 23.3 x 40 mW = 932 mW. Now, standard resistances in this range is 560 Ω, 680 Ω with dissipation rating of 250 mW, 500 mW. So, to enhance the power rating and matching the resistance value, this single resistor is comprised of multiple resistors in parallel combinations. For instance, if we parallel 4 pieces of 2.2kΩ/250mW resistors, the equivalent resistance will come as (2.2/4) kΩ = 550 Ω, allowing (23.3x23.3/2200) W = 246.8 mW dissipation in each resistor thus keeping it within the range.

The switch is used to isolate each chain from the other so that each can be disconnected in case a shorted fault occurs in any of them,

The best advantage of this circuit is that it is supplied with 24 V DC which can be further downgraded and the mains supply is limited to the wires to the power supply terminals, keeping remaining circuit in the safer range. But, it is electrical, so be safe and be supervised by some technical person while making it operational

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