Electronics 101 – Basic circuit design

by hacker

I know plenty of people who would like to get into electronics. Like to create new stuff. But for some reason, they all seem to hold back. You see, ideas never end. That factory in your head keeps churning them out, almost regardless of conscious thought or effort. But like most things left uncared for, they eventually wither away. This is something I learnt from watching too many movies. Tell me, am I lying when I say you dreamt of creating a suit like Tony Stark’s IRON MAN? At some point in your life you would’ve looked at some problem and said, “Hey! I could make something like I saw in that movie to solve this! ”. I’m not talking specifically about the suit that allows you to fly (that would’ve been cool though). No, I’m talking about any machine which you can control somehow or some device that you thought was ultra cool………..or the less glamorous but just as important, ultra practical device.

Well, why didn’t you make it? No, the problem wasn’t “I would’ve, it’s just that I didn’t have much time” or “I would’ve, it’s just that the work is too mechanical”. It’s actually much simpler than that. You just needed someone to guide you. It isn’t as easy as pulling out your keyboard and hammering away. You think too many things can go wrong. You’re right to an extent, but the main reason you didn’t follow through was because you needed someone to give you that initial push, someone who wasn’t there. So, I take it as my personal responsibility to try and do this to you:

push

Well, I guess I might have scared you a little there. No problem. I’ll try a different tack. Think of any idea. Even one you thought was silly or something whose execution has you intimidated. I am going to do the same. I wanted to build a clapping on-off switch and I’ve been putting it off for some time now. And there’s no better place to start than at the beginning.

So, today I’ll be discussing  how to find out the resistance required to drive a simple LED. I mean don’t you see circuitry and think to yourself, “How am I supposed to know what goes where?” Well, today happens to be your lucky day, because I’m going to tell you. Let’s get to it.

Okay, here is an LED. It is red in colour, just in case you lost your colour perception in a tragic accident.


I am going to call it John. You can call it whatever you like. So John is a diode. He’s also a happy diode. He practically glows with excitement. John is a Light Emitting Diode. Like any other electronic device, he requires a voltage difference to operate. Since there is already resistance built-in, he’ll require current too. So we have three parameters; the voltage, current and resistance. We know 2 out of 3 (voltage and current). How? Well I’ll let you in on a secret. It’s given in the data sheet. Data sheets are like birth certificates for any electronic device. Read it and you won’t have any surprise explosions or molten plastic on your hands. For now, I was able to compile a small table: (and I also like to rhyme)

Now, this answers a lot of questions. Namely, if you connect John to say “X” voltage, you can predict whether he’ll glow….or blow. Easy does it. So, what resistance can we use so that John’s kids don’t end up orphaned?

Wait, why do you need a resistance again?

To limit the current. It’s like the speed breaker for electronic circuitry. You slow down when you see one of them on the road. Well, you’re supposed to anyway. So, the resistor is like the current limiter. Just remember this, anything that has current passing through it, will have a voltage drop across it. Even in a simple wire. You always need a potential difference to make current flow through a device and if current is flowing you can be sure there is a potential difference across the two ends of the device. This potential difference is also called Potential Drop sometimes, so don’t get confused. Electronic terminology is more or less straightforward, unlike Law terminology. Now, back to the issue at hand, what value to choose?

This is how it is drawn on a paper. The circuit I mean:

The one that has a + on it is the battery. It provides the  ”X” volts that can “potentially” damage LED John. R1 is the resistance and John, well John is just John. Now, say I want to make him glow on a 5V battery. I don’t know R1′s value yet. How do I calculate it?

Easy: X = R1 * current through R1 + Voltage drop across John

Since X = 5v(voltage of the battery), current is approximately 18mA , and John has a drop of 1.7v. How did we figure that out? Let’s walk through the Math:

R1 * 0.018 + 1.7 = 5

R1 * 0.018 = 3.3

So that gives us: R1 = 3.3/0.018 = 183 Ohm.

Great, so now we have a value of the resistance. 183 Ohm. You may not always be able to purchase that exact value from an electronics market, so you’ll have to settle for something close to it. Usually something higher, say 200 Ohm in this case. If you want an upper limit, it’ll be 250 Ohm. But that’s it. No more. Any higher and it’ll just make John glow less.

Remember, too much resistance means too little current and most devices will not perform as well. Conversely, too little resistance and the excess current may damage the device. So if you have to pick, go with more resistance.

Now you know the secret. Try it on your own. Say we use a 12V battery(X is 12Volts).

So, you have R1 * 0.018 = 12 – 1.7 which gives us R1 as 10.3 / 0.018, which is a whopping 572 Ohm. Now you can find a 560 Ohm resistor if you look hard enough, but usually you’ll just go for a 600 Ohm.

Great, you’ve taken your first step on a long journey. You now know how to design circuits. Well, the simplest circuit in existence. But it’s a start. The next step would be to replace the LED with other components. But we’ll get into that later. I’ll just leave you with the thought, “Get an idea. Draw it. We’ll make it together”.

If at all you want to request any specific tutorial please contact us at “contact(at)9circuits(dot)com”. We’d love to hear from you. Questions may also be asked as comments, along with non-technical banter.

Also, pretty soon we’ll be out with our self help kits. Be sure to read this when you acquire those.


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