# How do you go about designing a circuit? [closed]

I'm a n00b^2 in the electronics world. Although many resources teach what each specific electronic component does (capacitors, resistors, diodes, etc), I have yet to find a "Why?" or "How?" as to the process of how one goes about designing their own circuits from scratch.

How do you know when/where/how many capacitors, resistors, diodes you need in a circuit, and at which junction points, where you need what in series/parallel?

Now before I get slapped -- I realize that this knowledge is absolutely something that comes with experience, and much research and understanding of electronics and electricity. I also understand the basic purposes of each component, but I don't know how to go about implementing them into my own circuits. I get dizzy when I look at a circuit board of a seemingly simple circuit/project see the 10's/100's/1000's of components that are reaching across the board, and especially connecting to multiple, separate components and points (the ever-tangled web of parallel circuit design stumps me).

What I have so far:

Let's say I want to design a motion-activated webcam. Well, first step, I'd work "backwards", because you never "start" with capacitors/resistors, as they're merely to clean and prepare voltage for the real "performers". So we'd need the two main functional components of the actual webcam (the action/output), and the motion sensor (the input).

Now for those to work, they need electricity. So I'd find their voltage/amperage requirements (still working "backwards"). Then based on their ratings, I'd pick out some components to properly limit the voltage/current to them (through resistors and voltage regulators).

Then I'd need a "switch", or a junction between them like a transistor.

I suppose that'd pretty much complete that circuit? But whenever I see even small hobbyist projects and circuits, it seems like they're just randomly painting the board everywhere with capacitors and resistors, and through such an expansive, almost "untraceable" web of parallel connections.

Can you share your process of how you plan and go about designing a circuit from scratch? And do you ever use software to plan and map your circuit design out?

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Question is too broad. –  spearson Mar 3 '12 at 2:19
Partial answer provided in question –  Michael Pruitt Mar 3 '12 at 5:48
I liked "noob^2" ...geek opening :D –  xsari3x Mar 3 '12 at 11:38

## closed as not a real question by Leon Heller, tcrosley, tyblu, Kevin VermeerMar 4 '12 at 0:53

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Designing a system is not easy. Specification of what a product should do is a whole processs of it self in any project. For DIY projects this is less the case, but nevertheless its important to realize what you're doing or want to do.

To take your example. Motion controlled webcam. Okay, seems to me like the webcam should turn on when it detects motion.

There are a few things in that sentence already: - turn on webcam - detect motion

Draw 2 blocks apart from each other: 'motion detector' and 'webcam'. These are seperate parts of your design and should operate and built up independently. The thing is, there are many different motion detectors and webcams. Figure out what you're going to use.. Then you also got to figure out how to interconnect these blocks.

Say a webcam is turned on by providing it power (however that doesn't have to be case, because the PC should start recording too! maybe you want it PC controlled). Alright, turning it on can be done via a relais or transistor. That's a circuit as is. What is its inputs? Probably digital. 1=on, 0=off. Typically you could say 1(on) is 5V signal, and 0(off) is 0V.

Alright, so that pretty much defined the input of your webcam block. The motion detector should fit to that. As I said , there are many different ways of detecting motion. Let's say we keep things simple and you just want to detect motion by seeing if something is in front of the webcam. There are IR proximity sensors that can do that. A sensor needs power and has an output. What type of output? There are once again, many different options, like a voltage (for example, 0-2V), current (4-20mA sensors..) or digital... which also has different interfaces and protocols; RS232, I2C, SPI, one-wire etc. As a DIY'er, pick what you can get and adapt. If you have some kind of digital sensor, you probably need some microcontroller to read the sensor, interpret the value and send a signal to your webcam (which required a 1, 5V or a 0, 0V). To interpret the value is also a question: what value of the sensor output do you detect as 'yes there is something in front of the webcam!'? Lets say its 50cm. Look up in the datasheet of the sensor when something is detected at <50cm. For a microcontroller this requires programming. For a sensor outputting an analog voltage, you maybe need a comperator with a reference voltage..

What you describe as a whole bunch of caps, resistors and other stuff is just the complete design. I just described 2 main function blocks, but there may be many more. You may have a digital circuit talking at 2.5V and your other part at 5V. There needs some converting or 'translating' between those voltages. You may also have a serial port that needs connecting to a modern PC. Maybe you add like a USB to UART bridge chip (Like FT232RL or something). That's another block on it's own.. input: USB, output: TX/RX as a TTL serial port.

Every circuit needs a power supply, which inevitable come with capacitors, and in most cases also other components like resistors, diodes, inductors and transistors. How to connect these? Look up the name of the IC's around the circuit and loook up their datasheet. The datasheet tells what a component does and typically has example circuits shown.

If you want to get started in electronics, don't keep staring at the whole picture. Try to pick up certain subjects of a device and figure out how it works. Build up simple circuits.. you will see that if they don't work you'll learn the most. You can later start trying to connect circuits between each other to create complete devices.

To KNOW how a circuit works, there is theory involved. Datasheets are full of figures that you need theory for to interpret them. That's just the way the technical world works.

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Sadly you can't approach it without some understanding of electronics and maths. It isn't as simple as "have a motion sensor and a switch or maybe a transistor connecting it to a webcam"

You need to know what the voltage and current requirements are, and these will determine power supply components. Do your webcam and motion sensor require different voltages? When your motion sensor turns "on", what does this mean? What is its output, and how does that translate into turning the webcam on?

When the webcam is on, how do you tell the PC to take a picture?

You also need to think about response times. Will the motion sensor respond to changing light due to clouds passing in front of the sun? Or do you need to have some sort of delay or timer built in?

So you can't just come at this as a beginner and expect to get it straight away.

I still swear by The Art of Electronics by Horowitz and Hill as a key primer. Once you have read all 1152 pages, you will have a pretty good idea how to design basic circuits. I still refer to it and my copies of Kraus' Electromagnetics, and Evans Engineering Mathematics on occasion.

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Thanks, great suggestions along the lines of what I was looking for -- the questions one asks him/herself in the planning and designing phase. –  Coldblackice Mar 3 '12 at 2:18

Since you said 'design' I assume this isn't a "What can I make with these items from my surplus pile?" project. So the first place I'd start is with requirements:

• The device must detect motion in it's range of vision.
• The device must begin transmitting images when image motion is detected.
• The device must operate over the normal range of indoor daytime illumination.
• The device should operate in full sunlight.
• The device may use a consumer-grade camera.
• The total parts cost must not exceed \$0.75.

... etc. The more you can define your requirements, the more you constrain your design. Limiting the freedom to decide "Oh, cool! We can add this feature and that neat part..." at any point in the design/prototype process is generally a Good Thing. The risk of over-constraining your design such that it becomes un-buildable is Bad Thing. Thus, this is an iterative process. As you specify requirements, you'll generate more questions and ideas. The more and better you answer the questions, the better your requirements become, and the better your product will be defined, giving you a better mental picture of the end result and making the way forward more and more clear, and giving you

Another, major advantage designing to requirements is that the requirements spec becomes the first draft of your product quality/performance test.

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