# What does what in a circuit?

Discussion in 'Electronics Resources' started by LED Man, Jan 16, 2008.

1. ### LED Man Thread Starter Active Member

Jan 15, 2008
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Hey everyone,

I have a pretty good understanding of EE, but I'm wondering what resources you guys use to figure out what a schematic does just by looking at it. I can pick out the general stuff, but looking at a schematic and figuring out exactly what each part does, and what it does as a whole is kind of confusing for me. Any ideas? Thanks.

2. ### Audioguru New Member

Dec 20, 2007
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My electronics education and experience allow me to read a schematic like reading a book in my language.

3. ### Dave Retired Moderator

Nov 17, 2003
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Yes, experience and education are the main points. Firstly you should familiarise yourself circuits symbols so that you can understand the rudiments of reading schematics. Then its understanding the interaction of these components that form the circuits/system. Sadly this requires quite a bit of studying and practising both circuits exercises and practical circuits.

Dave

4. ### mrmeval Distinguished Member

Jun 30, 2006
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Time. Build your own simple ones and work up. See if you have a local Make group or General Technics or other electronic hobby group.

5. ### hgmjr Moderator

Jan 28, 2005
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Schematic reading and circuit analysis are skills acquired by beginning with the study of simple unsophisticated circuits. As you become comfortable with the simple circuits, you can move on to circuits with more components and greater complexity.

It is a certainty that if your interest is genuine and you enthusiasm high, you will invest the time and effort to develop the skills you need.

hgmjr

6. ### LED Man Thread Starter Active Member

Jan 15, 2008
62
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See, I'm pretty good at identifying elements and their function, but that is for each piece individually. Looking at how it relates to other parts in the circuit, and why certain resistors or caps are put in a specific place, gets confusing. Like why would you chose to put a cap at the base of a bjt tied to ground (example)? I have the pieces, but not the whole.

7. ### hgmjr Moderator

Jan 28, 2005
9,030
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Perhaps if you can post a simple schematic that contains some of the components and circuit connections that have you baffled along with questions about the circuit, we may be able to provide several explanations that will give you a feel for the process.

hgmjr

Jan 15, 2008
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9. ### hgmjr Moderator

Jan 28, 2005
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What are a few of the questions you have about interpreting the schematic pictured?

hgmjr

10. ### Dave Retired Moderator

Nov 17, 2003
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Again this is down to knowledge and experience. Asking questions in a forum like this one, and reading up on the theory is the best start you could get. Everything you need is free and available from this site, so use it. We don't care how many question you ask as long as you understand what you need to (and obviously the thread must adhere to the forum rules! )

I learn something new from this site and the people who frequent it everyday.

Dave

11. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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If we look at the LMV431 Datasheet, on page 14, we see that the U2, Q1, and the switch-selected emitter resistor form a milliamp constant current sink. Different current values can be selected by choosing different resistor combinations.

I was able to figure out the above by noticing similarities in pattern between the circuit provided for discussion and the circuit on page 14 in the datasheet. As previously noted, it is a matter of having played with the "building blocks" enough to realize when someone else has used them.

And like Dave, I've learned something new. I'd never heard of an "adjustable precision shunt regulator" before. Now I have the opportunity to play with one if I want to.

12. ### jonkopp Member

Jan 17, 2008
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And the 555 timer IC is being used in mono stable mode, so the formula

t = 1.1(R x C)

is used.

R = 100K
C = 47u

so, t = 1.1(100,000 x 0.000047)

t = 5.17 sec.s

The values were chosen to give you around a 5sec output trigger.

13. ### jonkopp Member

Jan 17, 2008
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Basically, it's like software engineering.
When you write software, each language gives you a handful of native data types. The people who write the code over time decided they needed a wider set of tools to be able to handle more complicated, but still frequent, tasks. So data types like arrays and vectors came into existence. It's still just a bunch of numbers or strings lined up, but it's now in an easier to work with and communicate package.

So too the electronic engineers did roughly the same. Over time they found that by combining transistors, diodes, resistors, capacitors, and inductors in different formations, you get different, but consistent results. Some of them are very useful, some are more special purpose. Just by rotating the transistor, you can change the circuit all together. Some of the circuits were popular enough to become individual units themselves.

So, at least for me, when I look at a circuit, I don't see a jumble of individual components, I see smaller circuits, forming a signal flow, or protecting an input, or setting a reference, or whatever. Becoming familiar with all these different types of circuits is the best way. I recommend you read some basic text books. At least a couple. Learn about amplifiers, oscillators, modulators and build up to moving into digital and learning boolean algebra and how you can make some of these circuits. Then you'll know why they chose that resistor value, or why that transistor is being used base-emitter.

I know it's dry, but if you want a world of knowledge, starting with matter and the elements, and building up to RF comms and Radar principles, the Navy Electricity and Electronics Training Series (NEETS Mods) are an excellent source of beginner to intermediate level data. These books were all I was really given before being thrown through a slam fast crash course on electronics and being kicked on the but as they handed me my first screwdriver. These books made a big difference between me just mindlessly staring at circuit diagrams, and eventually being able to see what the hell I was doing.

Experience does go a long way. You need to see and interact with a large variety of these types of circuits. There is no such thing as the best amplifier. There is going to be at least a handful of amps that will efficiently meet your needs. So familiarity with different versions of what's out there is a must. May I recommend the Encyclopedia of Electronic Circuits vol 1-7. It's older stuff, but man is it a drool fest. I challenge any hardcore EE to flip through a couple of these and not want to immediately build something. Building a few of these and eventually stringing them together will get you on the road to understanding what is happening, and introduce you to multiple approaches to some common applications. The NEETS Mods are free online, the encyclopedia is not(that I know of) but they are cheap enough used on amazon.

I hope this is what you were looking for. Take care.

14. ### Dave Retired Moderator

Nov 17, 2003
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The NEETS modules jonkopp is referring to can be found here. And if you haven't already done so check our e-book.

Dave

15. ### LED Man Thread Starter Active Member

Jan 15, 2008
62
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So the battery is connected to pin 1 and 2 of the 555 right? after that what happens, I haven't had too much experience with a 555. Thanks

16. ### jonkopp Member

Jan 17, 2008
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555 ic's can be configured in three main ways, and then can be implemented in thousands more. Astable, Monostable, and Bistable are the three main types of configuration.

Pin 1 is Vss ground
Pin 8 is Vss positive

The 555 in this circuit has been set up in the monostable configuration. This means it has one constant output, being off, until being externally triggered. In this case applying ground to pin 2(the trigger pin) by closing the switch. This causes pin 3(the output pin) to go high. After being triggered, it will stay on for the time constant set by the RC network(explained in my last post), around 5 sec.s, and then return to it's off state.

The reset pin doesn't seem to be hooked up in this case(pin 4), but if it was attached to another switch, you'd be able to turn the circuit off during it's 5 sec timed output.

Check out these sites for a much more in depth look at the 555. Full pinout and tutorials for the rest of the possible configurations can be found here:

http://www.kpsec.freeuk.com/555timer.htm

17. ### FredM Senior Member

Dec 27, 2005
124
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http://rbgamble.net/Tester/ gives a description of the basic operation - when one is given notes on a circuit, it is sometimes a help (but beware! I have often had to evaluate circuits with circuit description provided, and found the description was wrong!)..

The notes say (and one can see that this is true) that the 555 is a monostable.. You can therefore split the circuit into 2 parts.. for evaluation, remove the connecton between U1/pin3 and the 300R (lets call this R5) resistor, and call the end of R5 which is now not connected "In".

Take the "in" to -V (this would, in fact, ve better called 0V, as it is the common from which reference is taken).. If there is any charge on the 2u2 C (lets call this C2), it will discharge via R5 - The base of Q1 will drop to 0V, turning Q1 off - No current is flowing through the circuit. (it is also a good idea to label all components prior to any analysis - R1, R2.. C1,C2 etc.. Otherwise one gets confused when making notes as to which component is which... Also, it makes communication far easier - Anyone with a notated schematic can find the component one is talking about.

Lets start with all switches open - Take "in" to +9V, C2 will charge quickly to 9V.
Close a switch.. The voltage on Q1 base will start at +9V, Q1 will be turned on, current will flow through the connected R, if the voltage across this R is > than the reference voltage for the LMV431 this IC will 'shunt' (act as a low resistance path from Q1 base to 0V) C2 will discharge via this shunt until the potential on Q1 base is 0.7V higher (Vbe) than the reference voltage.

Care should be taken (I do not know the spec of the 431, this design may be ok - but looks 'unsafe') when providing a discharge path for a capacitor through a semiconductor - a resistor to limit the current under worst-case conditions, is often a good idea.

With the base sitting at Vref + Vbe, the emitter is held at a constant voltage Vref, and the current from the +V to the -V terminal is fixed at Vref/R regardless of the voltage (within reason) between +V and -V terminals.

C2 acts to damp changes in the potential at Q1 base.. The other capacitor (0.01) is unlikely to do a lot - it is small and directly across a low impedence battery.

My advice to anyone learning circuit evaluation is this.. Dont waste your time too much on hobby contributions like the one above - they are often bad designs.. Go for designs from more competent sources.. Schematics from kits which are in production is safest.. To go into production, the circuits need to be repeatable. At a more serious level, books like "Art of Electronics" and (for analogue design) NS "Linear applications handbook" and projects in electronics magazines are a better place to start.

18. ### Pope John New Member

Jan 11, 2008
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Most electrical circuits use sections or stages to preform a task. A bit like a car has wheels gear box motor an a lot more to help it run. But you can also use the same wheels and parts to build a truck. Depends how you put it all together.

In the car motor there are pistons and pumps and a lot of individual pieces all put in a particular place but they can be changed around a bit and as long as you keep to the basic principals you can make it larger or smaller, a diesel engine or whatever.

Electrical circuits are similar in that they use a lot of building blocks or sections to make the thing go. Radio use a tuner section then an detection section to find one signal out of all the radio signals being transmitted. Then an amplifier to make a signal that can be used to drive the speaker section.

In the battery tester circuit you gave the caps to ground can be used as a filter to remove or block certain frequencies when in parallel with a coil or resistor. The small capacitors are used to take out any switching spikes and high frequency as this is testing a battery which is a direct current device.

Read the books on basic electronics at this site and others on the net.

Buy a soldering iron and a multi meter only a few dollars and get hold of a 555 timer.
For parts you can pull the resistors and capacitors out of old VCR or computer boards.
You don't have to buy much at all.

Make up some circuits and you will learn much more trying to get the things going than any other way. There are heaps of timers and buzzer circuits on the net and you can use a 555 to run them.

The best fun and fastest learning is by building things yourself and searching for the faults.

John

19. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Good suggestions John.

Further to Johns post, check the Experiments section of the e-book for circuits you can build and have a play around with. Where appropriate the experiments are linked to sections in the e-book which explain the under-pinning theory. This method has proved to work for many budding electronics hobbyists and engineers-in-training.

Dave