# Understanding Circuit Schematics

Discussion in 'General Electronics Discussion' started by Wotan, Nov 6, 2011.

1. ### Wotan

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Oct 8, 2011
Here is a schematic that I need help understanding-

On this circuit I'm assuming pin 2 is connected by a wire to pin 6 even though it doesn't make that clear to me on the circuit and I'm also assuming pin 4 is connected by a wire to pin 8. I'm also assuming pin 1 is connected straight to ground. How am I supposed to tell what resistor R1 is connected to? I don't know if it's connected from pin 7 to pin 8 or directly to positive. It's not clear to me looking at the schematic. Also I don't know how to tell if R3 is connected to positive or to pin 4 since it's not clear on the schematic. Is there some way to tell by looking at the schematic, after all schematics are designed to show you what connects where.

2. ### davelectronic

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Dec 13, 2010
Hi there Wotan.
I cant see what the problem there is, you have the posative rail and the negative rail, all the component rails have continuity, all components are giving there value, the capacitors are stating there type, i fail to see a problem.
Dave.
PS, rail status is ascertained by component orientation, ie led anode / cathode.

Last edited: Nov 6, 2011
3. ### Wotan

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Oct 8, 2011
Hi Dave, the problem is that I don't know how to make the cicuit using this schematic. It makes sense to you but to me I'm not sure how to understand the connections.

For example, I can't tell by looking at the schematic if R3 is connected directly to the positive rail or if it is connected to pin 4 ( i know the other end of R3 is connected to D1 ). I also can't tell if R4 is connected directly to ground or if it is connected to pin 5 with the other end of R4 obviously being connected to D2.

4. ### davelectronic

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Dec 13, 2010
Understanding schematics

Hi again Wotan.
Yes some circuits can be a bit hard to evaluate and understand, so the best way to look at it is from a components point of view and the orientation, so the 555 timer has pins doing different things in the circuit, depending on the circuit the function of that pin is going to be at a high level or low / or posative or negative, looking at the components pin out will tell you at what level any given part of the circuits rails are at.

The couple of links below might help, to visualize a circuit from paper to board construction can be puzzling for some circuits, but if you follow the rails and the components pin notations you wont go far wrong.

Anyway the links are below, the first is a good start for components understanding, the links on the page are along the top of the page in blue, the other link is a good site and that link explains some on understanding schematics.
Dave.

http://www.kpsec.freeuk.com/index.htm

Last edited: Nov 6, 2011
5. ### davennModerator

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Sep 5, 2009
that end of R3 is connected to both the + rail and pin4 along with pin8
That end of R4 is connected to the - rail of the battery along with the pin1, one side of C2 and the negative side of C1

One end of R1 and R2 connect to pin7, the other end of R1 connects to + rail along with Pin8, Pin4 and one end of R3 as discussed earlier

Anywhere where there are lines coming together and a dot joins them. just think of that dot as a solder joint joining those parts together

so for another example the lower end of R2 is connected to pin6, pin2 and the + end of C1

cheers
Dave

Last edited: Nov 6, 2011
6. ### davelectronic

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Dec 13, 2010
There you go that answered your questions, i chose not to point all the detail out as this is not learning how to read a schematic, but rather another member interpreting it for you.

Ok no big deal, but the only way to solve the problem i was tought was given the tools and instruction and figure it out, which years ago i thought a bit mean, but now i understand it was for learning purposes.

So in a word as an old engineer said to me, here's what you need, or do you expect me to do it all for you, meaning me.

As it was a general schematic read, and not a function question, i thought you would be able to see the way forward.

Questions answered, but it pays to learn the components function and why they are placed in that position and orientation, there can be many variables in a circuit, only by learning to understand yourself will you build promising circuits.

Having a complete circuit break down from another member will work, but not being very self sufficient .
Dave.

7. ### davennModerator

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Sep 5, 2009
it will help him for next time, dave

doesnt hurt to be a little bit helpful, we all were at the start one time or another
show a bit of encouragement and get some one well on their way, they are sure to come up with other problems in days to come

It really boils down to if the person is obviously trying to make an effort themselves with the project.
There are times when I, in a relatively polite way, will tell the person to go R.T.F.M. .... this wasnt one of those times

Sometimes, even these days after being into electronics for more years than I care to remember, someone occassionally still has to point out the "bleedin' obvious" to me haha

none of us would be where were are without a bit of kind mentoring along the way

cheers
Dave

Last edited: Nov 6, 2011
8. ### Wotan

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Oct 8, 2011
Here is a drawing of what I think the circuit should be. Is this correct? My battery is only charged to 4.6 v so I'm not sure if that is the problem since the circuit isn't working.

9. ### davennModerator

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Sep 5, 2009
hi Wotan....

ok a few probs and the reasons why it doesnt work....

is this built on a breadboard ... you know one of those white boards that you push the components into to make connections ?

first big problem... have a look at the positive lead of the battery ... and wonder why the cct doesnt work... do you see why ?

Dave

10. ### davennModerator

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Sep 5, 2009
have a look at a problem with the LED D1 as well

ohh the weak battery wont help but it isnt the main prob

D

11. ### Wotan

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Oct 8, 2011
Here is what I have now, is it correct yet, or at least more correct than the last drawing?

Last edited: Nov 6, 2011
12. ### davennModerator

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Sep 5, 2009
Thats better, it should work now. if still problems get a new battery

where did that cct originate from ?

D

13. ### Wotan

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Oct 8, 2011
www.circuitstoday.com, I don't know how I found it though. I think I was just looking on the internet for circuits and that one looked like an interesting one to do.

14. ### davelectronic

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Dec 13, 2010
Yes ok Dave i see what your saying, i was always tought old school, the answer was there the OP just needed to look a bit harder, not much, i thought it could be worked through and more satisfaction on a completed self tought project, as opposed to me giving all the answers.
Dave.
PS , looks like he got it going, well i think he must have done, unless a new battery was needed.

15. ### Wotan

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Oct 8, 2011
I got it going, but not how it was connected in the drawing. I connected D1 to pin 4 without a resistor and so D1 flashed. And I also removed the capacitor connected to pin 5 and D2 still pulsed very slightly but never got to full brightness. Yeah, so it wasn't perfect. I took it apart a bit and tried to get it going the way it is in the drawing but it didn't work. I think the connections on the circuit are a bit dodgy and that is the problem.

16. ### davennModerator

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Sep 5, 2009
but was that with the 4.5V battery ?

it was designed to work on 9V with the components listed

Dave

17. ### davennModerator

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Sep 5, 2009
hahaha thats funny mate I suspect I'm somewhat older than you

take care
Dave

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Dec 13, 2010