# My first circuit

Discussion in 'General Electronics Discussion' started by completenovice, Jun 23, 2013.

1. ### completenovice

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Jun 23, 2013
Hi
I'm brand new to electronics and no idea what I'm doing
I'm thinking of making this circuit. The power supply is a USB cable, about 5V and about 500mA. Each diode (they're LEDs by the way) are 20mA and 3-3.5 V each. Will this circuit work or do I need any sort of resistors or anything? Also if I add in a variable resistor will that allow me to change the brightness of the LEDs?
Any help appreciated
Thans
Sam

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2. ### davennModerator

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Sep 5, 2009
hi there

welcome to the forums

just take the top row of 4 LED's for a start
you said they are 3 - 3.5V each.
Work out the total voltage and then tell me why that wont work off 5 V

Dave

3. ### BobK

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Jan 5, 2010
Last edited by a moderator: Jun 23, 2013
4. ### completenovice

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Jun 23, 2013
I'd forgotten about the anode and catode on the diagram, I know the way the negatives and positives that should go in relation to eachother. Assuming all those are correct is would it work?
Thanks
Sam

5. ### davennModerator

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Sep 5, 2009
hi Sam

Yup OK so as Bob said, your sets of LED's are reversed .... just change the polarity of the battery to solve that

BUT still ... no it wont, read my post above again.
you have 4 x 3V what does that equal?
you said your supply voltage is 5V

do you see the problem ?

Dave

6. ### BobK

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Jan 5, 2010
Sorry dave, but you Ninja'd me. I was composing mine when you posted.

Bob

7. ### davennModerator

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Sep 5, 2009
no probs
just a hint tho, don't always give the full answer straight away let them work it out
it just helps them to learn a little more ... rather than being spoon fed

see with his response he hadn't read what I said about the voltages .... lets see if he figures out his problem

Dave

8. ### completenovice

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Jun 23, 2013
Ok, so I have 10 LEDs in total running at roughly 3V. Does that mean I need a thirty volt supply or what?
Sorry I'm a compete novice
Thanks for bearing with me
Sam

9. ### BobK

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Jan 5, 2010
No, it means you cannot put them in series. They have to be in parallel and have a reistors in series with each one.

Did you bother reading the tutorial I pointed you to?

Bob

10. ### completenovice

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Jun 23, 2013
Yes I did, I just became lost after a few lines. My only electronics experience is a short Unit on it in GCSE physics which isn't much use, I read the parts of the tutorial which were about what I needed and didn't understand, hence I came back here.
Thanks though
And thanks for that advice too, I understand what I need to do now and how it will work .
Sorry for my cluelessness (if that's a word)
Thanks alot again
Sam

11. ### completenovice

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Jun 23, 2013
after listening to what you've both said, had another look at that tutorial and after finding a couple of equations online. I have come up with this, does it look anybetter and does the resistance of my resistors look realistic ie will they work do you think?
Thanks a lot
Sam

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12. ### BobK

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Jan 5, 2010
Much better! That will work, assuming your power supply can supply 200mA.

Bob

13. ### duke37

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Jan 9, 2011
To get positive at the top,which is the norm, the battery has to be inverted.

The leds will then conduct. The arrows should point down to conduct in pnp or npn transistors or leds.

14. ### completenovice

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Jun 23, 2013
Thank you both for bearing with me
You've helped me a lot
I couldn't complete my project without your help
Thanks
Sam

15. ### BobK

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Jan 5, 2010
I can't believe I missed that the polarity was still wrong.

Bob

16. ### completenovice

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Jun 23, 2013
what it's wrong? I though the whole thing just needed flipping / inverting
Sam

17. ### completenovice

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Jun 23, 2013
Oh yes I've seen it now. It's just that the LEDs should be the other way right? + to + and - to -. I've got them the oppisite way
Is that the problem
Thanks
Sam