# COMMUNITY : FORUMS : SCIENCE

## Can someone look over this picture on Circuits with LED?

Ok, After countless stupid topics and stupid questions I just made a quick circuit in 5 minutes, to see if whatever I'm thinking in my head is correct. I really do need someone to verify, I plan on doing a bigger project after I understand how resistors work and all that nonsense. According to my calculations, I have 1 LED, Green, which runs 2.1V at 20mA. To power this, I used 2 1.5V AAA batteries. So that means I have 3V powering a 2.1 V LED. I have to drop .9V, so I did .9V/.02mA and got 45ohms. I need 45ohms of resistance to deliver 2.1V of power to the LED. (In the pic I used 3x15ohm resistors, again, I don't know how to wire them either...) Now This is my wiring, I am not sure how you link up resistors, I linked them all to the negative wire which I think was wrong...Someone please verify though.

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Goodhart9 years ago
Yes, it would be best to place the resistors on the "positive" side of the circuit. So called Positive ground circuits can be, um, funny (on such a simple one as yours, it probably doesn't matter so much, but best to learn the correct way from the start). Despite the way the symbols on a schematic point, voltage travels from - to + and not from + to ground as was once believed.
9 years ago
voltage travels from - to + and not from + to ground as was once believed. Pardon me please but voltage does not flow only current flows
9 years ago
Then let me put it another way: electrons (negatively charged particles) are said to move from neg to pos. Voltage is a measure of the energy carried by the charge. Current is the rate of flow of charge. Therefor, there can be voltage without current, but current can not flow without voltage.
GorillazMiko9 years ago
Just saying, you didn't have to have 3 resistors, you could have just had 1 with the same power of all 3 connected.
9 years ago
Power is a completely different thing to resistance, that'll just add confusion.
NachoMahma9 years ago
. Looks to me like it ought to work. As others have pointed out, electrically it makes no difference where you put the resistors, as long as they are in series. You could put two of the resistors on one side of the LED and one on the other.
. Most circuits I've seen will place the resistor(s) between the power source (battery) and the load (LED), but, as Goodhart and others point out, that's only convention ("That's the way the guy I learned from did it"). With your simple circuit, it doesn't matter.
. For simple proof-of-concept circuits such as your's, I like to use a potentiometer. Don't need to keep so many resistors on hand or have to figure out how to parallel/series what I happen to have.
. If you haven't already, get a decent digital VOM. You should be able to find a good meter for DIY work for 30-40 \$US. I prefer Fluke, but they can be a little pricey.
. I learned a lot from books by Forrest Mims.
9 years ago
And just in case you might not know what a "potentiometer" is... it's essentially just a variable resistor. Break open any discarded piece of old electronics that has a turning knob or a sliding dial on it and desolder those, or clip them off if they have some wire attached (leave as much wire on as possible - it may come in handy). Almost all of those will tend to be potentiometers. As Nacho suggested, If you don't have one yet, get yourself a cheap multimeter (aka VOM, or Volt-Ohm Meter) from your local radioshack or equivalent convenient neighborhood electronics store. Make sure it has a setting to measure resistance (Ohms, often indicated by a little greek Omega symbol). Crank the knob or slider all the way to the right and measure the resistance across the terminals. Then crank it all the way to the left and measure the resistance. If it's not already printed on the back of the potentiometer, write what you found on a slip of paper, and tape it to your new potentiometer.
9 years ago
the circuit im hopeing to build puts the resistor on the negative side. but normally, resistor go on the positive side.
Cygnis (author) 9 years ago
Ok, I see what you guys are saying? Just one more question, I'm a little confused. If I have a complete circuit, with a negative and positive wire, where do the resistors go? On the negative or positive wire? And If I introduced a switch, would it be smarter to put it on the positive or negative wire? Got a little confused by your answer Goodhart. After this I should be all set. Thanks in advance.
9 years ago
Sorry, put the resistors on the positive side of the circuit. Components normally go in the positive "leg" or side. Sometimes the Ground or negative side is called a Ground Rail. No interruptions or parts "inline".
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