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I've used this website to figure out how to wire up an LED project I am going to do, the only thing I'm not convinced by is the fact that on this diagram they have supplied me the resistors are after the LEDs. Is this correct, is this wrong, or am I just reading it wrong. I am reading it as I have 3 parallel sets of 4 LEDs in series with the resistors on the negative end of each set.

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  • \$\begingroup\$ The job of the resistor is to limit the current. When they are in series it is exactly the same. You could even put it in between two LEDs and it wont change a thing. \$\endgroup\$ – Andrés Sep 9 '17 at 22:05
  • \$\begingroup\$ Ah thank you. I get it. I had actually seen a video on this a couple hours ago and I completely forgot the concept. Is there any prefered practice as to whether I put them at the start or end, or is it just down to whatever is convenient for me? \$\endgroup\$ – Mikelong1994 Sep 9 '17 at 22:22
  • \$\begingroup\$ Wherever they are convenient. \$\endgroup\$ – Wesley Lee Sep 9 '17 at 22:42
  • \$\begingroup\$ Thanks. I may need to post this as a seperate question so please let me know if that is the case, but I'm looking at the Data Sheet for a Duracell 9v battery and it shows a graph that states that after ~4 hours of constant 50mA usage the voltage drops to ~7.5v, the wizard says this will draw 60mA, does that mean the LEDs will get dimmer and dimmer after ~4 hours use? \$\endgroup\$ – Mikelong1994 Sep 9 '17 at 22:48
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Current is the same for all nodes in a series circuit. As such, you can put the resistor anywhere in that series string. Each of those 3 parallel sets is a series circuit.

As to your other question, the leds will fade over the 4 hours, not just after the 4th hour, as the 9v battery is being drained. Since you are drawing 60mA, it will drain faster than the 4 hours you see.

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  • \$\begingroup\$ Since I want the LEDs to be powered constantly, would I be better off running it from a mains adapter rather than a battery? It's a clock I am designing. The clock kit will run from a single AA battery and I want to run 12 LEDs (My initial idea was to run them on a seperate 9v battery). Is there any way for me to have a mains adapter run my LEDs and connect into the AA slot of the clock so the entire project runs from that single source? \$\endgroup\$ – Mikelong1994 Sep 9 '17 at 23:19
  • \$\begingroup\$ @Mikelong1994 Yes. A 9V DC 200mA or higher Supply for your leds, and then a 9V to 1.5V voltage regulator. A normal linear regulator could work, like the lm317, as the current needed for a single 1.5V AA clock is pretty small. Simply wire the regulator output to the connectors for the AA battery. \$\endgroup\$ – Passerby Sep 9 '17 at 23:24
  • \$\begingroup\$ Awesome thanks. I assume the LEDs and the voltage regulator would be run parallel to each other from the supply? (Last question, I promise.) :D \$\endgroup\$ – Mikelong1994 Sep 9 '17 at 23:31
  • \$\begingroup\$ @Mikelong1994 Yes they are. \$\endgroup\$ – Passerby Sep 9 '17 at 23:42
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The resistors are shown in series with the LEDs. It doesn't matter what order the four LEDs and the resistor of each LED string are connected in. You can scramble them any way you want, as long as they stay in series, and you will get the same result.

One way to convince yourself of this is to realize the current is the same for all devices connected in series. Since the same current goes thru the resistor no matter where you place it in the series string, it drops the same voltage. That leaves the same remaining voltage for the LEDs, again regardless of order.

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