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I have made the following circuit. Using the multimeter,+ on first and - on last LED, I get 8.7 volts. What are possible reasons for no light? I have checked that I got the connections correct ( + to - ) for each LED. Since I have cut the leads to solder the LED together, I used lead frame orientation to double check that connections were correct. Any other possible reason that this might not be working? If one was damaged during soldering, do all stop working? Would current still go through ? 9V battery, 1.9 voltage drop on LED, 82 Ohm resistor, circuit was working on breadboard, no light after soldering.

9V battery

enter image description here

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  • \$\begingroup\$ What is the voltage drop across the resistor? \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 20 '15 at 0:56
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You need to test each LED individually. Use your battery in series with a 1k resistor and touch the leads to each LED in your string. If the LED doesn't light up, swap the wires to reverse the polarity and try again.

Yes: if one LED in your series string is open, all will be dark.

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It may be that your battery has dropped below the combined forward voltages, or that one of the LED's has failed in an open circuit.

To see how much current is actually flowing, measure the voltage over the resistor. Since the resistor is 82-Ohm, and I=V/R, every volt dropped across the resistor equals 12.2 mA.

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