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### FAQ for "Build a Simple Circuit" resource

Posted: Thu Aug 30, 2012 12:24 pm
The following FAQ contains frequently asked questions and answers about the "Build a Simple Circuit" resource (http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-simplecircuit.shtml). If you are having trouble with the procedure, you may find assistance in the answers below.

Q: The LED is not turning on or emitting light, and it has never turned on or emitted light. What do I do?
A: If your LED has never lit up, then your circuit is probably incomplete; electric current is not able to flow through the circuit. This could be because the circuit's connections are incorrect or because of a faulty circuit component. Check that all of the circuit components are connected to the correct holes in the breadboard. Check your circuit against steps 2-10 of the procedure and against Figure 4 in the Project Idea.

Pay particular attention to the LED; make sure it is inserted correctly. Look at the leads coming from the LED and then look at the LED body itself. You should see one flat edge on the base of the LED that lines up with the shorter lead. This is the cathode and should go towards the negative or minus side of the battery. The longer lead then goes towards the positive or plus side. Remember that direction matters for a diode like an LED. In addition, if you pushed a stripped connection wire too far into the breadboard, then the insulation will lead to a bad connection.

If your components are connected in the correct places and there is no insulation in the breadboard sockets, but the LED still does not turn on, then rebuild the circuit and check each of the connections with a multimeter as you do so. By this systematic approach, you should be able to find out where the problem is:

1. Start by disconnecting the battery from the battery connector and removing all of the components from the circuit board.
2. Starting with step 2 of the Experimental Procedure, add each component to the breadboard.
3. As you add each component, use a multimeter to check the connectivity between the first component (the red battery lead, in this case) and the component you added.
1. To do this, turn the dial on the multimeter so that it measures resistance and touch the two probes of the multimeter to the two circuit components you are testing.
2. If the resistance between the two components is 0 (or very close to it), then the two components are connected and the circuit is complete between the two points.
1. If the circuit is complete between these two points, add the next component and check the connectivity between the first circuit component and this new circuit component.
2. Once you have added the resistor, the resistance of the circuit should be about 220 Ω. If the resistance is about 220 Ω, then the circuit is wired correctly and the components are connected correctly.
3. If the circuit is incomplete, then there is a problem between the beginning of the circuit and the component.
1. Once you know where the problem is, recheck those connections and circuit components. One of the odd things about circuits is that sometimes just taking things apart and putting them back together again can fix a problem!

Q: The LED was lighting up, but it suddenly stopped working. What should I do?
A: This could happen for a few reasons. If your LED was lit and then suddenly turned off, then the LED may have burned out. Double check your wiring to make sure you have the resistor connected in series with (in between) the battery and the LED. Make sure the resistor is the correct value. For a 9 volt battery, a 220 ohm resistor (red-red-brown) will light the LED safely.

To identify the source of the problem, start by using the multimeter to measure the voltage coming from the battery. To do so, turn the dial on the multimeter so that you are measuring DC volts (a 10 volt scale would work if you have to set a range). Touch one multimeter probe to each of the battery terminals. If the reading is much less than 9 volts, then the battery could be the problem. Try a new battery. If the voltage is ~9 volts, then the battery probably is not the problem. Use the steps 3a-c in the previous FAQ answer to check the connection between the resistor and LED.

Q: What do the colored bands on a resistor mean?
A: The colored bands indicate the value of the resistor and its tolerance. That is, they tell you how much resistance the resistor nominally provides, in ohms (Ω), and how close the actual resistance of your particular resistor is to the stated resistance value. For example, if the colored bands indicate a value of 220 Ω and a tolerance of ±5%, then the actual resistance of that resistor will be within 5% of 220 Ω, or somewhere between 209 and 231 Ω.

There are 10 possible colors on a resistor. Each color has a value from 0 to 9. You can think of the handy mnemonic "Bright Boys Rave Over Young Girls But Veto Getting Wed". It is a nonsense phrase, but the first letter of each word gives the colors in order from 0 to 9: Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, and White.

On a standard resistor there are 4 colored stripes. One is a metallic stripe such as silver or gold. Start at the other end of the resistor, which will be one of the non-metallic, colored bands. The first two bands are digits of the resistor value. The third is a multiplier and tells you how many times to multiply the first two digits by 10. The fourth colored band indicates the precision, or how accurate the resistor value is likely to be.

In this project, we used a resistor with red-red-brown stripes. Red means 2, and we have two red bands, so the value so far is 22. The multiplier stripe is brown which means 1. Multiply 22 by 10 one time, to get 220. The fourth stripe is gold which means the resistor will be within 5% of the stated value. Precision bands of silver (10%) or no 4th band (20%) are uncommon.

You can read more about the resistor color code on the http://www.csgnetwork.com/resistcolcalc.html website, which has a table explaining what each band and each color means, along with a nifty resistance color code calculator.

If you have other questions about the procedure or need assistance troubleshooting the "Build a Simple Circuit" resource (part of the Science Buddies Electronics Primer, http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-intro.shtml), please post your question in the forum at Ask an Expert: http://www.sciencebuddies.org/science-fair-projects/phpBB3/viewforum.php?f=71. Our team of volunteer Experts is available to assist. We attempt to reply to questions within 24 hours. Please note that you will need a free Ask an Expert account in order to post questions.