Science Buddies: "Ask an Expert"

Hi,

My son and I are new to building circuits, and we are not understanding the procedure for how to build the light detection circuit for his science fair project (How Does LED Brightness Vary with Current). Number 2 where you build the circuit on the breadboard seems confusing and a bit circular to me. Can you help me understand what is meant by:

2b. "Use the 2-inch wire lengths to connect each terminal to a power bus." I can't find a definition of what is meant by "red terminal" and "black terminal" on the breadboard video. If it just means to plug the battery pack into the buses, are they just restating it over and over again? I also wonder if it means I need to use the 2-inch wire lengths we cut...my battery pack wires are like 4 inches long.

2c. Why are we putting the power pin and a wire connected to the positive bus in the same column? My understanding is that holes in any one column, for instance G7 and G8, are not connected electrically. The directions say to connect each of the pins to other wires by putting it in the same column, so I'm really confused. Do they actually mean rows the whole time?

2d. "Connect the ground wire from the battery pack into a different column." Isn't this wire already supposed to be in the negative power bus? Are we to move it for some reason? Why is that? And same question...columns aren't connected, so why are we moving this wire to the same column as the ground pin?

Overall, I'm quite frustrated by the confusing directions. They seem to use the same term for different concepts. I am already invested financially in this project, so if you can help us make it successful, I'd really appreciate it!

AmyBrewster - Sorry you are dealing with confusing instructions. I don't know where the instructions are that you are following, so I can't be real specific. Using a standard breadboard is pretty straightforward. I don't have a picture of the breadboard you are using, so can't comment on your references to G7 or G8, but I think you inherently understand the way they work. Attached is a picture of a typical 'breadboard'. Let's use the convention that looking at the breadboard held horizontaly, we will call the horizontal connections 'rows' and the vertical connections 'columns'.

The connectors along the edges, with red and blue lines, + and - symbols are your power busses. Here is where you would connect your battery pack, observing polarity. All 50 connections in the + 'row' are connected for example. You can obtain power from any one of these available connections.

In the working area of this breadboard, all 5 connections in any column, A-E are all connected. F-J are all connected, but the next column is isolated.

Wire lengths used on this breadboard aren't critical. They can be 2", 4" or any convenient length.

Hope this helps. Write back if you still have questions. Good luck.

Rick Marz

Thanks so much for your reply. I think we figured out our circuit, but now that we are testing the voltage across the resistor to see if it is all working, we are not getting any variance across the resistor when we shine a light at our light-to-voltage converter or when we cover it up. It is supposed to vary from 0V (when we cover the light detector) to close to 5V when a flashlight shines on it. We are getting like 4.15 the whole time. Do you suppose our circuit is wrong, or could the converter be bad??

By the way, when I switch the resistor to a stronger one, I do get less voltage across the resistor.

Any help trouble-shooting? Again, I THINK our circuit is correct, so what options are there for not seeing a change in the light detector as I expected?

AmyBrewster - Again, not knowing what your experiment looks like I'll hazard a few guesses. It sounds like you have a light emitting diode as the illumination source. You are changing the current powering the LED by changing the value of a resistor that is placed in series with your power source and the LED.

Secondly, you probably have some form of light detector like a photo diode or similar sensor. This also may be in series with a resistor and power source.

When light falls on the detector, you can measure voltage across the series resistor as representing the light brightness, and as current flows through the light sensor.

If you can send a link to the description of your experiment I'm sure I can be more helpful.

Rick Marz

You are right! That’s a great description of what we are doing. Here is the link for the science fair project, and we are trying to follow it exactly.

https://www.sciencebuddies.org/science- ... th-current

I believe after some feedback that our light to voltage converter is connected correctly, but I keep getting a consistent 4.15 reading across the resistor no matter if I shine a light on the detector or cover it up with my finger. We are in the testing of phase of “building the converter onto the breadboard”. I am expecting a reading of zero when the light detector is covered, so it’s making me wonder what it wrong.

If we can be sure our equipment is working, we’ll move on and use the LED and various resistors to see how everything changes. At this point, I’m wondering if the converter is malfunctioning or if this is user error.

Great news! A friend told me the light to voltage converter might be saturated with light. He recommended I take readings in a dark room. It worked! This experiment must be done in a dark a room as possible to avoid getting ambient light in the reading. We are so happy to be back on track!

These directions could really use some re-writing. I love the concept, but there are many pitfalls because of confusing language and helpful info such as the experiment won’t work in a bright room. Hopefully someone will look into some little updates.

Thanks for your help!

AmyBrewster - Congratulations! Great observation by your friend.

Rick Marz