## Linear, Logarithmic:What works bestfor human senses helpASAP

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### Linear, Logarithmic:What works bestfor human senses helpASAP

My partner and I were following the sciencebuddies guide for the project here: http://www.sciencebuddies.org/science-f ... #procedure
We ordered all of the materials in the material section.
One thing that we don't understand is how the multimeter works. In the procedure, it tells you to put a 330 Ohm resistor, but later it tells you to connect it to a multimeter (voltmeter). However, in one of the pictures they took, the multimeter was not connected. Also, we aren't quite sure how to measure the output. When we turn the LED on, which setting should we use for the multimeter connected to the 10 kiloOhm resistor? The multimeter seems to be showing 0 voltage, even when we adjust the LED.
Thanks
handydandyscibook

Posts: 1
Joined: Fri Feb 08, 2013 4:11 pm
Project Question: Linear vs Logarithmic Changes: What works best with human senses
Using an light detection circuit to figure out if linear or logarithmic changes are sensed better in the human senses
Project Due Date: Tomorrow
Project Status: I am conducting my experiment

### Re: Linear, Logarithmic:What works bestfor human senses help

Welcome to the Forum handydandyscibook.

For general help using our multimeter 1) read the manual, 2) read it again, 3) read the SciBuddies help section on multimeters at

http://www.sciencebuddies.org/science-f ... eter.shtml

4) also look at

>In the procedure, it tells you to put a 330 Ohm resistor, but later
>it tells you to connect it to a multimeter (voltmeter).

I’m not entirely clear what this sentence means. The proceedure section tells you to connect multimeter #1 so as to measure the voltage across the 330 ohm resistor, i.e. put the aligator clip going to the VΩmA input (red lead) on the side of the resistor connected to pin 2 and the common (black lead) probe on the side of the resistor connected to ground. You will be measuring voltages of up to 5 volts; set the MM to DC Volts and select a suitable range (not necessary if your MM is “autoranging”). With the power on, as you turn the potentiometer from max to min you should see a matching drop in the voltage. While conducting your experiment you should have MM #1 connected like this. Another, separate measurement you should make before conducting your experiment is to remove the 330 ohm resistor from the board (with the power off of course) and then to use the resistance function of the MM to find the actual resistance of your nominal 330 ohm resistor — that’s the value you should use when converting the voltage across this resistor to current.

>However, in one of the pictures they took, the multimeter was not connected.

That may have been to allow you to see the board wiring more clearly. In use, the multimeters should both be connected.

>Also, we aren't quite sure how to measure the output. When we turn the LED on,
>which setting should we use for the multimeter connected to the 10 kiloOhm
>resistor?

The datasheet for the TSL14F says “Irradiance responsivity is characterized over the range VO = 0.2 to 4 V.” So you should crank up your LED to maximum brightness, then adjust the distance between the LED and the sensor so as to get about 4 V at the positive side of the 10 K resistor. You should set up MM#2 to read up to 4V across the 10K resistor for the experiment as well.

>The multimeter seems to be showing 0 voltage, even when we adjust the LED.

Have you checked the MM & test leads? If you look at the +5, +15, and −15 V DC power supplies you should see values within 10% (probably less) of the nominal values. With the probes clipped to each other you should see perhaps 1 mV or less jittering around with an average of 0 mV. If you see 0 for all the supplies, either your MM, your test leads, or the input 110 AC circuit of the test board are toast. Try different test leads (these are often faulty). Check the fuse on the test board. (I don’t have one handy to see if they have a fuse, but they should.)

I hope this helps. Let me know how you are doing.
John Dreher
Expert

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Occupation: Astronomer, Professor of Physics, SETI Researcher (retired)
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