Science Buddies: "Ask an Expert"

What is in this Water? Experiments with a Homemade Turbidity Meter
Here's a link to the project I'm doing : https://www.sciencebuddies.org/science- ... j&from=TSW

I was following the procedures for this particular project and I came along some difficulties. For example : I was building the circuit for my project and the end result was that it didn't preform properly. I checked all the wires and jumper leads so they were all connected correctly ... I'm guessing one of my problems were that the multimeter wires were too thick to fit into the breadboard so I used jumper leads and wires to connect them instead. Will that affect the circuit ? I also had a hard time trying to do step 6 under adding the potentiometer.
I checked all my wires and jumper wires and did it according to the experimental procedure, but my circuit didn't preform as I would have liked it to. I checked if the battery was charged and it was brand new, I also tried different batteries and it still came out the same, so I don't think that that was the problem. I used a green laser light, I don't know if that will affect the outcome. When I tried testing the circuit, it always read the same, 0.00 (DCV 20V) and I also tried (DCV 2V) and it still didn't work. I tried testing it in dim light and bight light conditions, but as I said, it didn't work.
Please help me solve this problem and answer the questions I have. Thanks for taking the time to read this . . .

JessiCa.P - Sorry you are having difficulties. This circuit is pretty simple, so you shouldn't have too much trouble checking your wiring. Using a breadboard with jumper wires is usually pretty reliable, but it us mostly utilized when using many electronic components that have solid wire leads that plug into the matrix (resistors, capacitors, diodes etc). Make sure you are using the breadboard correctly and that connections are really 'connected'. Rows are often isolated into islands so that power and grounds are available, and that you can isolate components with several connection points. You may think that two adjacent holes are connected in the breadboard, but they might not be. You can use the ohmmeter function of your multimeter to verify that your connections are established.

The use of jumper wires to connect your meter isn't a problem assuming you have made a good connection to the leads of the meter. When you start your measurement, set the potentiometer at the middle of the range.

I couldn't find any electrical specifications on the Radio Shack site for this CdS photoresistor (RadioShack part #276-1657). A specification sheet for a similar cell indicated that it's resistance ratio from dark to bright light is as much as 200:1. The absolute ranges in resistance are from say 5,000Ω in bright light to 500kΩ in darkness. These values should easily give you a wide range of measured voltages on your meter with this setup.

I think there is something 'disconnected' in your circuit. I don't know how you attached the wires to the potentiometer, for example. Were the wires soldered or just wrapped around the connectors a few times. Can you find any 'short' circuits? Was all the wire used as jumpers bright copper for the connections or could there be some insulation issues? Try some of these things to see if you are successful. Post again with any results.

Rick Marz

First I connected the wires to the potentiometer by twisting them together, but not twisting them around a few times. Then I tried conecting two wires to the potentiometer by twisting them together and twisted the wires around the connectors a few times.
By 'middle range' what do you exactly mean by that ?..

I checked the photoresistor to measure resistance with the multimeter and it seems to be working. I checked all the connections and I haven't been able to find the problem. My project is to be turned in Tuesday and I still haven't found a solution. Please reply with the utmost urgency, thank you.

JessiCA.P - By 'middle of the range' I meant to set the shaft about half way through its rotation. It is supposed to be a 1 meg ohm value, so that will set it at about 500 K ohms. I looked at the diagram again and hope you weren't confused by the absence of a connection to the negative terminal of the battery. The negative terminal must be connected to the COM, or black lead of the multimeter to complete the circuit. If you got a valid measurement across the CdS cell when checking its resistance I maintain that you have an open circuit and something is disconnected.

Rick Marz