Wednesday, March 30, 2016

Week 11

Blog sheet Week 11: Strain Gauges

Part A: Strain Gauges:
Strain gauges are used to measure the strain or stress levels on the materials. Alternatively, pressure on the strain gauge causes a generated voltage and it can be used as an energy harvester. You will be given either the flapping or tapping type gauge. When you test the circle buzzer type gauge, you will lay it flat on the table and tap on it. If it is the long rectangle one, you will flap the piece to generate voltage.

1.       Connect the oscilloscope probes to the strain gauge. Record the peak voltage values (positive and negative) by flipping/tapping the gauge with low and high pressure. Make sure to set the oscilloscope horizontal and vertical scales appropriately so you can read the values. DO NOT USE the measure tool of the oscilloscope. Adjust your oscilloscope so you can read the values from the screen. Fill out Table 1 and provide photos of the oscilloscope.

Table 1: Strain gauge characteristics
Flipping strength
Minimum Voltage
Maximum Voltage
-3 V
3 V
-10 V
10 V

2.       Press the “Single” button below the Autoscale button on the oscilloscope. This mode will allow you to capture a single change at the output. Adjust your time and amplitude scales so you have the best resolution for your signal when you flip/tap your strain gauge. Provide a photo of the oscilloscope graph.

Image 1: Single change output

Part B: Half-Wave Rectifiers

1.       Construct the following half-wave rectifier. Measure the input and the output using the oscilloscope and provide a snapshot of the outputs.

The input is 10Vpp at 1kHz
Output is 4.56Vpp
Image 2: Half wave rectifier output

2.       Calculate the effective voltage of the input and output and compare the values with the measured ones by completing the following table. 

                       Table 2: Effective (rms) of input and output voltage                         
Effective (rms) values
3.53 V
3.54 V
1.61 V
1.58 V

Equation used for calculated values: Vrms = (Vpp/2) (.707)

3.       Construct the following circuit and record the output voltage using both DMM and the oscilloscope.

Table 3:  Output voltage readings with 1 µF

Output Voltage (p-p)
2.32 V
Not possible
Output Voltage (mean)
2.89 V (not possible..?)
Not possible

The mean shouldn’t be higher than the peak to peak value. It should be somewhere between the peak to peak voltage. It's also impossible to measure the peak to peak value and mean value on the DMM.

4.       Replace the 1 µF capacitor with 100 µF and repeat the previous step. What has changed?

Table 4: Output voltage readings with 100 µF

Output Voltage (p-p)
160 mV
Not possible
Output Voltage (mean)
3.28 V 
Not possible

What changed after replacing the capacitors is that the output voltage peak to peak was smaller than the mean output voltage.

Part C: Energy Harvesters

1.       Construct the half-wave rectifier circuit without the resistor but with the 1 µF capacitor. Instead of the function generator, use the strain gauge. Discharge the capacitor every time you start a new measurement. Flip/tap your strain gauge and observe the output voltage. Fill out the table below:

Table 5: Output voltages at different tap frequencies
Tap frequency
Output voltage
1 flip/second
10 seconds
1.15 V
1 flip/second
20 seconds
1.16 V
1 flip /second
30 seconds
1.27 V
4 flips/second
10 seconds
2.14 V
4 flips/second
20 seconds
2.2 V
4 flips/second
30 seconds
2.3 V

2.       Briefly explain your results.
According to our chart,the more taps you do, the higher the output voltage. When you do a steady tap, the voltage generally stays the same. When you do multiple taps per second, it slowly increases.

3.       If we do not use the diode in the circuit (i.e. using only strain gauge to charge the capacitor), what would you observe at the output? Why?

When we didn’t use the diode, the output wasn't consistent. When you use a diode, it only allows positive current to flow; without it the capacitor wouldn't really charge. 


  1. Overall great job on your lab this week. I do believe that in parts 3 and 4 of part b that you can measure the peak to peak voltage of the signal using the DMM. You may want to look at this again to make sure.

  2. Nice detail to your work. I agree with Tyler, that you'd be able to record some more data using the DMM. Just remember to consider RMS vs Pk-Pk.

  3. On Part C, how did you get your output voltage to be 1+ volts? ours never broke 200 mV.


  4. I actually believe that the mean value for the DMM is able to be measured, not peak to peak. This is something we could figure out together.


  5. Thanks for the comments. We were told that you can't do the parts asking for the DMM measurements, hence why we said not possible. However it is something that we could go back to and look at.

  6. We also were told that the output peak to peak and mean cannot be measured by DMM. It's interesting that for problem 1 people got negative voltages for their minimum. We got 1V and 5V. However, we had issues with our strain gauge so I am sure that is what explains that. Your blog is very organized and easy to read.

    1. Thanks, Laura. Just looking at the other blogs you can kind of tell which strain gauge each group had based on their table. Not sure if anyone had success with the flipper though.

  7. I'm amazed at your voltage in part C, for our 1 tap per second we never got near 1 volt! Those are some strong taps!

  8. Good job. Thanks for responding to comments.