Well, I am almost sure that the problem is that your device does not produce enough power to lighten an LED. Although you got 8V from the device, you connected it to an oscilloscope, which will typically have 1 or 10 MOhm input impedance, therefore, in the best case scenario of 1MOhm, you have about 8^2/1000000=64uW of instant power produced by the piezoelectric device, which is not enough to lighten an LED, plus the loss in the rectifier and leakage of the diodes.

Do you have any idea of your piezoelectric coefficient (d33 or d31)? The energy produced in a piezoelectric device by a force F(N) is relatively to charge Q(C): Q=d33xF

It means that the voltage it generates is dependent on the area of the electrodes and the external load, so with your 1MOhm oscilloscope it reached 8V, but could reach higher if your load was higher. Try it with the oscilloscope probe in x10 (which also multiplies the load by 10, i.e. 10MOhms) and you will see the voltage rising slightly. It won't be 10 times more because of the capacitor created by the electrodes, but might be one or two volts more. When you use an LED, the impedance will be very high until the voltage reaches about 1.5 V and then the impedance will dramatically fall down.

I suggest you to eliminate the rectifier from the circuit and connect the piezoelectric device directly to the LED, and check the polarity of the device and the LED to make sure that you are polarizing the LED correctly. Also, use a small green or red LED, because that's the LED colors that are more efficient (I think green is the best one, but I am not completely sure, but anyway avoid yellow, blue, white, etc.). If you are using through hole, pick a 3mm one instead of a 5mm, but better if you use a very small SMD one). Also, pick an LED with a 25 degree viewing angle instead of 150 degree to direct all the light intro one beam. And finally, do the experiment in a nearly dark room. Maybe with a very strong hit in the piezoelectric device you might be able to see a very dim light. If even so you cannot see anything, you can connect the oscilloscope to the LED terminals and you'll see the waveform in the oscilloscope rising up to 1.5 or 1.8V, it will stagnate there for a while and then will fall back to zero. That proves the LED is conducting, but the light it produces is so small that your eyes cannot see it.

Thank you for your detailed explanation. I think you are right. My textile based piezoelectic sensor did not produce enough power to lighten LED bulb. We should store the energy in capacitor as my discussion with my colleague. I will firstly connect capacitor to my device and the capacitor will be connected to LED bulb. I will use different capacitors, 100 microF, 470 microFarad and 1000 microFarad. I connect the capacitor and LED bulb to breadboard. I will work in dark room. I hope I can get lighten LED bulb.

I don't know piezoelectric coefficient of my material? I have no idea about this. I will search. I will learn how can I check the polarity of the device. I used PVDF nanofibers. I think electrospinning process should provide the polarisation.

I am using red LED bulb.

I will use different pressures and impact. I am jumping on the device for human movement sensing :)

I read 8 V at 10X probe in oscilloscope.

I will try to connect oscilloscope to LED terminal.

Thank you so much. I am looking forward hearing your suggestions.

Best Regards

Nurhan

Nurhan Onar Çamlıbel The capacitor is not a solution. Unless you use a circuit to charge the capacitor with several impact events, and then discharge all the energy at once in the capacitor.

If you are using PVDF, in the best case scenario you have a piezoelectric coefficient (aka, d33) of about 30 pC/N, but since you made it by electrospining, my guess if that you have maybe 20 pC/N. And yes, electrospinning provides polarization. It would help to have a d33 meter (like this one: https://www.americanpiezo.com/standard-products/d33-meter.html) to confirm the real piezoelectric coefficient.

The capacitance of the electrodes of your piezoelectric device is already limiting the maximum voltage that it can generate. For example, if you hit your device with 100 N of force, you get:

Q=d33xF=20E-12*100=2nC

of charge built up on the electrodes of the device.

Now, suppose you have capacitance of 600pF in the electrodes of the device (just like I had in this article Article Environmental friendly printable piezoelectric inks and thei...

V=Q/C=2E-9/600E-12=3.33V

in the terminals of the piezoelectric device.

However, if you put a 100uF capacitor in parallel with the device, then you have 2E-9/(600E-12+100E-6)=20uV instead. This is not any helpful, unless, as I said, you hit the device many times and use a rectifier to keep storing the energy in the capacitor. But even so, the leakage current of the diodes in the rectifier will mostly never allow you to keep any charge in the capacitor at all.

So, my suggestion is still to connect the LED directly to the piezoelectric device, and picking up an LED with the mentioned characteristics to increase the chance of lighting it up. Then try to hit it at full force. To have the correct polarization, just simply test with the oscilloscope which is the positive and negative terminals of the device for a certain hit. Then, connect the positive terminal to the anode of the LED and the negative to the cathode. Or, the one I prefer, just trial and error, and invert the LED once in a while.

Alternatively, you can use an amplifier circuit, just like I did in that article. but I know, it is not the same :-)

Hope you have some good results. If you need anything else, don't hesitate to write again.

Best,

Sérgio

Hi Sergio

Thank you for sharing your information. I will try your suggestion and will share my results. I also hope so getting good results. I will try to find d33 meter in the other laboratories. I will try to connect the LED directly to the piezoelectric device. I will try using different capacitors.

Actually I have another question. I read 8 DC voltage in oscilloscope with my device. This is DC voltage under impact in very short time. Still do you think I need rectifier?

Thanks a lot

Nurhan

Dear Nurhan Onar Çamlıbel

I think you would get better results without a rectifier because the rectifier would also cause a voltage drop in the diodes, plus they have a current leakage, therefore the energy you can apply in the LED is lower.

When you hit your piezoelectric device, it generates a positive peak, and right after it generates a negative peak as the material relaxes back to original state. However, only the initial peak due to hitting is the one you can use because it is much higher (but also much shorter). You only have to be sure that you connect the LED with the correct polarity for the energy of that peak can conduct through it. The only advantage of using the rectifier would be to not care about the polarity, but it will cause at least 1V drop in the rectifier. If you really want to test it with the rectifier, use schottky diodes because they have a lower voltage drop then ordinary diodes. And also be careful in picking a low-leakage ones, although there must a compromise because the lower the leakage current, the higher the voltage drop.

You can also use an ordinary PZT disc with an LED and see how it works. There you can lighten up the LED easily. In your piezoelectric device it will be the same procedure, but only with a hit about 10 times stronger, since it has a piezoelectric constant about 10 times weaker.

Hope this helps, and if you need something else, don't hesitate.

Best,

Sérgio