This thin silvery ribbon is an interesting sensor. It’s made of 2mm wide by 600mm long piezoelectric film sandwich. Piezo film can be used to detect motion or vibration - when shifted around or touched, it creates a small current pulse across the terminals. If a 1 Mega-ohm resistor is placed across the two terminals you will see the current converted into a voltage that can be read by an analog-to-digital converter (ADC).
Hi Antony
I very much doubt this would supply enough current to operate a LED directly.
I think you would have to interface an OpAmp (and power it) to supple this current.
Exactly how you would go about this would depend on exactly how you want to use this device and exactly how much voltage and current you get from it under the conditions you want to use it.
Unfortunately this sort of information is not available on the device description page. But having said that such info would be extremely difficult to provide as there probably would be a vast number of scenarios which would put this sort of information into a non practical category.
You would probably have to try it out but would benefit from an oscilloscope to observe the output while doing whatever you have in mind for it.
Cheers Bob
Hi Bob,
Thanks again for all your help so far. I managed to get access to an oscilloscope, and just wanted to check my plan for hooking it up to the piezo film before I try it, since I’m still fairly new to this.
Here’s what I’m planning to do:
Use a standard scope probe (or clip leads if I don’t have a proper probe) — probe tip to one piezo lead, ground clip to the other. Since the piezo output is AC, I don’t think polarity matters here.
Set the coupling to AC, since the output is small and centred around 0V.
Start the vertical scale around 1–5V/div, since I’ve read piezo spikes can hit fairly high voltage even with very little current — I don’t want to start too sensitive and clip the signal.
Set the timebase to somewhere around 10–50ms/div, since a tap or flex should produce a quick spike that decays over a short time.
Then tap or flex the piezo film while watching the screen, and hopefully see a voltage spike.
A few things I wanted to confirm with you:
• Am I right in thinking I should measure it with no load connected first (open-circuit), just to see the peak voltage — and that this won’t be the same as what I’d actually get once it’s driving something like an LED?
• If my scope has a peak-to-peak voltage measurement function, is that the number I should be recording for my folio?
• Would it be worth testing a few different tap strengths and comparing the readings, to show how output scales with force?
Let me know if I’ve got any of this wrong, or if there’s anything I should be doing differently given I’m working with pretty basic equipment.
Thanks again for your patience with all my questions!
Cheers,
Antony
No, Connect the 1MΩ load as in the text.
Use the scope and probe in 10:1 mode as this will be more realistic. In this mode the scope will look like 10MΩ so will not have much effect on the 1MΩ load.
I still don’t think it will be capable of driving a LED directly but I could be proven wrong here. Only trial will tell.
The forward voltage across the LED will be 2 - 3V @ about 10mA depending on LED colour and type. There are sensitive LEDs that will be fairly bright at about 1mA but you will have to find them. They are probably not in the general assorted colour packs. Special buy. Should use a current limiting resistor but the value would depend on lots of things. Namely the results of your trials.
“Peak” value or half P-P as the LED is still a diode and will only respond to one direction of pulse. Be a bit careful here as LEDs don’t take too kindly to too much reverse voltage. The data sheet for the particular sheet will list the Max reverse voltage.
Yes but what you are going to do in practice is the pertinent point.
Cheers Bob
