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Topic: Piezo question (Read 2414 times) previous topic - next topic


Hi everybody,

I've posted before about this project- I'm trying to make a disdrometer. I have a specific piezo question, though.

So I started my project with these little piezo elements from Sparkfun (http://www.sparkfun.com/products/10293)

for my application, I needed a bigger sensing disk (I wanted a bigger area to capture raindrops on, essentially), so I went ahead and started working with these 44mm piezo elements from digikey:

After a month of trying to amplify tiny voltages, I went back to my Sparkfun piezos and realized that they create a bigger voltage / force (raindrop impact) than those digikey ones  :smiley-eek-blue:

I don't understand anything on those datasheets - like what the impedance or capacitance of the element have to do with the voltage they put out. I'm an environmental engineer and I've been struggling for this project for a long time. Can anybody tell me what values on those datasheets to look at for determining voltage outputs? Where do I get better piezos? I looked at the company that sparkfun uses and apparently they are based in Japan.

any help/feedback would be appreciated. I'm a total n00b and I've got a month to finish my project! thanks!

thanks a billion (raindrops),


It's unclear what you meant by "trying" to amplify small voltages, but the guy on this
thread has an amplifier that looks like it will work ok.


You shouldn't have to worry too much about the impedances and capacitances of the piezos,
just amplify the signals using a simple voltage amplifier. A hard rap on a piezo will generate
many volts. like 5-20V, and one imagines a raindrop is just a tiny rap, and the piezo only puts
out a few millivolts.


Thanks for your response. So I've already used that circuit, and it doesn't amplify the signals that I need. With my 'bad' (Digikey) piezo elements, the noise is bigger than the signals for my smaller drops.. basically, I can get a signal to amplify when it's larger than a certain threshold, but otherwise it doesn't work. I've tried a number of ADCs and a number of circuits with no luck.

Basically I'm looking to see if there are more sensitive piezo elements out there (I know that some are more sensitive than others from my experience with the SparkFun piezos); where I am getting ~20-30 mV for a large drop, I know of a researcher in the Netherlands that can get 1V from the same diameter drop at a slightly higher speed. So clearly there is some difference in the quality and sensitivity of these piezo elements. His was off-the-shelf, but I don't know where he got it from.

He gets my big signals (20-30mV) for drops that are ~0.2-0.4 mm, which is pretty much just a light drizzle. I think for my piezos those would come out as microvolts but I don't really know.

Any other ideas?


I think for my piezos those would come out as microvolts but I don't really know.

Well, I didn't spend a month on this project, but this what comes to my mind after reading:
1. Piezo is a current source. When you talking 20 mV or 1 V it doesn't make any sense w/o R-load. It's simple, sensor generate a current proportional  to mechanical force, you setting resistor in parallel and getting voltage to amplify father.
2. There is a trade. Setting as high R as you could, you increasing voltage output ( V = I x R ). But the same time frequency response become sloppy, sensor would be high sensitive to single drops but completely lost in heavy drums, where pulses would "overlapped".
3. High R also source of noise.
4. You need to look at C (capacitance) in data sheet. Than, make estim how many water drops per second you need to count - max, it would gives you an Freq. Make a slack, 3-5 times coefficient.  For example, 50 raindrops/sec = 50 Hz. x coeff 5 = 250 Hz.
5. You setting an R = F / C, where F from 4 and C from data sheet
6.  Than you make a measurements of voltage, ( unfortunately, I'd not find Force-Current chart in data sheet), and based on that you make a search an amplifier that satisfy requirements : Gain, Noise, Freq., etc, (don't look at any crap some dumb landscape-designer post on-line),


I can imagine the noise is larger than the signal for small drops. If you're intent on using piezos,
and you've already tried a lot of different amplifiers, well there are many different piezos
on the market, so you might just buy one or two of every one you can find and test it.

I imagine piezos pick up background sounds and vibrations, so possibly placing the pickups in
some kind of sound-deadoning box might help.

OTOH, if the noise problem is just be too great, well raindrops are water and should have some
conductivity, so you might try going to some kind of conductivity-sensing device.

A couple of other ideas also come to mind. One, is to build a little reservoir so the raindrops
will "integrate", so one large drop will flow out the outlet when enough small drops come in.
A simple funnel might do, as the drops will tend to congregate due to surface tension and a
larger drop will drop off the bottom of the funnel. Plop!

The other idea is some kind of raindrop light-beam interrupt pulse-counter.


The problem with these disc sensors is that they are a piezo element on the side of a disc of metal - the larger the metal disc the less impact a raindrop is going to have on it - a group of smaller sensors will likely be more sensitive since less metal has to move for each raindrop impact (and therefore responds more)

However don't expect them to stay working for ever, the piezo material may not tolerate water or acids in the water (some research would be needed to find out).

You may find an electret microphone fairs better for signal-to-noise (though amplification is then required).
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