Electrical – the characteristic of piezo transducer

piezo

I want to detect the tap/sound, I am using piezo transducer (circular) and connected it to raspberry pi using ADC (MCP3008, SPI communication). When I tap on the transducer the output value should increase somewhere between 0 to 1023 depending on the tap vibrations. But I am getting reverse output values, the output value read is 1023 when I do not tap and when I tap it continuously the output value reaches to 0(Zero). Could anyone tell me the reason for this? I have connected red wire (+ve) of transducer to the CH0 (channel 0) of ADC and black wire of transducer to GND of raspberry pi. I think the connection is perfect. Is that characteristic of Piezo transducer, to send output low signal when tap is detected and high signal when no tap? Below is the code 

#!/usr/bin/python

import spidev
import time
import os

# Open SPI bus
spi = spidev.SpiDev()
spi.open(0,0)

# Function to read SPI data from MCP3008 chip
# Channel must be an integer 0-7
def ReadChannel(channel):
  adc = spi.xfer2([1,(8+channel)<<4,0])
  data = ((adc[1]&3) << 8) + adc[2]
  return data

# Function to convert data to voltage level,
# rounded to specified number of decimal places.
def ConvertVolts(data,places):
  volts = (data * 3.3) / float(1023)
  volts = round(volts,places)
  return volts

# Define sensor channels
piezo_channel = 0
piezo_level = ReadChannel(light_channel)
piezo_volts = ConvertVolts(light_level,2)


  # Print out results
  print "--------------------------------------------"
  print("Light: {} ({}V)".format(piezo_level ,piezo_volts ))

  # Wait before repeating loop
  time.sleep(delay)`

Best Answer

Piezo-electric transducers approximate, to a first order analysis, a capacitor.

Assuming you have hooked up the device like this:

schematic

simulate this circuit – Schematic created using CircuitLab

As a transducer of this type has no current flow in a static situation, then with no vibration at the transducer, the voltage read in the ADC is indeed the voltage at the positive side of the transducer.

The reason you even see other readings when you tap the device lies in the input circuitry of the ADC you are using:

MCP3008 Input Stage

On the first measurement (no tapping) the input sampling capacitor will charge up to Vpos; when you tap the transducer (assuming the channel is connected for sampling), a small ac current will flow from Vpos and the sampling capacitor in the ADC, changing the voltage on the sampling capacitor, so you read a value other than full scale.

Note that the sensitivity of the input will depend on the sampling frequency (which I do not know); the sampling capacitor is in circuit for 1.5 clock cycles.

Tap it often enough and the sampling capacitor will completely discharge (where you will read 0 from the ADC result).

To properly read it, you should really hook it up like this:

schematic

simulate this circuit

The value of resistor here is not necessarily accurate; there are only small currents induced in most of these devices.

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