I just arrived at Project 12 from the guide book of the arduino starter kit. Some day I had the grasp of everything and some day I didn't understand a thing.
Today sadly is the day where I don't understand the use of the 1MOhm resistor.
The project guide says that I need to use a 1mOhm resistor between GND and the A0 Input Pin wire. The other end of the piezo is directly connected to 5V.
That works well. When I analogRead @ A0 it gives a pretty good range of values between 0 and 200 depending of the hardness of the vibration.
What I did not understand is why it depends of the resistors strength? If I check in a 220 Ohm resistor no values are received. It reads only 0 all the time?
Can anybody explain it to me? Does it has something to do with the piezo's resistants?
The piezo generates lots of voltage but very little current. With no resistor you can get 50V from it just by flicking it with your finger. The 1M resistor absorbes the current so that you only get about 5V across it. If you put a lower resistor then it will absorbe too much current and will not be able to develop enough voltage across it to cause any effect.
Hmpf...I thought if the 50V hit a resistor without any resistor coming after that one, the voltage before the resistor always has 50V and no loose in this voltage.
The correct explanation is that piezo elements generate constant current, and without a
resistor across as much voltage as is necessary to make that current flow (until leakage
absorbs it).
The resistor means the voltage is propotional to the current, with a much more sensible
range of values. Without it the voltages typically exceed 5V and the chips protection diodes
carry the current (which is thankfully rather small and not destructive).
Actually piezo elements are ceramic capacitors that are designed to be bent, its the same
type of material - bending forces displacement of charge in the atomic lattice which shows
up as charge at the terminals proportional to the bending strain. Current is proportional
to the rate of change of bending strain.
This is why you don't use ceramic capacitors in high gain audio circuitry, they are microphonic.
Falke88:
Hmpf...I thought if the 50V hit a resistor without any resistor coming after that one, the voltage before the resistor always has 50V and no loose in this voltage.
True but that is not the case here, the other end of the resistor is going to ground. The piezo is a voltage generator and has a high impedance and so that impedance and the resistor form the potential divider to reduce that voltage.
This is true for other abbreviations where both mega- and milli- prefix are commonly used (in cases there only one is in common use, like megahertz, people aren't careful on capitalization, and both mhz and Mhz typically refer to megahertz )