Piezo Buzzer Buffer circuit?

[rocky79]

I am trying to use the piezo speaker as a sensor. If i touch it i would get an output voltage. But i want to include a buffer circuit between it and the micro controller. I used the common buffer configuration and the op amp output keeps latching to almost the supply voltage.
I know the piezo is a really high impedance element so i connect it to an opamp input it is as if there is nothing there because its high impedance and that's why the output latches. How to fix this problem?

Thanks

[Gary Warner]

I am trying to use the piezo speaker as a sensor. If i touch it i would get an output voltage. But i want to include a buffer circuit between it and the micro controller. I used the common buffer configuration and the op amp output keeps latching to almost the supply voltage.
I know the piezo is a really high impedance element so i connect it to an opamp input it is as if there is nothing there because its high impedance and that's why the output latches. How to fix this problem?

Thanks
Are you using an open-loop (zero feedback) setup? It shouldn't lock to a rail voltage if the gain is controled with a negative feedback loop (gain) network. Wire diagram would help here.

[Dan Baldwin]

A schematic of what you're doing would certainly help.

As Gary said, in order to use an op amp in a linear application, you will need negative feedback, meaning that you will need a large resistor between the output of the op amp and the inverting (-) input. Generally, for an inverting amplifier, the non-inverting input is tied to 1/2 of the supply voltage (you can use 2 equal value resistors as a voltage divider), the input signal is tied to the inverting input through a resistor. The ratio of the value of the feedback resistor divided by the value of the input resistor gives you the gain of the amplifier.

Dan

[rocky79]

A schematic of what you're doing would certainly help.

As Gary said, in order to use an op amp in a linear application, you will need negative feedback, meaning that you will need a large resistor between the output of the op amp and the inverting (-) input. Generally, for an inverting amplifier, the non-inverting input is tied to 1/2 of the supply voltage (you can use 2 equal value resistors as a voltage divider), the input signal is tied to the inverting input through a resistor. The ratio of the value of the feedback resistor divided by the value of the input resistor gives you the gain of the amplifier.

Dan
Hi Dan and Gary.
Here is what i have as a simple buffer. I also made another amplifier circuit but if i can get this to work then i am ok.
There is one important note. That is the Piezo acts as a capacitor parallel plates. So impedance is really high.

[Discharger]

Maybe try an inut load resistor from pin 3 to ground? 1m - 100k?

[MatC]

if by "common buffer configuration" you mean the voltage follower rocky79 posted up, you're on the right track.
A few points though:
- Piezo devices are AC, you can't measure constant pressure but they are great for measuring pulses
- Piezo devices output very high voltages, which may overload the opamp inputs and cause latchup, amongst other horrors. They also tend to give negative as well as positive pulses, causing similar problems.

The easiest solution is to check the opamp specs, and if they allow 0.6v beyond the power rails (most should), put diodes between the input and the power rails: diode from ground to opamp input (stripe on input) and diode from input to V+ (stripe on V+). Careful to get these the right way round :)
A resistor (eg 10 to 100k) after the above protection circuit and before the opamp will help further.
/Mat

[Dan Baldwin]

The piezo basically looks like a capacitor in that it is an open circuit to DC. If you want the output to be about half of the power supply voltage, you will need to put in two resistors of equal value, one from the positive rail, and one from the negative rail hooked to the non inverting input of the op amp where the piezo is connected. You can use large value resistors, perhaps 50K or so, depending on the input impedance of the op amp.

Dan

[rocky79]

if by "common buffer configuration" you mean the voltage follower rocky79 posted up, you're on the right track.
A few points though:
- Piezo devices are AC, you can't measure constant pressure but they are great for measuring pulses
- Piezo devices output very high voltages, which may overload the opamp inputs and cause latchup, amongst other horrors. They also tend to give negative as well as positive pulses, causing similar problems.

The easiest solution is to check the opamp specs, and if they allow 0.6v beyond the power rails (most should), put diodes between the input and the power rails: diode from ground to opamp input (stripe on input) and diode from input to V+ (stripe on V+). Careful to get these the right way round :)
A resistor (eg 10 to 100k) after the above protection circuit and before the opamp will help further.
/Mat

Mat, I think what you mean is a zener diode not just a regular diode?.
But i got the idea !