How to Wire a Piezo as an Impact Sensor

[holdred]

This is a lot of good info. Thank you.

@jremington

Incidentally, the first circuit (if series) provides no discharge path for the piezo (which is a capacitor) except through the input protection diodes.

Ah, you've explained the criticism of that diagram in my first forum thread (resolving a library conflict between a servo and speaker).

One of the hardest parts of getting started with Arduino, for me, has been finding good models for a given project. That diagram comes from the Tutorial Book enclosed in the Arduino Starter Kit. And apparently, there are similar problems with the Alarm Clock tutorial from Elegoo. (I'm aiming to make an alarm that can be silenced with a nerf gun, combining a simple clock with input from a piezo.)

As I learn more, it'll be easier to sort out good instructions from poor ones, and even to make use of the good parts of flawed code and plans. But when you're starting out, it's sure tricky to fix the plans you're using as a guide.

@jremington

Arduino inputs are limited to about 5.5V and -0.5V by internal diodes, which can be destroyed by excessive current flow. Hence the suggestion for the additional series resistor (10K to 100K).

I'll do that. Thanks. I wonder why none of the cited diagrams include it?

@JohnRob

I personally would choose the 2nd circuit (with the 470k ohms). The 470k could go to 1 meg if you are not getting a consistent signal. But you should add a 10k in series with the input to the Arduino.

I have a 100K and a 1M - will either do for that purpose? And to improve my understanding: if I run them in series, do 4 x 100K Ohm resistors provide 400K resistance?

And can you confirm placement of that additional resistor? I'm new to reading schematics, and what I see here is ground on the right, and if diodes point against the flow of current, current must flow from the top left (#1), so that means I'd add a resistor in series above the words "Analog Input." Is that right?

I have 5.1v Zener diode and several kinds of fast-recovery or Schottky diodes. Which are well-suited for that second diode (above the Zener in this schematic)?