Triggering a 555 timer with a piezo disc

Other Hardware General Electronics
1

Greets,

I made up a little circuit (see attach.) with a piezo disc and a 555 timer that will use to trigger an external interrupt. The circuit works, i.e. the piezo triggers the 555 timer but I was wondering if there is a simpler (fewer components) circuit for triggering a 555 IC with a piezo element?

piezo zener 555 trigger circuit (with values).jpg
piezo zener 555 trigger circuit (with values).jpg669×540 42.8 KB

2

Lose the zener, make the 220 ohm resistor 10k so it limits current to safe levels?

3

if I take out the zener the spikes go to 20V and over, I don't know if that's good for the 555 trigger pin. I'm hitting the piezo pretty hard, tapping tempo. I've tried different resistor values for R1, up to 300K, works the same, so I'll just use a 10k instead of the 220.

4

a pair of clamp diodes to rail and ground would do the job - 1N4148's would be fine

rehards

Allan

5

Actually yes that would be wise, especially with the CMOS version the 7555 (which is superior in many respects).
Schottky clamp diodes conduct first and protect better against latch-up. The standard '555 inputs are not
rated for min input voltage AFAICT.

6

a pair of clamp diodes to rail and ground would do the job - 1N4148's would be fine

Like this?

piezo zener 555 trigger circuit (with values and clamping diodes).jpg
piezo zener 555 trigger circuit (with values and clamping diodes).jpg669×540 53.2 KB

7

I would use a ground referenced circuit, something like this, but adding the two clamp diodes:

8

Any reason to DC bias it?

9

My idea is enclosed... - much as post #6..

The reason for the bias is that the 555 only triggers on a negative going edge..

regards

Allan

piezo.pdf (15.5 KB)