Percussion

Interesting.

I happen to have some LM393 chips.  I might have to dig them out and try this circuit.

Thanks

DAB

Why don't ZD1 and ZD2 have any current limit?

I would expect that the diodes can handle more current than the piezo produce.

Do you have examples of current limiting use?

Sorry, I am not sure what the Zeners are for.  Nor do I know Vcc.

The zeners are there to clip the signal as it could go higher than the input range of the comparator. It's explained in some detail in the link from the original post.

Vcc is the postive power rail, which was +5v in my case. The logic levels on the beaglebone are lower, at 3.3v so you would want to use a lower level supply and also 3.3v zener diodes if you were to adapt the circuit for this challenge. The LM393 would work at that lower voltage without issue.

Why not omit D1?

Don Hersey wrote:

 

Why not omit D1?

That's a good question, you probably should ask the designer of the circuit, I'd guess that it's either because the behaviour of the diodes is not symmetrical or because it provides 2 paths of protection.

It's definitely not the only solution, but it did work nicely in my case and it should also (with the above adjustment for 3.3v) work for this challenge.

voltage clipping - How would I design a protection clipper circuit for ADC input? - Electrical Engineering Stack Exchang…

Protecting Inputs in Digital Electronics | DigiKey

Microcontroller Input Protection Techniques - Kevin's Projects

If you have a oscilloscope you could try different options and let us know if there is a difference or not.

Normally schottky diodes are used in this configuration instead of zeners - they provide better clamping and it doesn't matter what the rail voltage is. As you mentioned, a zener could be used to eliminate one diode in a specific voltage scenario, if it helps with the cost.

You are correct Andy - they clamp before the internal protection diodes of the chip, which would typically conduct at about 0.7 volts, so the Schottky diodes take the current instead and the chip is not stressed. If you want to rely on the internal diodes and eliminate the external diodes, check that internal diodes exist, then check their current rating, then place a resistor in series with the input to ensure the current does not exceed the rating. This is still not considered good design practice, but if you are stuck without diodes and need to get it working, it should work.