[PD] (OT) safely connect piezo transducer to soundcard as microphone

[Martin Peach]

On 2012-04-21 11:07, Matteo Sisti Sette wrote:
> By the way, thinking about it again, I think the sound card applies a DC
> voltage to its input (about 3V) (in order to polarize an electret
> microphone), so I'm afraid the parallel diodes can't work because the
> one pointing downwards would always be "on".........


That sounds unlikely. If it had 3V on the input you would always get a 
high value with nothing connected. Anyway, it's better to use a line 
input with piezos because their output is in volts, not millivolts.

If it really has 3V on it, putting a 10k resistor in series with the 
input will protect it and still pass the piezo signal, which is very 
high impedance, so won't be diminished much:

IN----+----+----/\/\/----OUT
       |    |
       ^    V
       |    |
GND---+----+-------------

Martin


>
> On 04/21/2012 05:03 PM, Martin Peach wrote:
>> On 2012-04-21 10:16, Matteo Sisti Sette wrote:
>>> On 04/21/2012 12:38 AM, Martin Peach wrote:
>>>> You could put a resistor in series to limit the current or a pair of
>>>> diodes in parallel to clamp the voltage. Probably a pair of 1N4001s
>>>> like
>>>> this would work:
>>>>
>>>> IN---+------+-----OUT
>>>> | |
>>>> ^ v
>>>> | |
>>>> GND--+------+-----
>>>
>>> Hi,
>>>
>>> Thanks for the suggestion. If I understand correctly, here the
>>> non-ideality of the diodes is what does the trick, and this would limit
>>> the voltage to below the diode's drop, right? which is about 1V or so...
>>>
>>> Now, thinking about it, wouldn't the following be even better? (this
>>> hadn't come to my mind before)
>>>
>>> IN---+------+-----OUT
>>> |
>>> v
>>> |
>>> |
>>> ^
>>> |
>>> GND--+------+-----
>>>
>>> with two ZENER diodes of appropriate reverse voltage? Facing each other?
>>>
>>> So within the "allowed" voltage range there would be no (or much less)
>>> distortion?
>>>
>>> Maybe I'm being too naive here?
>>>
>>
>> Yes that's the next step, if you want a larger voltage range. Or use
>> strings of ordinary diodes to add about .6V per diode. I think if you
>> only want to detect hits it doesn't matter too much. Ideally, you would
>> know the voltage range of your audio input and set the levels to fit
>> that.
>>
>> Martin
>>
>
>
>