[time-nuts] Re: electronics question or how not to fry my raspberry pi

[Bob kb8tq]

Hi

There also are logic families that are 5V tolerant when run off of 3.3V. That
makes finding a “translator” the same as finding any chip from that family. 
This may or may not make things easer to do / easier to find. 

One of many families is the NC7SZ series. One common gate:

https://www.onsemi.com/pdf/datasheet/nc7sz125-d.pdf <https://www.onsemi.com/pdf/datasheet/nc7sz125-d.pdf>

Should be pretty easy to find.

The gotcha with resistive dividers is the “lowpass” filter you create between
the R of the divider and the C of the input. That can make the process a bit
noisy / flakey.

Bob

> On Jan 28, 2022, at 3:20 PM, Andrew Kalman <aekalman at gmail.com> wrote:
> 
> I find that the best way to handle these translations is to use one of
> TI's level translators ... each chip has two power supply rails, and
> translation is done transparently across the chip, and there is good max
> voltage overprotection on both sides  as well. I use them a lot to handle
> 5V <-> 3.3V level issues.
> 
> Try the SN74LVC8T245PWR for unidirectional level translating . They also
> have some bidirectional ones ...
> 
> --Andrew
> 
> --------------------------------
> Andrew E. Kalman, Ph.D.
> 
> 
> On Fri, Jan 28, 2022 at 11:41 AM folkert <folkert at vanheusden.com> wrote:
> 
>> Hi,
>> 
>> I bought a GPSDO. It outputs somewhere around 3V. This is connected to a
>> picdiv and then to a raspberry pi. The picdiv is happy with 3.3v, the rpi
>> as well. All good.
>> 
>> Now I bought a "Square Wave Amplifier" by BG7TBL (
>> https://www.aliexpress.com/item/4000192799858.html?gatewayAdapt=glo2nld&spm=a2g0o.9042311.0.0.3d764c4dMZPAX8
>> ). Documentation I could find was a bit vague about the
>> output voltage but I measured 5v with a scope (see
>> https://vanheusden.com/permshare/scope.png - the scope software says
>> 2MHz but output is really 10MHz).
>> 
>> I did not study electronics, am only a electronics-hobbyist so bare with
>> me when this is a dumb question.
>> 
>> The RPI doesn't like 5v on its GPIO pins.
>> So I wonder:
>> - can I feed the picdiv 5v on its GPIO pin while giving it a 3.3v
>>  voltage so that it outputs 3.3v as well to the rpi pins?
>> - or should I use a voltage divider? I was thinking of a 4.7k ohm and
>>  8.2k ohm resistor giving slightly less than 3.2v - will that work? or
>>  will that attenuate the signal too much? The 50 ohm bnc cable between
>>  the amplifier and the rpi is 3m long. Anything else I should be aware
>>  of?
>> 
>> 
>> Regards,
>> 
>> Folkert van Heusden
>> PD9FVH
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