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

[Andy Talbot]

Those diodes are so robust that a PIC connected the wrong way to a 5V 1 Amp
PSU was protected by all these diodes conducting in parallel and current
limiting the PSU.   The PIC appeared to have survived (although I chucked
it anyway, just in case)

Andy
www.g4jnt.com



On Sat, 29 Jan 2022 at 18:56, Robert Atkinson via time-nuts <
time-nuts at lists.febo.com> wrote:

>  You can run the PicDiv on 3.3 V and connect the 5V signal to the PicDiv
> input via a series resistor between 1k and 10k. Put the resistor at the
> PicDiv end of the connection.
> The PIC has protection diodes on it's input These clamp the input to the
> supply. The series resisor limits the current. This is robust. There are
> thousands of devices out there with a pin conneted to the mains with just a
> series resistor. t's used for zerocrossing detection or monitoring the
> mains frequency.
> Robert G8RPI.
>
>     On Friday, 28 January 2022, 20:06:20 GMT, 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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