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

[glen english LIST]

I endorse Dave's suggestion (below) .

The resistive divider is simple but slows down the edge rate which may 
be undesirable especially if the input isnt a schmitt.

The MOSFET solution is the most appropriate. take a look what is inside 
packaged translators... just that.

On 29/01/2022 10:33 pm, Dave B via time-nuts wrote:
> You can actually use a single small N channel MOSFET (2N7000 or 
> similar) with it's Gate connected to the lower Vcc via, say, a 1k 
> resistor.  (Not strictly needed, but with long leads, it helps prevent 
> HF transient oscillation.
>
> Then use it's Source as the lower voltage data line, and it's Drain as 
> the higher voltage data line.  You get two way communication like that 
> too.   (Hint:  Consider the parasitic diode between the Drain and 
> Source.)
>
> Action:  everything floats high to 3.3V or 5V.  FET is not conducting.
>
> Pull the 3.3V side down, FET turns on (Gate is +3.3V wrt Source) and 
> the Drain pulls down the 5V side, note the 5V side pull down current 
> will pass to the 3.3V side, so take care that the 3.3V pull down port 
> is capable of sinking that current as well...
>
> Pull the 5.5V side down, Parasitic diode conducts, pulling down the 
> 3.3V side that in turn also turns on the FET, providing a lower 
> impedance path "backwards" though the device.  As above, the 5V side 
> "sinks" the 3.3V pull down current.  It all works magically well.
>
> Many of the simple multichannel bus voltage translators that do NOT 
> provide galvanic isolation, are just like that internally, but with 
> pull up's to the two Vcc rails, and some extra protection diodes etc.
>
> I've done that between 3.3V and 5V systems, and between 5V and 9V 
> logic systems with discrete parts, It works very well indeed.
>
> To a limited degree, the same "trick" also works with many bipolar 
> transistors, with a diode from Emitter to Collector, and a better 
> choice of resistor on the Base.
>
> Have Fun.
>
> Â Â Â  Dave G8KBV