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

[Dave B]

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


On 29/01/2022 08:30, time-nuts-request at lists.febo.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

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