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.
Dave G8KBV
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