> I have the allegro 3209 which runs at 3V and although there are numoreous
> hall sensors this is the only one working for me.  The hall needs to give
> pulse on any magnet pole N or S and no signal when there is no magnet (I
> think it is called omnipolar).  If I could find the same sensor running at
> 5V another problem would arise as I need to run this circuit from either a
> 3V or 5V supply.
>
> The hall sensor is powered from 3V, the open drain output is connected to
> the + side of the reed relay (of course the relay needs 5V and it does not
> work with this setup), the - side of the relay goes to ground.  This is
the
> basic circuit.

You can do this with 1 transistor and 1 resistor. Size would be minimal
Works for any reed relay voltage you wish to use .

PNP
Emitter to V+
Base via resistor to hall sensor output
Collector to reed relay (say 10k to 100k)
The ultra cautious might add a 1M resistor from base to V+ but it will work
without it.

Only caveat is that the hall sensor open collector needs to be able to
withstand the supply voltage used by the reed relay.
The 3209 is actually rated at 5v abs max on the output and they don't give a
normal operating voltage. Adding a zener of at least 2 volts rating in the
hall sensor output (anode to hall sensor output) would make it safer. Or a
few diodes facing in the opposite direction.
Add one more R and an NPN and you have a bullet proof design and still very
compact using SM parts (2 resistors, 2 transistors). A dual npn/pnp may be
available.




        RM

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