I am trying to build a switching circuit using a hall effect transistor.

The circuit that needs to be switched (I'll call it circuit "A") 
runs 2.5 volts. It is in between two pins of a larger circuit, 
so the "negative" pin (following current flow from pos. to neg. ) , 
   is *not* grounded -- it goes back into the circuit.

One option for me would be to use a reed switch. This would
simply connect the two wires when a magnet is passed near the switch.
The reed switch does not have to be connected to ground.
Using the hall effect transistor would prevent me from having to replace
broken reed switches, or ones that have over time become magnetized
and therefore "normally closed" instead of normally open.
However the hall effect switch, at least the one that I bought for testing,
requires that it be grounded. It is a 3 pin with input, output and ground.
I have built a test circuit with a hall effect transistor as shown here: 
http://www.physics.lsa.umich.edu/demolab/demo.asp?id=387
where an led lights up between the hall effect output pin and
power. 

I am not sure where to add circuit A into this,
because circuit A has no ground, all the example circuits

What is circuit A, you might ask, and what is this all for?

Circuit a is an electronic organ brain, which I have removed
from the original housing and intend to use to midi-enable
an old hammond organ pedalboard. 
(one of these: http://pages.infinit.net/yumbambe/jpegs/A100_41576/Pedalboard.jpg )

The hall effect switches would be for playing each of the notes.

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Clarification of Question by rseaman-ga on 09 Dec 2004 12:43 PST

Seeing an unfinished sentance in there --

"I am not sure where to add circuit A into this,
because circuit A has no ground, all the example circuits" 

should read: 

"I am not sure where to add circuit A into this,
because circuit A has no ground, all the example circuits I've found
for using a hall effect or normal transistor as a switch are grounded
-- none for controlling a separate, isolated circuit."

Your choice of hall switch is quite reasonable. Not a ?transistor?
BTW. Good reed switches last for millions of operations though but
*never* discharge a capacitor through them. Even 0.1uF charged to 5
volts, instantly discharged, will rapidly destroy or weld the
contacts.

What resistance is acceptable for the switch? If you can tolerate a
few tens of ohms, analogue switches should be ideal. Cheap too. All
you need to do is enable them from the hall switch but you may have to
do some level shifting or voltage inverting. Easiest to use are 4000
series CMOS such as the 4066 (also known as bilateral switch), a quad
device. For lower resistance go for the 74HC4066. There are several
other ?4066?s so peruse manufacturers web sites and check out the
characteristics and voltage rating etc. Those numbers are prefixed by
CD and SN etc but 4066 is the important number. Bear in mind that you
probably don?t want surface mount.

You can get a slight click with analogue switches (due to charge
transfer) but I suspect in your application you won?t hear it. And the
hall switch doesn?t bounce, so depending on the rest of the circuitry
the hall effect may sound better than reeds.

You don?t say your country so that means you are in the only country
in the world -- apparently ;-) So I had a brief look at Digikey cat
and found several devices. Have a look yourself. Any questions (or if
you can?t find what you are looking for) just fire them back. Let me
know how it?s going.

Best

I'll order one of these and give it a shot!

Thanks a lot.

You'll need quite a few of course, but cheap. Any probs, there are a
few techies (other than myself) pleased to assist with future queries.
Good vibrations :-)

Best