I recall finding something like this a while back, not positive on the details, but it's a start for you. I'm sure some others will recommend something better, I doubt this can provide a lot of current too.
Connect a couple of equal resistors in the hundreds of k range across the battery and drive an op amp connected as a follower from the centre tap.
Call the op amp output ground, and you now have + and - lines of half the battery voltage. Good for a few milliamps.
Ron H gets in with the diagram while I'm writing.
If it's a 9V pile type battery, you could measure half way up the side and hammer a nail in, hoping to make contact with 4.5 volts.
Maybe not, although I do remember screwing a self-tapper into the exposed links on the battery in my old Rover 105 to get 6 Volts to power a portable cassette player!
Personally, i would just connect a couple of 10K resistors across the supply, and a cap (maybe 100uF) in (edit)parallel with each resistor, and the point at where they meet in the middle is gnd, the - terminal of the battery negative, and + is +
People will frown upon this - they always seem to - as the voltage can be a bit unstable, but I've used this method quite a few times for op-amp based projects with no problems at all.
If you take the outer jacket off a 9v you can see all the little cells in there. I'm not sure how accessible the connections are. It's been a long time since I've done it, but it seems to me like it looked like a string of little sausages. I wouldn't recommend this. Check out "rail splitters." I think the 7660 is one but I can't find the data for it. I have one at home but it has been useless to me because it's easier to use two batteries, or just use single supply op-amps.
But I'm not an analog guy. The spec for most of my analog projects is just, "Must not catch fire while I'm not watching it."
lol, yes, well spotted. I wrote that post in a bit of a hurry!
There have been quite a few threads on this subject in recent times. A forum search should turn up a few things, although I guess most of them will have been mentioned here already.
Quote:
If it's a 9V pile type battery, you could measure half way up the side and hammer a nail in, hoping to make contact with 4.5 volts.
If you take the outer jacket off a 9v you can see all the little cells in there. I'm not sure how accessible the connections are. It's been a long time since I've done it, but it seems to me like it looked like a string of little sausages. I wouldn't recommend this.