Switching from +ve to -ve (Rather than +ve to 0ve)

I am trying to build a system using a 555 timer and logical gates and I have
come against a problem. I need my circuit to be activated by one switch. I
need the connecting or breaking of this switch to cause a drop in voltage
from +ve to –ve.
I am powering my circuit using a 9V battery.
I know how to connect the switch to my battery to cause the voltage to vary
between +9V and 0V, or connected slightly differently between 0V and –9V.
I have been recking my brains and I can't find out how I can make the output
voltage go from +9V to –9V depending on the state of the switch. If possible
I would like this to happen with low or negligible current flowing as this
would drain the battery.
Can anyone point me in the right direction?
Cheers
Robert

 

[Aaron Hughes]

Well what you are trying to do is pretty tricky, but here are some ideas.

If we neglect the battery limitation for a second, as assume we have all the
supplies
in the world, here are two options.

1. If this is a slow circuit (under 100kHz) you can hook up an opamp in a
"comparitor"
mode. Then if input voltage is above 8.5V, have the output swing to 9V.
If the input
voltage is below 8.5V, have the output swing to -9V.

above I am assuming that the 555 is giving this 9V, 0V, 9V, 0V, 9V, 0V, etc
etc

2. You could also setup an opamp circuit as a subtractor circuit. Then the
output
voltage has 4.5V subtracted from it. Then the output would be
4.5V, -4.5V, 4.5V, -4.5V, 4.5V, -4.5V, 4.5V, -4.5V, etc etc
Then you could have another stage that amplifies the voltage by 2,
making
9V, -9V, 9V, -9V, 9V, -9V, 9V, -9V, 9V, -9V, 9V, -9V, etc.

Now the really tricky part here is that you only have a 9V supply, and it is
a battery.
Problems here are taht the 9V only stays at 9V for a while, and then it will
begin
to drift to 8V, and then 7V and so on as it "dies".

In a nutshell to do what you want then, you need to pretty much make a tiny
power
supply that is driven by your battery. The output should be 9V, 4.5V and
ground.
Then in your circuits, all of the grounds will be connected to the 4.5V
point. Now
this will make your ground be "seen" as -4.5V.

Finally, in order to get the 9V and -9V, some amplification will be needed,
but this
is pretty tricky as well. Opamps will not help you now, but RLC circuits
will. You can
build amplifiers this way, that are based on "tank" circuits, but don't ask
me how, it
has been too long. The only problem is that while the voltage will be right,
there will
not be much power, i.e. once you connect a load to this output, it most
likely will
drop back to 4.5 and -4.5V anyway.

Anyway, I hope this gives you some ideas.

--
Aaron Hughes
http://www.canerdian.ca
<fake@reply-address.com> wrote in message
news:c3nd8c$sbs$2@newsg1.svr.pol.co.uk...

I am trying to build a system using a 555 timer and logical gates and I
have
come against a problem. I need my circuit to be activated by one switch. I
need the connecting or breaking of this switch to cause a drop in voltage
from +ve to -ve.
I am powering my circuit using a 9V battery.
I know how to connect the switch to my battery to cause the voltage to
vary
between +9V and 0V, or connected slightly differently between 0V and -9V.
I have been recking my brains and I can't find out how I can make the
output
voltage go from +9V to -9V depending on the state of the switch. If
possible
I would like this to happen with low or negligible current flowing as this
would drain the battery.
Can anyone point me in the right direction?
Cheers
Robert