"Bill" wrote in message
news:870Gi.13845$bf1.9609@trnddc04...

"ian field" wrote in message
...

"John Fields" wrote in message
...
On Tue, 11 Sep 2007 22:51:12 GMT, "Bill" wrote:


"ian field" wrote in message
...

"Bill" wrote in message
news:gOxFi.6585$d01.6442@trnddc08...
"John Fields" wrote in message
...
On Tue, 11 Sep 2007 08:31:49 -0500, John Fields
wrote:

On Mon, 10 Sep 2007 02:30:56 GMT, "Bill" wrote:

Anyone know of a way to reverse polarity (9 to 12V DC) each
time
the
power
is removed? In this gadget I have there is a small 12V supply.
When
I
turn
the gadget off and then turn it back on, I wish to have the 12V
reverse
polarities. For another example, I have a black box with 2
posts,
the
top
post is +12v and the bottom post is 0v. I turn the power off and
the
next
time I turn the power on, I would like the top post to be 0v &
the
bottom
post to be +12v. Is there like a simple flip flop chip or ??
than
can
accomplish this task? All comments most appreciated. Thanks in
advance.

---
Please crosspost instead of multi-posting.

I've crossposted this to seb and sed.

You could use a 555 pulsing on power-up to toggle a DPDT
dual-coil
magnetic latching relay by using one set of the relay's contacts
to
steer the pulse and the other to switch a DPDT single-side-stable
relay on and off to get your polarity reversal. The caveat is
that
the pulse to the latching relay has to be short enough so that
it'll
be gone before the armature gets to the other contact or both
relays
will buzz or, at least, switch back.

You could also use a 555 to drive a solenoid which would toggle a
DPDT alternate-action pushbutton switch.


John ... This is all low current operations. How about something
like
a
divide by 2 triggering a 555? Can this give alternate highs and
lows
on
pin
3, the output? I can figure what to do after pin 3. I also wonder
if
I'm
explaining the problem adequately.

Bill
--
JF



A good old fashioned 2 transistor bistable with input coupling
capacitors
and steering diodes and a small NiCd battery charged when the PSU is
on
to
retain the bistable state when its off, a driver transistor to power
a
relay
double pole changeover when the PSU is on.

I think you got it! Sounds like a viable plan. I will study it. Thanks
so
much Ian. This is in the realm of what I need gents, so if there are
any
other ideas along this line, please tell me.

---
While seeming attractive at first blush, the down side of Ian's
scheme is that you'll need to build the whole thing using discretes,
you'll need a charging circuit for the battery, and you'll need to
replace the battery when it fails.


--
JF

The same basic configuration could be assembled with a CMOS quad 2 I/P
nand,
it might even be possible to retain the state with a supercapacitor.

I know what you mean by a cap retaining the charge but I never heard the
term super cap. What size and type cap would I need and how long would the
charge last? All ears, er, eyes..

Bill

Super caps range from about 0.22F to several F, some smaller values are
available with voltage rating of 5.5V but many of the very large values only
2.5V. They are typically used for settings memory in things like cash
registers and VCRs etc.

Another possibility is a lithium coin cell like the 2032, these last for
many years in PC RTC+NVR including the older PCs which often used a 74C04 as
the RTC crystal oscillator.