Re: Reverse Polarity

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Fixed the list of crossposted groups.

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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

No.

The 555 is a timer with an output that goes high for a predetermined time when its input is triggered, and then goes and stays low until it's triggered again.

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You may not be.

If you could get into a little more detail about your application it
would help.

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.

reverse

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I'm

relay

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.

Bill

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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.

Your description is murky and incomplete with the biggie being just what you mean by turning what on and off..

There has to be a retained 'memory' of what the previous state was so if one assumes you mean that all power is removed from 'everything' then the 'memory' device must retain it's 'memory' without power. So you get suggestions like a magnetic latching relay.

If the power supply itself can remain on, disconnecting the 'red and black posts' for 'off', or if there can be 'standby power', like what keeps your PC clock alive when the PC is 'off', then a whole world of powered solid state devices opens up.

But none of your NE555 and flip flop ideas are going to work if all power is removed because then there's nothing to 'remember' the previous state. They go 'brain dead' with no power and will power back up the same way each time.

A small microcontroller with EEPROM or similar would be a simple solution, but only if the OP has some experience with them.

Jim

I thought about mentioning that but, with programming, it's an order of magnitude more complex.

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.

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It might...

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nand,

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

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http://www.google.com/search?hl=en&q=supercapacitor

The charge would last for:
 
          (Vp - Vih) C
     t = ---------------
               I

Where t is the time in seconds,
      Vp is the peak voltage the cap is charged to,
      Vth is the logic\'s Vih(min),
      C is the capacitance in farads, and 
      I is the current drawn by the circuit

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.

Cheers! Rich