I really like the simplicity of this idea.  It's very easy to understand
how it works.  The only drawback I see is that this circuit will be
triggered about 10,000 times a day, 200+ days a year.  I expect the
relay contacts to burn up within a few months or years with that much
activity.  But, maybe not.

In any case, I can easily visualize a solid state version of this using
an SCR and transistor as someone else suggested.  Thank you everyone for
your input.  I will post a followup message once we find a circuit that
works well for us.

-Pat

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of M. Adam Davis
Sent: Tuesday, June 01, 2004 8:44 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Sequential Logic Circuit - two inputs / one output

The easiest and most intuitive method is to use two relays.  Easy to
wire up, parts available at any electronics shop.

The first relay is wired up so that it powers itself through its
normally open contacts.  Use A to power the coil, and the relay closes.
The relay stays closed because the contacts are now closed and it
continues to power itself in the absence of A.

The second relay provides power to the first relay with its normally
closed contacts.  B powers this relay - when it's powered it cuts power
from the first relay, which unlatches.

If you really need to have power drop on the falling edge of B, use two
diodes to the output, and let B power the output as well as the first
relay.

I've attached a very small schematic to illustrate.  5v low current
relays are readil available, and can switch much larger loads if
needed.  Sometimes I think we overthink some problems where a simple
solution such as this may be perfect.  The main failing here is that it
can be bulky compared to an few surface mount devices, but reed relays
are fairly small considering what they do.

-Adam

Patrick Richards wrote:

>I am looking for help in designing what I am sure is a very simple
>sequential logic circuit.  The circuit will have two inputs and one
>output.  Lets say the inputs are A and B and the output is X.  A and B
>will be at either 0VDC or +24VDC.  I have +5VDC and +24VDC available to
>the circuit, so X can be high at either voltage, but I would prefer
>+5VDC.  The initial state is A, B, and X are low.  On the rising edge
of
>A, X goes high.  A is the output of a pushbutton switch.  This could
>actually be rising-edge-triggered, or high-level-triggered.  A will
>return to low.  On the falling edge of B, X will go low.
>
>
>
...

>My main concern is to keep the circuit as simple as possible.
>
>

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