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New problems with the solved RS-latch problem

I have now solved the problem with the SR-latch, but I ran into a new problem (which regards what I think is oscillation) instead.

Block-circuit for this project:

------------------------- --------------------------

1 | Triggers (S1, S2, MR) | ---> | SR-latches (trig-lock) | --->

------------------------- --------------------------

--------------------------------------------------------

1 ---> | OR-gate (made of two diodes and a pulldown resistor) | -->

--------------------------------------------------------

-------------------------------------

2 ---> | 555-trigger (monostable, 50 secs) | --->

-------------------------------------

----------------------------------------

2 ---> | 555-trigger (astable, 50% duty, 1Hz) | --->

----------------------------------------

-------------- ----------------

2 ---> | Mech relay | ---> | "Alarm-lamp" | -------------- ----------------

Block 1 and 2 works as a charm separately, but when connecting them together the both (actually one, but it triggers the other automatically) 555:s just for a second and then the SR-latches gets reset and they in turn resets the 555:s.

I don't have a scope so I can't dive down into the circuit and see what happens and where, but I *think* the 555s cause a heavy oscillation for the Vcc- and GND-rails that makes the RS-latch spike on its R-input, and resets the outputs.

On play-hookey.com I read about the OR-gate I made (a DL-gate, diode-logic gate) and it said that DL-gates sometimes can cause a lot of distorsion in signals. Therefore I tried to use a "real" OR (74LS32) instead.

The error did however remain for the 74LS32 as well. The 555s run a second or two and the whole circuit gets reset. I tried to add a cap, sized 100nF, to the output of the 74LS32 to see if it was the OR-gate that wasn't able to fully source the 555. When I did this, the circuit seemed to work fine. I'm not sure this means there was oscillation in the circuit, since I haven't been able to dive down under the hood and investigate because the lack of a scope.

Conclusion: If I use a proper OR-gate and add a cap, the circuit works.

But, the problem in doing this are two. First of all, I would rather not waste a whole IC just for a simple gate, and second of all there will most likely not be enough room on the veroboard which this circuit will be residing after the prototyping is done. The space where the veroboard will be is physically very limited, and that's the reason I reached out for the solution to build an OR-gate of my own.

What I'm now wondering is if anyone, with the scenario above in mind, can take a good guess in why the RS-latch is acting as it does when I use my homebrewed OR-gate, or what on earth is resetting the circuit.

--
Sincerely,                      |                http://bos.hack.org/cv/
Rikard Bosnjakovic              |         Code chef - will cook for food
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Reply to
Rikard Bosnjakovic
Loading thread data ...

--
Why don\'t you post a schematic of what you\'ve got so we can see what
we\'re dealing with instead of wasting a lot of time having to ask
you questions?
Reply to
John Fields

You're right, ofcourse. The problem was that I didn't have any schematics, just the breadboard-layout. I have now transferred the breadboard into a proper schematic, "as is".

The schematics:

formatting link

Component-list:

formatting link 

--
Sincerely,                      |                http://bos.hack.org/cv/
Rikard Bosnjakovic              |         Code chef - will cook for food
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Reply to
Rikard Bosnjakovic

It says I'm using two 556. I have no idea why the application did it that way. I'm using a single 556.

--
Sincerely,                      |                http://bos.hack.org/cv/
Rikard Bosnjakovic              |         Code chef - will cook for food
------------------------------------------------------------------------
Reply to
Rikard Bosnjakovic 

--
What strikes me immediately is that the monostable isn\'t being
triggered properly.  What it\'s looking for is a short, low-going
pulse on the trigger input which doesn\'t last as long as its output
pulse.  Also, the circuit can be implemented without the latch, I
think.  I\'ll work on it and post back in a little while.
Reply to
John Fields

--- Here ya go... :-)

Vcc Vcc | | [1K] [10K] | | +--[0.1µF]--+--> TO U1-6 | | | O--+--O | BELL (NO) | | MAIL (NO) | O--+--O | | | Vcc | | | [1K] | | C |O--+--B DOOR (NC)| E |O--+ | | | GND GND

-- John Fields Professional Circuit Designer

Reply to
John Fields 

--
Looking at your schematic again, it may be even simpler than shown
above.  Something like this: 

                  Vbat        Vbat
                   |           |
                 [1K]        [10K]
                   |           |
                   +--[0.1µF]--+--> TO U1-6 
                   |
                   |
              | O--+--O |
    BELL (NO) |         | MAIL (NO)
              | O--+--O |
                   |
                  |O
        DOOR (NC) |
                  |O
                   |
                  GND

That is, if I\'m reading the logic right.

What I get from the schematic is that if the DOOR  switch is closed,
then when either the MAIL or the BELL switch is closed, you want an
alarm cycle to start.  However, if the DOOR switch is open you want
the other switches do be disabled and prevented from starting an
alarm cycle if either one (or both) is pressed.  Right?
Reply to
John Fields

maybe you beed powersupply decoupling, try putting 100nF capacitors between VCC and ground on yourr chips.

--

Bye.
   Jasen
Reply to
Jasen Betts

You want to operate a relay for ~ 50 seconds when either the door or the mail button is pressed? If so this will save you a bunch of PCB area vs the or-latch-timers:

1N4001 +12 ----+---+------+------|
Reply to
ehsjr

As soon as the door-switch is opened, the alarm timer (if started) should be stopped. It does not matter if the alarm gets started if the bell or the mail-switch gets closed while the door is open, because as soon as the pulse from bell and mail stops (they can't be "locked", thus shooting a pulse only), the alarm will stop immediately.

--
Sincerely,                      |                http://bos.hack.org/cv/
Rikard Bosnjakovic              |         Code chef - will cook for food
------------------------------------------------------------------------
Reply to
Rikard Bosnjakovic

--- That's a completely different problem, but one which can be solved by using the door switch to reset the timers in the 556, like this:

Vbat Vbat | | [1K] [10K] +---- - - - | | 6|_ +--[0.1µF]--+-----O|T1 | | | | | O--+--O | | BELL (NO) | | MAIL (NO) | | O--+--O | | | | GND 4|_ +--O|R1 ____ | 10|_ Vbat---O O--+--O|R2 DOOR | | | +---- - - - [10K] 556 | GND

However, in reviewing your schematic again, I found some more errors which I'll correct, then I'll post a new schematic to abse later on today.

-- John Fields Professional Circuit Designer

Reply to
John Fields 

--
Please disregard the circuit shown above.  The right one is on abse
under the same subject as this thread.
Reply to
John Fields

Here's a revised circuit, after you added that opening the door should shut the alarm relay off immediately:

1N4001 +12 -+-----------+---+------+------|
Reply to
ehsjr 

--
From his schematic, once the thing is activated, he wants the relay
to go on for about half a second every second, for about a minute.
Reply to
John Fields

Ahhh. I didn't realze that's what he wanted. Thanks!

Ed

Reply to
ehsjr 

--
You\'re welcome.

My pleasure, :-)
Reply to
John Fields

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