Re: Need help with intermittent circuit failures - JK Flip Flops

On Sat, 10 Jun 2006 20:51:43 +1000, "Jason S"
<jst3712@xxxxxxxxxxxxxx> wrote:

Thanks guys heaps. You been a great help.
I haven't had a chance yet to try your schematic John (Petrus and ehsjr, I
have noted your observations and advice as well), but I'll let you know as
soon as I've totally rewired my breadboard...ugh.

However, do I really need to add another IC to my board (the 2nd 555)? This
board has to be as small as possible... I didn't think I'd need so much
electronics to do this simple task. Maybe 1x 556 (dual 555 IC) instead of
2x 555 would save a little more space??

---
The second 555 isn't necessary. After thinking about it for a
while, Petrus' suggestion to use unused gates to get the delay
turned out to be a good one, so that's what I did. I've posted a
new schematic for you on abse which has that change (as well as
others) on it. After looking the schematic over, if you need a
circuit description I'll be happy to post one here.
---

By the way - in relation to the way I configured the monostable.... is it
still a bad thing to want to be able to trigger the monostable at power on
(ignoring the reed switch for arguments sake)? Assume pins 1 and 8 were
connected directly to the supply rails... so when power is turned on, the
monostable triggers automatically (due to C2).

---
And _stays_ triggered, so its output can never go low, U2-2 can
never go high, and the motor will never be able to turn!

For a 555 to operate properly, the signal on pin 2 must go more
negative than 1/3Vcc and then go more positive than 2/3Vcc before
the the output timeout period elapses.
---

I understand the correct
method of triggering pin 2 with a negative pulse, but then I would need a
separate circuit to achieve that as well - the negative pulse.

---
No, you wouldn't, all that would need to happen is that the circuit
be reconfigured. See my schematic.
---

That is the
only reason why I connected the reed relay the way I did - When the magnet
comes into contact with the reed, it triggers the 555, doesn't matter how
long the reed is in contact with the magnet for. In my circuit, if I made
it so that the reed switch triggers pin 2, the 555 would freeze (on 'high')
because the motor would have turned off and stopped right in front of the
magnet indefinately.

---
That's not true. Assuming a normally open reed switch, the way you
have it wired now keeps the 555's input low, which keeps its output
high until the magnet makes the switch, then the switch closes and
forces the outout of the 555 low until a switch bounce cause it to
start timing out again. That's one of the reasons for the source of
your problems.
---

I was just wondering that's all.

---

The two 15V zener diodes across the motor opposite each other is critical
yeah?

---
Maybe not critical, but you need them in there to clamp any
transients caused by a change of current in the inductance of the
motor.
---

Is my Q1 transistor overheating because these are absent? Or because
of the motor reactivating too soon before the relay switches over??? The Q1
is rated 0.8A.

---
More than likely because you're not providing enough base current to
drive the transistor into saturation. That's why Ed suggested that
you change R2 to 1K. The problem you may have with that is that
4000 series CMOS can't supply much current without going out of
rail-to-rail. I've paralleled three of the 4069's inverters in
order to circumvent that, and you should be able to drive the
transistor nicely with that "buffer"
---

P.S: John, I really appreciate your schematics... makes it a lot easier to
understand what you are talking about =).

---
Thanks, my pleasure. :-)


--
John Fields
Professional Circuit Designer
.

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