Power Latch Circuit Question

[Cauley Felps]

Hi,

I'm building a PIC-based MIDI timecode LED display from the web. I'm
mounting it in a small enclosure I have, and would like to use the
existing power switch (a SPST tactile button).

I found a schematic for latching power using 4 transistors here:

http://www.edn.com/article/CA472837.html

I changed the resistor values for my 5v supply and plugged it up on a
proto board. It seems to work OK, other than the fact that it powers
up in the "on" state (I didn't think that was normal).

There's a 7805 regulator in the PIC circuit with a 2200uF and 330nF
cap parallel across the input and ground, and a 10uF and 100nF
parallel on the regulator's output and ground. There's also 4- B40C800
diodes, so it can take AC or DC, but I've been testing it with a nice
clean 12vDC supply going straight to the 7805 and filter caps.

I planned to use a small 120AC to 12V transformer when I put it
together. I may also stick a DPDT 120V slide switch on the back panel
to kill the power completely.

Does all this sound like the most logical way to go about it? If so,
is the transistor/latch circuit OK after the regulator and filters, or
would it dirty up the power feeding the PIC and LED's?

I'm open to any other suggestions on the power supply or latch
circuit.

Please reply in the newsgroup
(and thanks a lot)

George

FWIW- my values came out to
R1- 10M R6- 2.3M
R2- 470k R7- 5M
R3- 2.3M R8- 500k
R4- 1k R9- 5K
R5- 50k

the cap stayed at 1uF and the transistors were BC547's and 557's.
R1 started at 2.3M, but it would switch back on if you held the button
for more than a split second while turning it off.

 

[Larry Brasfield]

"Cauley Felps" <cauley@nomail.com> wrote in message
news:j5lc21ho5ku4u4o9cn53n91hhh715or0ss@4ax.com...

Hi,
Hi.

I'm building a PIC-based MIDI timecode LED display from the web. I'm
mounting it in a small enclosure I have, and would like to use the
existing power switch (a SPST tactile button).

I found a schematic for latching power using 4 transistors here:

http://www.edn.com/article/CA472837.html
....
I'm open to any other suggestions on the power supply or latch
circuit.

There is a somewhat simpler approach, at least from
a hardware perspective, when you have a general
purpose (micro)computer available.

Imagine you have these hardware elements:
1. An electronic switch to connect or disconnect
power from all of the circuit except as noted here.
2. A mechanical, momentary switch which, when
depressed, forces the electronic switch to be on.
3. A programmable system with an input by which
to read the state of the mechanical switch and an
output which can force the electronic switch on or
off, but which, as reset is asserted, does not force
the switch on or off. (An output that is tri-state by
default upon power-up or reset is easiest.)

Then, use this algorithm:
Upon power-up reset, wait until the mechanical
switch appears to be depressed for a time longer
than any normal bounce, then assert the output
that holds the electronic switch on. Monitor
the mechanical switch for non-depression, lasting
longer than a normal bounce, before beginning the
next step. Monitor the mechanical switch during
normal operation, (via interrupt or polling), often
enough to be able to notice subsequent depression.
Once depression is observed lasting longer than a
normal bounce, enter a pre-shutdown state where
all or most indications of power being on are not
displayed. In this state, wait for the mechanical
switch to be non-depressed for longer than a
normal bounce. Then negate the output holding
the electronic switch on.

This may seem involved, but it requires somewhat
less hardware than the article you cited.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Cauley Felps]

Larry,

Thanks! I think I get most of what you're saying, but may not be
capable of pulling it off at this point. You're talking about using
the same PIC to monitor the button presses right? I guess this would
still leave the PIC powered up and just turn off the display output.
I'm also not sure that the PIC and program I'm working with could
handle any additional functions. I'll save this info for later. I'm in
the process of getting familiar with PIC code, but it will take me a
while. All the stuff where I got the circuit is open source and he
sort of encourages changes and enhancements.

I'm still wondering about the rest of it, if you or anyone here knows.
(the location of the latch circuit and the reason it powers up
latched)

-Thanks!