Power Latch Circuit Question

Hi.

....

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
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Reply to
Larry Brasfield
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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:

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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.

Reply to
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!

Reply to
Cauley Felps

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