OT: Tactile Momentary Switch for System Power

[jecottrell]

Hello All,

I'm awaiting membership in the Circuit Cellar forums as recommended by several others and thought I'd try here for some ideas in the meantime.

I'd like to use a single momentary tactile switch to power up and down a system and was trying to figure a simple way to do this. I found a EDN article with a 10 part circuit that can power up a system but not down.

I just thought of controlling the on/off pin of a regulator with the mom switch (ON) and also driving it w/ a IO pin (OFF).

Any suggestions on the following would be greatly appreciated:

Search terms to find what I'm after.
Specialized ICs that handle this in a single package for simplicity's sake.
Your favorite approach that you're willing to share.

Thanks,

John

[PCM programmer]

Here's a previous discussion:
http://www.ccsinfo.com/forum/viewtopic.php?t=27580

Here's one example of how to do it. It's not necessarily the best one.
http://elm-chan.org/works/mp3/report_e.html
Click on the link for: "Main board circuit diagram (Jun. 29, '00)"

[jecottrell]

PCM,

Thanks, I had recalled that thread you referenced but couldn't find it in the search. (I find the phpBB search tool extremely painful.)

The reference to the EDN article in the previous thread was the one I had found. I was hoping there were more ideas floating about....

I ran simulations in PSpice of both circuits in the article and couldn't get the circuit in figure 1 to work. Finally after re-reading the text I saw that there must have been an error in their diagram. The R1-S1 node wasn't connected to the Q1-Q2 node. So.... If anybody ends up here in search of a solution to a problem similar to mine... beware of that typo...

Thanks again,

John

[jma_1]

Greetings,
I have not read the Circuit Cellar article yet, but perhaps you could use a supervisory / reset chip. Maxim makes a couple MAX6467 and MAX6468 that seem interesting. Depending on the application the MAX6453 - 6 might also work. Maxim will also gladly send you some samples. :-)

http://www.maxim-ic.com/products/supervisors/switch_debouncers/

Cheers,
JMA

[treitmey]

Note that circuit cellar made me go to another web site as part of the setup, so they can avoid spammers. Its all in the emails.

[future]

Why not use a flip-flop?

[jecottrell]

Sorry. I'm new to all this and I'm not familiar with all the options out there. I read up on flip-flops and that sounds like exactly what I need.

However, is there such an animal that allows 20V in on one input and 3.3V in on another?

I'll start looking....

Thanks for the idea.

John

EDIT:

On first glance it apears as though they all are limited to a max of 7V Vcc. That won't work being on the upstream side of the regulator.

Or is there some other way to do it with a flip-flop?

[future]

you can use it to control your power relay/switch.

[jecottrell]

So are you saying that the flip-flop is downstream of the regulator (requiring that the regulator be powered 100% of the time?) If that's the case then it's not workable. I can't afford powering the regulator and the flip-flop when the PIC is powered down. I apologize if I'm still missing the gist of your approach.

I was also wondering how high a voltage the clamp diodes on the PIC inputs could handle through a 100K+ resistor? Or should I just put a better clamp to be safe?

Thanks,

John

[asmallri]

Hi John,

What is your power budget for the powered down system?
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Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!

[jecottrell]

Hi Andrew,

Sorry about the typical squirrelly customer answer of "I'd like to keep it as low as possible." But, in the proof of concept/prototyping stage where I am now, it's not too important. Eventually if everything works out, I'll contract the final design work to a pro. (That will be both hardware and firmware...)

Current PSpice simulation shows about 25uA with my current solution.

Thanks,

John

[kender]

There's a family of on/off pushbutton controllers made by Linear Technology. LTC2950 through LTC2954. Most of them come in a SOT-23-8 package.
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Read the label, before opening a can of worms.

[mskala]

[Guest]

This works,

edn.com/contents/images/6317068f1.pdf

So does this,

edn.com/contents/images/6372827f1.pdf

You may need to add a 'w w w dot' in front of the URL (they won't let post a real link).

HTH - Steve H.