auto power on/off & battery saver circuit ideas?

Javalin

Hello all,

I've got a requirement to run a small project that I'd like to use a ON/OFF SPST switch on the box.

BUT,·I'd like the project to be able to turn itself off (i.e. Low Battery) and to·switch on·when the power is applied.

Currently I have a two push button solution - the "ON" push holds the reguators on until the propeller boots - then it holds them on.· The "OFF" button breaks the circuit - and it all goes off.

Im struggling for idea's.· I can get the circuit to auto-turn off, but cannot see how to get the project to switch on without a push button or something to hold the regulators on....

thanks for any help or ideas,

James

Mike Green

How about showing us what you have so far so we can tell how you're controlling the regulator?

Javalin

Hi Mike

Sorry - yes that would be clearer!

Im using LDO regs with a shutdown pin.· Attached is what I've got working thus far.· The user starts the regulator (and prop) via the ON push, and the propeller holds the EN high when it starts.· OFF collapses the circuit.

I'd like just to have a ON/OFF SPST switch and the propeller will switch on and run when the switch moves to ON, turn OFF when the switch moves to OFF and be able to turn off (deep sleep maybe?) if the battery is low.

As a bonus - It would be nice to be able to program the propeller without the device switching off.· Not a huge problem as you just hold the power on - but.....

Cheers,

James

Mike Green

You could use a micropower comparator as a set-reset flip-flop as on page 11 here: www.national.com/ds/LM/LMC6762.pdf. You'd need a SPDT center off momentary contact switch to provide the set (for on) and reset (for off) signals. By adding another 100K resistor to the reset input, you could drive that with the Propeller output to provide a processor controlled off switch. The reason for using a comparator instead of a logic gate is that the comparator can run off a much higher unregulated input supply voltage (up to 15V in this case).

parsko

James,

This sounds like what I've tried to do with my Automotive Power supply. My concept is that my prop is dead (powerless) until it see's Main power (ignition switch), and then keeps itself going (via a battery pack) until it (Prop) decides to turn itself off.

Schematics and many long winded verses from me can be found at:
http://forums.parallax.com/showthread.php?p=718998

-Parsko

Javalin

Hello,

Thanks both for the ideas - i'll look into it!



James

Beau Schwabe

Javalin,

Here is a derivative of a 3nA Standby power circuit·I modified for use with a Propeller. Originally this was designed using a BS2 on a doorbell for my daughters playhouse.

Reference:
http://forums.parallax.com/attachment.php?attachmentid=38912
http://forums.parallax.com/showthread.php?p=598707

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 6/10/2008 5:46:30 AM GMT

Lee Harker

I used this exact concept in a microcontroller circuit recently·and it·is pretty simple. You·power your circuit using a·voltage regulator that has an on/off pin; there are many to choose from. You put a high value resistor on it to keep it normally off and then you connect a pushbutton to turn it on "momentarily". This pin will also be connected to one of your port pins, possibly through a 220 ohm resistor to protect the processor. When the program starts, one of your first instructions will be to activate the pin that will hold the regulator on. That will be fast enough to beat your finger leaving the pushbutton. When your program has completed, you shut off the regulator and the "hold off" resistor will keep it deactivated until next time.

In the project I used this, the on switch was a magnetic reed switch and the circuitry was in a waterproof enclosure with a clear lid. I used a magnet to turn it on so I didn't have to open the case.

Beau Schwabe

Lee Harker,
·
"When the program starts, one of your first instructions will be to activate the pin that will hold the regulator on. That will be fast enough to beat your finger leaving the pushbutton." ...· That won't work with the Propeller because there is about a 1-2 second delay to load the program after power is applied, before the program starts running.· The schematic that I posted uses an SCR made from two discrete transistors.· A third transistor is used to turn the SCR off.· Since SCR's turn on hard and latch, it·works as a good solid switch.· However, because they turn on so hard, they can also be tricky to turn off once they are on.
·
···

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Franklin

Could you not use a 555 to strech a pulse for three seconds to hold the propeller on until it boots....just a thought... might be overkill.

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- Stephen

Beau Schwabe

Franklin,

You probably could use a 555, but using the Propeller, you'd still need some sort of latch to keep power to the 555 so it would stay "alive" unless the 555 were powered up all of the time. In which case, if the 555 were powered all of the time, I don't think you could get down into the single digit nano amps of current draw for standby mode.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Javalin

Mike,

I've prototyped the IC you suggested in the circuit you mentioned.· I cannot get it to work.

I've connected up as shown in the attachment - as the PDF (attached see P11) shows.

The only difference is that the 50k's are actually 51k's·- I could'nt seem to purchase a 50k in quantities < 5000....!· The resistors are all 1% tollerance's.

The A- pin holds at 4.35v, the A+ at 35mv and the Ao at 40mv or so.· Its a slightly dead 5v supply - about 4.6v really.

Any ideas?· Am I missing something (probably!)

Cheers all,

James


Post Edited (Javalin) : 6/27/2008 8:28:55 PM GMT

Paul M

James,

Have another look at the schematic in the pdf; the SET and RESET inputs are normally at 0V and momentarily taken to V-BAT to set/reset the latch. In your implementation the inputs are floating when the pushbuttons are not pressed. R26/R27 and R24/R18 form voltage dividers which set the switching point of the comparator. The In- is held at 0.5V-BAT. In+ is at 0V when the output is OFF and, when the output is ON, the In+ is at about 0.66V-BAT. This is only true if R27 and R18 are tied to ground in the quiescent state. I would try say 10k pulldowns to ground - this shouldn't upset the switch points too much.
Hope this helps

Paul

Javalin

Hi Paul,

I'll check it out tonight!

Cheers,

james