on/off circuit

You'll need to provide a bit more information. What is being switched? What's available to sense the "being used" state? What control signals, if any, are available to control power? Is any warning of impending power termination required/desired?

The more information you can provide up front, the more targeted the solutions can be. As it is, there's really no answer to your question.

--
Rich Webb     Norfolk, VA

A monostable is what you need. Press the switch and it will stay on for X minutes. An improvement if required would be to use a retriggerable monostable activated by whatever the device does when it's doing something useful which would extend the period but still switch off when the activity ceases.

Graham

On Thu, 29 May 2008 17:38:01 +0100 in sci.electronics.basics, Eeyore wrote,

How would one design that in such a way that there would be zero (or practically zero) battery draw after it shuts itself off?

A CMOS monostable itself draws negligible current. Use its output to drive a mosfet powering up the circuit being switched. Or even use a relay if mosfets are too complicated for the beginner.

Graham

Power the "power up" circuit from the switched line, and from the 'on' switch.

If power is off, and the on switch isn't pushed, current draw will be the leakage current of the pass MOSFET, which will be minuscule.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

A led is to be switched off.

A simple circuit is used to switch the led on/off with 9 v.

After the led is first switched on, the switch remains activated/ closed.

However, the led should turn off after a fixed time (even though the switch remains activated/closed), without being turned on again.

That makes little reals sense to my mind but is perfectly technically acheievable.

Why not use a momentary switch ?

Graham

Gotcha. Take a look at Don's explanations of using a 555 timer chip as a monostable.

That should do exactly what you're looking for - holler if you have additional questions once you've looked that over.

--
Rich Webb     Norfolk, VA

You never heard of a ONE_SHOT ?

I have, ONE SHOT is all it takes to get you all baffled.!

"

--- If you're not going to use the switch for anything else, View in Courier:

Vcc>-+---------+-----+----------+--------+ | | | 8| | [10K] [10K] [Rt] +---+---+ [100K] | | | 2|_ Vcc _|4 | +-[100nF]-+-----|-----O|T R|O---+ | | 6| | | Vout | +------|TH 555| [100nF] / | | 7|_ |3 | / | O S1 +-----O|D OUT|----|--+-[Rled]--+ | O +| | GND | | | | [Ct] +---+---+ | [LED] | | 1| | | GND>-+---------------+----------+--------+------------+

T = 1.1 RtCt

Vout - Vf(led) Rled = --------------- Iled

JF

--
Oops...

Since the switch will bounce on break as well as on make, the LED will
flash when the switch is closed, then again when it\'s opened.

The fix:

          
.        | --O    [100K] [Rt]  +---+---+   |
.                |     |  2|_ Vcc _|4  |
.                +-----|--O|T     R|O--+
.                |     |  6|       |   | 
.                |     +---|TH  OUT|---|----+ 
.                |     |  7|_      |3  |    |    
.             [100nF]  +--O|D  7555|   |  [Rled]
.                |    +|   |  GND  | [10K]  |
.                |    [Ct] +---+---+   |  [LED]
.                |     |      1|       |    |K
.GND>------------+-----+-------+-------+----+

JF

--
And, just for grins:

Version 4
SHEET 1 880 680
WIRE -320 48 -432 48
WIRE -80 48 -320 48
WIRE 288 48 -80 48
WIRE 336 48 288 48
WIRE -320 112 -320 48
WIRE 336 112 336 48
WIRE 32 176 0 176
WIRE 288 176 288 48
WIRE 288 176 256 176
WIRE -320 240 -320 192
WIRE 32 240 -320 240
WIRE 288 240 256 240
WIRE 336 240 336 192
WIRE 336 240 288 240
WIRE 32 304 -224 304
WIRE 288 304 288 240
WIRE 288 304 256 304
WIRE -224 336 -224 304
WIRE 336 336 336 240
WIRE -80 368 -80 48
WIRE 32 368 -80 368
WIRE -80 416 -80 368
WIRE -432 432 -432 48
WIRE -320 432 -320 240
WIRE -224 448 -224 416
WIRE -432 560 -432 512
WIRE -320 560 -320 496
WIRE -320 560 -432 560
WIRE -224 560 -224 512
WIRE -224 560 -320 560
WIRE -80 560 -80 496
WIRE -80 560 -224 560
WIRE 0 560 0 176
WIRE 0 560 -80 560
WIRE 336 560 336 400
WIRE 336 560 0 560
WIRE -432 624 -432 560
FLAG -432 624 0
SYMBOL Misc\\\\NE555 144 272 R0
SYMATTR InstName U1
SYMBOL res 320 96 R0
SYMATTR InstName R1
SYMATTR Value 1e6
SYMBOL cap 320 336 R0
SYMATTR InstName C1
SYMATTR Value 2.2e-6
SYMBOL voltage -432 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 3 24 104 Invisible 0
SYMATTR Value PULSE(0 9 1 1e-6 1e-6 10)
SYMATTR InstName V2
SYMBOL res -240 320 R0
SYMATTR InstName R2
SYMATTR Value 270
SYMBOL cap -336 432 R0
SYMATTR InstName C2
SYMATTR Value 1e-8
SYMBOL res -304 208 R180
WINDOW 0 -38 67 Left 0
WINDOW 3 -39 33 Left 0
SYMATTR InstName R3
SYMATTR Value 1e5
SYMBOL res -96 400 R0
SYMATTR InstName R4
SYMATTR Value 1000
SYMBOL LED -240 448 R0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
TEXT -408 592 Left 0 !.tran 15

JF