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Science Forum Index » Electronics - Basics Forum » on/off circuit...
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| steve... |
 Posted: Thu May 29, 2008 2:22 am |
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I require a circuit that switches off.
It is similar to the pocket calculator that will automatically switch
off if not used in a few minutes.
Could anybody please provide.
Thanks.
Steve. |
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| Rich Webb... |
| Posted: Thu May 29, 2008 8:15 am |
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On Thu, 29 May 2008 05:22:07 -0700 (PDT), steve <kvsteve at (no spam) gmail.com>
wrote:
Quote:
I require a circuit that switches off.
It is similar to the pocket calculator that will automatically switch
off if not used in a few minutes.
Could anybody please provide.
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 |
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| Eeyore... |
| Posted: Thu May 29, 2008 11:38 am |
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steve wrote:
Quote: I require a circuit that switches off.
It is similar to the pocket calculator that will automatically switch
off if not used in a few minutes.
Could anybody please provide.
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 |
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| David Harmon... |
| Posted: Thu May 29, 2008 12:12 pm |
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On Thu, 29 May 2008 17:38:01 +0100 in sci.electronics.basics, Eeyore
<rabbitsfriendsandrelations at (no spam) hotmail.com> wrote,
Quote: A monostable is what you need. Press the switch and it will stay on for X
minutes.
How would one design that in such a way that there would be zero
(or practically zero) battery draw after it shuts itself off? |
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| Eeyore... |
| Posted: Thu May 29, 2008 1:26 pm |
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David Harmon wrote:
Quote: Eeyore wrote:
A monostable is what you need. Press the switch and it will stay on for X
minutes.
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 |
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| Tim Wescott... |
| Posted: Thu May 29, 2008 1:48 pm |
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David Harmon wrote:
Quote: On Thu, 29 May 2008 17:38:01 +0100 in sci.electronics.basics, Eeyore
rabbitsfriendsandrelations at (no spam) hotmail.com> wrote,
A monostable is what you need. Press the switch and it will stay on for X
minutes.
How would one design that in such a way that there would be zero
(or practically zero) battery draw after it shuts itself off?
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 |
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| steve... |
| Posted: Fri May 30, 2008 10:46 pm |
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On May 29, 6:15 pm, Rich Webb <bbew... at (no spam) mapson.nozirev.ten> wrote:
Quote: On Thu, 29 May 2008 05:22:07 -0700 (PDT), steve <kvst... at (no spam) gmail.com
wrote:
I require a circuit that switches off.
It is similar to the pocket calculator that will automatically switch
off if not used in a few minutes.
Could anybody please provide.
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 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. |
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| Eeyore... |
| Posted: Sat May 31, 2008 5:27 am |
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steve wrote:
Quote: 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 |
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| Rich Webb... |
| Posted: Sat May 31, 2008 9:39 am |
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On Sat, 31 May 2008 01:46:16 -0700 (PDT), steve <kvsteve at (no spam) gmail.com>
wrote:
Quote: 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.
Gotcha. Take a look at Don's explanations of using a 555 timer chip as
a monostable. http://www.doctronics.co.uk/555.htm 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 |
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| Jamie... |
| Posted: Sat May 31, 2008 11:59 am |
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Eeyore wrote:
Quote:
steve wrote:
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
You never heard of a ONE_SHOT ?
I have, ONE SHOT is all it takes to get you all
baffled.!
http://webpages.charter.net/jamie_5" |
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| John Fields... |
| Posted: Sat May 31, 2008 6:55 pm |
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On Sat, 31 May 2008 01:46:16 -0700 (PDT), steve <kvsteve at (no spam) gmail.com>
wrote:
Quote: 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.
---
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 |
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| John Fields... |
| Posted: Sun Jun 01, 2008 10:36 am |
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On Sat, 31 May 2008 18:55:34 -0500, John Fields
<jfields at (no spam) austininstruments.com> wrote:
Quote: On Sat, 31 May 2008 01:46:16 -0700 (PDT), steve <kvsteve at (no spam) gmail.com
wrote:
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.
---
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--+-----+-------+-------+
.. |S1 | | 8| |
..VCCIN>--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 |
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