automatic shut off circuit

[Albert]

I need an automatic shut off circuit for battery powered electronic
items. Over the years, I've spent way to much money buying batteries
because the previous user forgot to turn the power switch off. Just
this week, I threw out a set of C cells from my daughters electronic
keyboard.

I wonder if anyone has designed something like this before.....

I want to incorporate it into existing items, so it needs to be small,
cheap to build and be powered from the existing batteries. Ideally,
I'd like to have 20 to 60 minutes of on time once the unit is powered
up. The length of the ontime is not critical, high precision timer
chips are not needed. Quick and dirty should be ok.

Any suggestions??

Thanks,

A

 

[James Sweet]

I need an automatic shut off circuit for battery powered electronic
items. Over the years, I've spent way to much money buying batteries
because the previous user forgot to turn the power switch off. Just
this week, I threw out a set of C cells from my daughters electronic
keyboard.

I wonder if anyone has designed something like this before.....

I want to incorporate it into existing items, so it needs to be small,
cheap to build and be powered from the existing batteries. Ideally,
I'd like to have 20 to 60 minutes of on time once the unit is powered
up. The length of the ontime is not critical, high precision timer
chips are not needed. Quick and dirty should be ok.

Any suggestions??

Thanks,

A

Sounds like a good use for a 555 timer circuit or an 8 pin microcontroller,
the latter offering more flexibility but a steeper learning curve.

 

[Bob Parker]

Hi Albert,
I've seen circuits to do that in some of the books of collections
of Elektor magazine projects.
You might also find something suitable at
http://www.commlinx.com.au/schematics.htm
Good luck.

Bob

 

[Martin Riddle]

You can use a 555 with its output driving a pass transistor that feeds the
555 supply and the rest of the circuit. A power on switch shorts the
transistor resetting the circuit.

I've used this method for a battery operated antenna pointer with the pass
transistor driven by a micro port. Worked beautifully.

Cheers

 

[Albert]

You can use a 555 with its output driving

A 555 and it's support components is a major undertaking.

I was hoping for something much smaller and simpler, perhaps a
momentary contact switch, a resistor, cap and a mosfet.

Would a cap connected to the gate of a mosfet keep the mosfet turned
on if the cap was charged through a momentary contact switch to the
plus side of the battery?

 

[Luhan Monat]

A 555 and it's support components is a major undertaking.

I was hoping for something much smaller and simpler, perhaps a
momentary contact switch, a resistor, cap and a mosfet.

Would a cap connected to the gate of a mosfet keep the mosfet turned
on if the cap was charged through a momentary contact switch to the
plus side of the battery?

I think that 555 circuit is about as simple as it gets.

 

[Rolavine]

Albert wrote:

Yes, I've used this before, put an N channel enhancement mode logic level
mosfet between your circuit and ground. It is a bit sloppy during the turn off
but maybe that is OK? Test it yourself!

Rocky

 

[Bob Parker]

That's the kind of thing I was thinking of. It depends on exactly
how the power switching of the existing equipment works, available
supply voltage, etc...

 

[Sam Goldwasser]

That's the kind of thing I was thinking of. It depends on exactly
how the power switching of the existing equipment works, available
supply voltage, etc...

One problem might be that as it discharges, the MOSFET won't turn off
like a switch but will gradually reduce the current, possibly with bad
consequences for both the MOSFET and powered device. So, there should be
another switch in between the MOSFET and the device such as a relay or
something else with hysteresis.

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[Bob Parker]

Happy New Year Sam,
I agree. However it depends on what's being switched and what
currents and voltages are involved. Some equipment wouldn't have a
problem with a very simple arrangement like this, but it could cause
major problems with other stuff.
Another difficulty is physically fitting a power-off circuit into
the device which needs it....

Bob

 

[Mike Page]

I need an automatic shut off circuit for battery powered electronic
items. Over the years, I've spent way to much money buying batteries
because the previous user forgot to turn the power switch off. Just
this week, I threw out a set of C cells from my daughters electronic
keyboard.

I wonder if anyone has designed something like this before.....

I want to incorporate it into existing items, so it needs to be small,
cheap to build and be powered from the existing batteries. Ideally,
I'd like to have 20 to 60 minutes of on time once the unit is powered
up. The length of the ontime is not critical, high precision timer
chips are not needed. Quick and dirty should be ok.

Any suggestions??

Thanks,

A

Why not a wall wart (battery eliminator) ? Unless it's for fun; but it
might turn out harder than you think - what's your minimum voltage and
maximum current ?

 

[GPG]

Happy New Year Sam,
I agree. However it depends on what's being switched and what
currents and voltages are involved. Some equipment wouldn't have a
problem with a very simple arrangement like this, but it could cause
major problems with other stuff.
Another difficulty is physically fitting a power-off circuit into
the device which needs it....

To speed up switch off you could try something like this:
V+
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created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
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