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555 timer

round

Apr 13, 2016
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Hello all,
I want a circuit powered by a 9 volt battery that when a momentary switch is pressed will light an led for 30minutes.

Is a 555 capable of the 30minutes?

Ignore the attachment I can’t delete it.

Thanks
 

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HellasTechn

Apr 14, 2013
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Audioguru

Sep 24, 2016
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The 100uF and 16k produce a pulse duration of only 1.8 seconds. 30 minutes has 1800 seconds.
The maximum allowed resistor value is 20M so then the capacitor must be an extremely large and expensive low leakage film type.
 

round

Apr 13, 2016
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It would need to be 1000uf. I suspected a typo.
Cheers
 

BobK

Jan 5, 2010
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1000 uF and 1.6Meg according to the calculator @HellasTechn posted.

Simplest circuit is a 6 or 8 pin microcontroller (PIC10F.) One chip, 1 decoupling capacitor, 1 MOSFET to switch the LED. But of course you need to the tools and the experience to do it this way.

Bob
 

AnalogKid

Jun 10, 2015
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Another way to go is with one 2N7000 or 7002 MOSFET and nothing else other than the R and C. No schematic capability right now, but here is the circuit:

R (2.2 meg) and C (1000 uF) in parallel from transistor gate to GND.
Switch from gate to Vcc.
Source to GND
Drain to LED to resistor to Vcc.

When you press the switch, the cap charges up to 9 V almost instantly. After the switch is released, the resistor slowly discharges it. This is in addition to the capacitor self-discharge due to leakage current, which is why the resistor value is larger than expected. When the cap voltage gets down to around 2 V, the transistor will begin to turn off. The LED will turn off slowly over about a minute.

ak
 

HellasTechn

Apr 14, 2013
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1000 uF and 1.6Meg according to the calculator @HellasTechn posted.

Simplest circuit is a 6 or 8 pin microcontroller (PIC10F.) One chip, 1 decoupling capacitor, 1 MOSFET to switch the LED. But of course you need to the tools and the experience to do it this way.

Bob

Yess well the Microcontroller is always the best option for such things...
 

HellasTechn

Apr 14, 2013
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The 100uF and 16k produce a pulse duration of only 1.8 seconds. 30 minutes has 1800 seconds.
The maximum allowed resistor value is 20M so then the capacitor must be an extremely large and expensive low leakage film type.


I miscalculated...
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Is a 555 capable of the 30minutes?

Almost certainly not (sorry). OK, a 555 is capable of this, but the resulting circuit will be unreliable.

For the time constants you're talking about, the input current of the 555 and the leakage current of the capacitor will likely interact to prevent the capacitor from ever reaching the 2/3Vcc if you follow the simple calculations. Even replacing the 555 with a CMOS equivalent is insufficient if the limitation is the capacitor leakage rather than input currents.

I would refer you to this, and this thread. (And thanks to @Ian because I would never have located that thread again if it were not for the improved search!)

Go with the counter solution or a microcontroller.

The circuit given by @tmetford is sometimes seen with a 555 and other dividers (which don't incorporate their own oscillator). The magic trick employed by these is that the oscillator is stopped when the chosen output changes state. This can be extended to something which can delay for years if necessary :)
 
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