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# 555 astable?

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#### qyc88

##### New Member
:? I'm doing a school project on a astable timer using the 555 timer. I need it to have the time capicity from 0 to 2 hours. ehh...can anyone help me?

well, about using the 555 as a 2hou period it is quite a bit difficult......
i mean, it is possible, but the problem is that for long period of times, the precision descreases....
you can use a counter do divide the timer frequency......
somthing like this......

thanx a lot

Well, if it's the heat that causes timing degredation, then just put a heat sink on it, that should fix the problem, I mean sure it's not space efficient, but I'm sure you could find a small IC heat sink.

nope, heat it not the problem.....
well, it is, bot not in this case......actually it is about the temperature, not the heat.
the problem is that for large timing you need large values for the cap and resistors......
wich makes it more unstable.

Ahhh... I see, because A lot of timing problems and hardware degredation problems are heat related, I want to see about finding a liquid cooled metal heat sink, get a freon pump going in one of my projects, would be nice. Until then, a good heat sink will do me.

well, it is a timing problem if the surrounding temp variates much.
but also if the IC or whatever you have is heating. but i don't think that your 555 will heat too much, since you will probably connect the load trough a transistor and/or relay so you will not draw much current from the 555.
if you use a 555 as an astable mode, the HIGH time is t=1.1R1C, where R1 is from Vcc to pin 7.
so if you try to time something like 2 hours=7200 seconds, you will probably need something like this : C=4700uF, R1=1.39Mohm, or.....
C=1000uF, R1=6.55Mohm.
you realize that the values are quite big.
think that the value of the resistor is so big that if you were to accidentaly touch it, the timing will drop quite a lot.

bogdanfirst said:
well, it is a timing problem if the surrounding temp variates much.
but also if the IC or whatever you have is heating. but i don't think that your 555 will heat too much, since you will probably connect the load trough a transistor and/or relay so you will not draw much current from the 555.
if you use a 555 as an astable mode, the HIGH time is t=1.1R1C, where R1 is from Vcc to pin 7.
so if you try to time something like 2 hours=7200 seconds, you will probably need something like this : C=4700uF, R1=1.39Mohm, or.....
C=1000uF, R1=6.55Mohm.
you realize that the values are quite big.
think that the value of the resistor is so big that if you were to accidentaly touch it, the timing will drop quite a lot.
The trouble with using a 555 for a long timer is that large RC values must be used. There is an upper limit of a few dozen megs on th R because of PC board surface leakages which means the C has to be large.

A large C of 4700uF usually means an electrolytic (tantalum is too expensive for very large values) and these invariably have an internal leakage current which varies with age, the voltage when charged, and when it was last charged to that voltage. That means that as fast as current is flowing through R to charge the C, some of that charge is leaking away.

The nett result is that the charging time is usually longer than calculated. To give you an example; some years ago I made a timer for a friend's burglar alarm to cut the siren off after 3 minutes. It worked OK as made. When the alarm went off in ernest a year later, the siren was still wailing after 20minutes because the caps had had no volts on them for a long time and had deformed.

The 555 gives very accurate times if you stay away from electrolytics. If you want a long time, run it as an astable to pulse a divider.

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