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The 555 IC has been desribed in hundreds of milions of publications and they are easily available on the net. Try for example this site: **broken link removed**
There you will find a free calculator on astabile configurations of the 555 IC.Try several combinations of RS and Cs .. and see how it goes .. **broken link removed**
The biggest problem with long times is the leakage currents of the timing capacitor, board surface, and the IC itself. These are functions of temperature. It is usually best for longer times to use the 555 to run a counter. The MC14536 and the 5555, by Siemens I think, are designed for very long time intervals and have a counter and oscillator inside.
Many thanks for your posts.
I was after about 1/600s frequency and this I could be done using just single 555.
I also checked 74HC5555: timer + frequency divider; very interesting option.
There is also a 7555 IC that has CMOS input for lower lekage.
Never used it for time longher than a minute; Take care that the problems will arise at the high temperatures.
Maximum time will be manufacturer dependant and is a function of the comparator input characteristics and of the discharge circuit.
But operating that close to the limits of a device exaggerates its idiosyncrasies making repeatability over time, humidity, temperature, different part batches and solar flares an iffy proposition. The counter is a better bet to get 2 hours out of a 555.
Walter Jung had a nifty little timer cookbook once upon a time, though the chip isn't all that complicated, there are a lot of novel ways to hook everything up.
Depending on your application, and it looks that you have already checked ICs with built-in timer and divider (5555), sometimes for more accurate time counting it is just easier to use mains frequency (50Hz or 60Hz) and devide it by as many stages as you need. Also, if this application requires some "inteligence" you may consider employing a small microcontroller. This option gives you enormous advantage as if you want to change sequences or functionality of a system you will be changing (in most cases) only software code ..
maximim period obtained by 555 is of order of few minutes, to obtain longer delay you need to use high value low lekage capacitots, these are expensive, accurecy of delay is poor,and circuit becomes somewhat prone to noise on pin 5.
Electrolytic capacitors have very poor tolerance (+/-50-100% or more).
Is there any way of keeping circuit based on 555 simple but for long times not to use electrolytic capacitors?
I am not sure if this is what you are after, but here is a circuit that multplies capacitance. It is based on an opamp (suitable will be a FET opamp such as TL071/081) and has some limitations such as max frequency and serial resistance.
The values can be calculated as follows:
C = (R2/R1) *C1
R ≈ R1
So, for example for 47µF, you can select the following values:
C1=47nF
R1=10Ω
R2=10kΩ
Good luck and regards ..
IanP
According to LM555 datasheet from National Semiconductor, it can be used for timing purpose from microsecond to hours.
However, I read on one book for 555 applications (I don't remember the true name of the book), it can work at 300kHz the maximum.
But in my opinion, 300kHz is for the good LM555, that comes from any welknown manufacturers such as National semiconductors, Fairchild, ..., while with the IC of the same code, but from unclear manufacturers, the maximum frequency must be much lower than 300kHz or it won't work.
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