555 timer setting

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Hi All.
I am using the 555 timer for a basic circuit.  I am using the circuit in the
back of the Dick Smith catalogue Can anyone help me with the values of the
Cap and resister.  The time I want is 2 minute (120sec) +- 10 seconds.  I
have tried various values, but it is unreliable, and on most occasions stays
on for a long time, and I can't wait that long and normally manually reset
it.  If someone can give me the right values, then I may be able to see if
the time problem is somewhere else in the circuit.
Thanks in advance for any replies.
Rob
Melbourne.



Re: 555 timer setting



"Fonz"
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** Lemme guess  -  you have 2.2 Mohm resistor and a 100 uF electro cap ??

Forget it.

The cap has as much resistance (leakage) as the resistor so never charges up
fully.

SOLUTION:

Get say three *tantalum" caps 10 uF, 25 volt, & use them in parallel with a
3.3 Mohm feed resistor.

This will give about 70 - 80 seconds delay.

Then put a 10 kohm trim pot from pin 5 to the + supply and adjust to get the
exact time you want by adjusting the comparator trip voltage.




.......  Phil



Re: 555 timer setting



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Try this

http://ourworld.compuserve.com/homepages/Bill_Bowden/555.htm



Re: 555 timer setting


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555 is not suitable for 2 minutes, try 4060 instead.

--

Bye.
   Jasen

Re: 555 timer setting



Fonz
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**  It is for the OP's spec  -  you fuckwit, sheep shagger.




........   Phil



Re: 555 timer setting


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You didn't specify the off time. The solution depends on both the
on and off times for a  oscillator configuration.

For 120 seconds on (high level at the output) and 107 seconds off,
the values could be 36K for R1, 330K for R2 and 470uF for the
capacitor.

-Bill


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