555 timer setting

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.

Reply to
Fonz
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"Fonz"

** 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

Reply to
Phil Allison

Try this

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Reply to
Tom Smyth

555 is not suitable for 2 minutes, try 4060 instead.
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Bye.
   Jasen
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Reply to
Jasen Betts

"Jasen Betts" Fonz

** It is for the OP's spec - you f****it, sheep shagger.

........ Phil

Reply to
Phil Allison

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

Reply to
wrongaddress

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