555 timer setting

[Fonz]

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.

 

[Tom Smyth]

"Fonz" <r@removeme.com.au> wrote in message
news:43e03882$1@dnews.tpgi.com.au...

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.

Try this

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

 

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

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