Hello,I made a simple astable 555 timer circuit the same as the one on
the 555 web site,but im having trouble working out the values of the
two resistors and the capacitor so it will turn on for 3 minuts and off
for 5 minuts.Could someone please point me in the right direction as i
found the data sheet a bit confusing.thankyou in advance
You seem to be asking for a 3 / (3 + 5) = 37.5% duty cycle. Most
555 example circuits, etc do not use such low duty cycle, but this is
possible. Also, most examples do not show such low frequencies, but I
have managed this with the 555, and you can ultimately find examples.
I am very sure you can do exactly what you are wanting with a 555.
I found good and bad ways to accomplish the task you are now attempting.
There are two books by two different aithors: Walter Jung and another by
Howard M. Berlin. I own the latter book and it shows some simple circuits to
accomplish this. The basic concept in the circuit I am most familiar with manages
this using a couple of diodes in parellel, but in opposing polarities,
that connect to the timing cap. There are also two separate potentiometers,
either both or only one in series with each diode. The diodes and pots are
connected between pins 6 and 7. The circuit allows independent control over
the charge and discharge intervals. If you can find the Berlin book "The
555 timer applications sourcebook with experiments", have a look at
the chapter on the astable configuration, and particularly figures 3-8
and 3-9. These allow control over the frequency AND duty cycle. I suspect
you can achieve exactly what you are wanting from those examples.
Dominic
