# 555 timer help

Discussion in 'The Projects Forum' started by 98QuadCabV10, Dec 30, 2012.

Oct 20, 2012
6
0
Hello,
I've been reading on the 555 timer and would like to build my own circuit. I've built one before, but was only able to do so by copying a schematic. What I would like is to have the circuit turn on a relay for 5-10 sec every 60 seconds. How would I go about finding the proper resistors and capicators for this circuit?
Thanks for any info,
Jesse

2. ### Wendy Moderator

Mar 24, 2008
20,766
2,536

Oct 20, 2012
6
0
Thank you for your help. So if I'm understanding this correctly to get the time high (on) I would take .693 x (r1 + r2) x c1 and for low (off) I would take .693 x r2 x c1. Am I correct so far? So do I just guess and put numbers in there till is get the time I'm looking for? Or is there an easier way and am I missing something?
Thanks again.

4. ### #12 Expert

Nov 30, 2010
16,298
6,811
There are calculators online that do 555 circuits, but you will get done just as quickly by just guessing and learning as you go. Reading the datasheet for the 555 is allso a good place to start. Several seconds takes a fairly large capacitor like 100 uf or 470 uf. There. That's a first guess. Calculate the resistors and see if you come up with a number that is below 1 million.

5. ### AfdhalAtiffTan Active Member

Nov 20, 2010
117
11
Yup, you're right...
Don't forget that both R and C are in SI unit; as example, 100uF = 0.0001F.

But, if the on and off time dissimilarity bugs you, you could add a diode between 6th and 7th pin of the 555, with the cathode being at 6th pin.
This will bypass R2 for charging the cap...

So the on and off time will share the same equation (roughly) t = 0.693*Rx*C. 'R1' for on time and 'R2' for off time.

Check this out:-
http://wolfstone.halloweenhost.com/TechBase/com555_555TimerCalc.html

6. ### Dodgydave Distinguished Member

Jun 22, 2012
4,992
745
use the hyserical oscillator design using a 555, and set the time to 60secs off 6sec on

Oct 20, 2012
6
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Thank you everyone for all your help! Could you tell me if I did this correct. So using the diode between pin 6 and 7 this is what I came up with. R1 - 33k ohms, R2 - 330k ohms, and C1 - 220 uf. Which would give me about 5 sec time on and 55 sec off. Correct?

8. ### AfdhalAtiffTan Active Member

Nov 20, 2010
117
11
Yup, you're very close, but, how did you calculate it to get 55 sec? I get around 50 sec...

Make sure the capacitor is in a good condition, it might have too much leaky current; messing up with the timing. (especially with very old caps)

Remember, this is astable circuit, it will oscilate endlessly (with provided power of course).