555 long timing

[e44-72]

Hi there
How bigger capacitor and resistor do you think I would need to make a 555 timer wired as astable flash every once a day or hour?

Grateful for any replies.

 

[Reloadron]

First, there is considerable difference between an hour and a day of 24 hours. Second, the 555 timer is not at all suited for an application like this. You may want to start by reading this link and consider a circuit using a ripple counter. You are not going to get where it seems you want to go using a 555.

Ron

 

[Jugurtha]

Hello,

The 555 isn't really suited for long timing (even the manufacturers advise to use other "schemes" to achieve that. I'm thinking using counters, etc).

Maybe if you can say for which you need it, more experienced users could help proposing a better solution.

All my best,

EDIT: Reloadron answered your question better while I was writing mine

 

[e44-72]

Thank you for the replies and the link given.
I am not exactly sure where I would use it yet, I just wanted to know if it was possible to achieve timscales like this with a 555.

 

[Reloadron]

No, and the problem with trying to use a 555 is in good part due to the 555 relying on R*C timing. When looking at long times the problem is capacitor leakage. So, that big problem is a matter more of capacitor leakage than the 555 itself. Just the nature of the beast.

Ron

 

[KMoffett]

There are some good long period timer circuits here: **broken link removed**

Ken

 

[e44-72]

Thank you for the comments you have provided, they have helped a lot

 

[Dvinchy]

set the capacitor about 10 000 uF with resistor about 10 mega

 

[Reloadron]

set the capacitor about 10 000 uF with resistor about 10 mega

Won't work! The OP mentions 1 hour to 24 hours of timing. I suggest you read this application note. Then place a focus on Frequently Asked Applications Questions around pages 6 & 7. Then note this:

7. What is the longest time I can get out of the timer?

Answer: Times exceeding an hour are possible, but not always
practical. Large capacitors with low leakage specs are quite
expensive. It becomes cheaper to use a countdown scheme (see
Figure 15) at some point, dependent on required accuracy.
Normally 20 to 30 min. is the longest feasible time.

That was covered in the thread.

Ron