# 555 Cycle time

Discussion in 'The Projects Forum' started by liteace, Jul 13, 2013.

1. ### liteace Thread Starter Member

Mar 7, 2012
75
0
Hi All, I did not want to high-jack someone else's thread so started a new one. I have here a an ST 555, I have:

R1 = 2700 Ohms
R2 = 330 Ohms
C1 = 47 uF

with that Im getting a cycle time of 104.1ms with a duty cycle of 94.6% I have been trying all day to get a cycle time of aprox 1.1s with a slightly lower duty cycle of say 85%, on time of 990 ish ms & off time of 100 ish ms but Im not having any luck , could some please help me out here

Thanks

EDIT, Sorry I think I have worked it out, the 47uF should be a 470uF should it not ?

Last edited: Jul 13, 2013
2. ### tracecom AAC Fanatic!

Apr 16, 2010
3,878
1,396
According to my calculations, when R1 = 2700, R2 = 330, and C1 = 470, the cycle time is 1097 ms. and the duty cycle is 90.18% on.

But when R1 = 2400, R2 = 470, and C1 = 470, the cycle time is 1090 ms. and the duty cycle is 85.93% on, which seems a little closer to your goal.

However, I just woke up from a nap and may have a brain fog.

liteace likes this.
3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
The datasheet is your friend, although it could confuse the hell out of you. The TI and old National datasheets say that the duty cycle is Ra/(Ra+2Rb). This is true for Doff=(off time/period).
For the ON time, Don=(on time/period)=(Ra+Rb)/(Ra+2Rb).
If you want an 85% dutycycle (Don=.85), you can calculate that
Ra=4.67Rb.

The period T is T=0.683(Ra+2Rb)C.
If you let T=1.1, and C=47uF, then Ra=23.6kΩ and Rb=5.06kΩ.

Closest 5% R values are Ra=24k and Rb=5.1k.

See attachment

File size:
27.7 KB
Views:
12
• ###### 555 astable.asc
File size:
1.7 KB
Views:
6
liteace likes this.
4. ### liteace Thread Starter Member

Mar 7, 2012
75
0
Thanks guy's everything confuses the hell outta me, as for brain fog I get that as well but it seems like its there from the moment I wake up and stays with me all day

Thanks