• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

555 Timer long off - short on times

Status
Not open for further replies.

mrwizney

New Member
Greets all;

Its been a while since I played with the 555 timer. I have got one to work with long time periods, years ago (yep, old fart now). Now I need an astable circuit that is on for 1 second, off for 60 seconds, or close to these times. I will figure it out eventually on my own, but I figure some of you are current with 555 timer applications, so I'll ask for your assistance and see what fly's.

Thanx in advance for your help should you reply to this.
 

MikeMl

Well-Known Member
Most Helpful Member
Sorta close. The first pulse is slightly longer.
 

Attachments

Hero999

Banned
What are you trying to do?

If it's just flash an LED or power a relay then connect the load from the + rail to the output and the duty cycle will be automatically inverted.
 

MikeMl

Well-Known Member
Most Helpful Member
If it's just flash an LED or power a relay then connect the load from the + rail to the output and the duty cycle will be automatically inverted.
Yes, but the first timing period is twice as long as the others, and since it the longer of the two periods, it is more noticable than the previous circuit.
 

Attachments

Last edited:

mrwizney

New Member
555 Circuit values

Hi again Mike;

I ran some numbers on the circuit you suggested. And with some digging, I found pretty much the same circuit, but your suggestion just looks more reliable...

Anyway, I used the following values and get the given signal specs as follows.
R1 = 280K ohm
R2 = 18.42M ohm
C2 = 4.7uf not electrolytic

Frequency = .016Hz
Duty Cycle= 1.7%
Time High = 1.008 Sec.
Time Low = 60.014 Sec.

If I may ask your opinion once again, do you see any problems with these parameters? I know the resistor values are not stock, other than that, do you think this is a stable configuration?

Thanks again for allowing me to pick at your brains...
 

MikeMl

Well-Known Member
Most Helpful Member
For a bipolar 555, I like to keep the resistors below 5meg, and use larger capacitors.
For a NE555, TI says keep the sum of the two resistors below 10meg. For a LM555, National says 20meg. The issue is that for an astable, the resistor has to supply the input leakage/bias current for TWO pins at once.

If your relay coil draws less than ~200mA, you can drive it directly from the 555 (either way). If it draws more, you will have to add a transistor or Fet or buy a more sensitive relay.
 
Last edited:

mrwizney

New Member
I thought there was some sort of limits that are the conventions. I was concerning myself about the timing cap. As I recall, it should not be an electrolytic because the stability of the timer is effected.

Knowing these details, I will go back and try again, attempting to stay within those parameters. As for the relay, yes I am aware of that. I am using a transistor driver circuit anyway because at some future time, a larger relay may be employed. And I am aware of the need to select components that properly saturate the transistor... etc.

Thanks again for your help...:)
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top