Hello dear friends.

I need your valuable help for a little circuit I'm designing.

It is a small circuit whose output I want to keep ON for one second and then stay OFF for 8 seconds. I've tried with a 555 in astable mode, but in all settings I've read the Ton> Toff. I have been testing with two LEDs connected to the 555 output (one connected to VCC and one to GND).

For simulation purposes, I am using two resistors of 47k and 10k with a capacitor of 220uf which turns on the red led for about 8 seconds and the green led for 1 second. But the behavior I want is the red led 1 second ON and then the green 8 seconds ON.



I know you will recommend me to use a PIC or AVR. I have similar circuits that work perfectly, programmed in PIC 12F675 or ATTiny85, but I prefer to leave the uC for more complex projects.

Any suggestions on how I can use a 555 for this?

Thank you very much in advance.

 

Swap the LEDs and recalculate the RC to provide a Ton of 8 seconds and Toff of 1 second.

Then Ton > Toff

 

Hello,

Do you want the leds to alternate?
Why is R6 connected to the TH and TR pins of the 555?
I would expect that connection of R6 to be connected on ground.

Bertus

 

You can make the on and off times independent by connecting a diode across R5, anode to the '555 pin 7 and cathode to pin 2 and 6.

Why is R6 connected to the TH and TR pins of the 555?
I would expect that connection of R6 to be connected on ground.

I certainly agree with that. It is not going to work properly as drawn.

 

Ooops.

I will review that.

Thanks a lot!

 

You can make the on and off times independent by connecting a diode across R5, anode to the '555 pin 7 and cathode to pin 2 and 6.


I certainly agree with that. It is not going to work properly as drawn.

Excelent!

I tryed that and my circuit works fine now!

Thanks a lot!

 

Wendy's Index
.The 555 Projects
555 Hysteretic Oscillator

 

Hello,

@Wendy , The OP wanted an asymetric on/off.
You circuit will be about equal on/off times.
When you double the timing post and use some diodes, the on and off time can be adjusted independently:



Bertus

 

Wendy's Index
.The 555 Projects
555 Hysteretic Oscillator

Great! Thanks a lot!

 

Hi!

Thanks to the excellent advice from all of you, in particular from @Wendy and @bertus, I have achieved what I needed. I have a timer that activates a led for about 0.69 seconds and another led every 1.15 minutes. Actually, what I want to do is activate a device to chase animals, which I do not want to be retriggered for 1 minute.



The timer works fine, but I have not been able to activate it with the PIR motion sensor. The cycle is activated every time the PIR is triggered, but I can not get it to work properly.

Any suggestions?

Thanks in advance.

 

Hello,

@Wendy , The OP wanted an asymetric on/off.
You circuit will be about equal on/off times.
When you double the timing post and use some diodes, the on and off time can be adjusted independently:

View attachment 128962

Bertus

I think Bertus called it.