Astable 555 Circuit with Ton < Toff

[ingalfsan]

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.

 

[Veraxis]

You can connect a diode across R5 to achieve duty cycles beyond 50%.
by doing this TH~= 0.693*R4*C
and TL= 0.693*R5*C

 

[AnalogKid]

Swap the positions of the two LEDs.

ak

 

[cowboybob]

Elegant, AK. Sometimes the trees obscure the forest...

 

[ingalfsan]

You can connect a diode across R5 to achieve duty cycles beyond 50%.
by doing this TH~= 0.693*R4*C
and TL= 0.693*R5*C

Excelent!

I tryed that and my circuit works fine now!

Thanks a lot!

 

[MikeMl]