NE555 delayed recovery

[bajticzek]

Hi guys,
I have a simple question about NE555...

Need a simple delay circuit for use in my car. One LED iluminated symbol on my dashboard keeps blinking for 100 ms each 10 seconds or so. So i need a circuit, that will switch that LED after 200 ms. I changed whole pre-facelift dashboard with newer one. The old one had bulbs instead of newer which has LEDs. So that 100 ms pulse couldn't even be seen. But now it's drivin' me crazy.

Googled a little bit and found this: **broken link removed**. I'll use the circuit shown in first half of the picture. I won't use switch, but will apply voltage directly to diode and input pin (these two are connected according to picture). My only question is what resistor should I alter to get different time constant? 10k one or 1M one? Is the 10k just a pulldown resistor and 1M resistor + capacitor gives the time constant? Or is it 10k resistor + capacitor?

 

[ZekeR]

You are correct in the first statement: the 10k resistor is just a pulldown which keeps the TRIG pin held low until it's triggered by the pushbutton, and which quickly turns the output off when the button is released. The 1Meg resistor sets the time delay before turn-on.

I'm not so sure if this is the right circuit for you, however. The way you worded your question, it seems like your goal is to lengthen a pulse from 100ms to 200ms, whereas this circuit is essentially shortening the pulse. You probably want to use a one-shot (also known as a monostable multivibrator), such as seen here.

Hope this helps!

 

[bajticzek]

Thanks for reply. I'll just change the 1M resistor and 1uF capacitor to suit my needs. And maybe I expressed bad. I need the circuit to omit all pulses shorter than delay set by RC. So if I set the time constant to be 200 ms, 555 will not change it's output until the time of input pulse exceeds this constant. So I think it's solved. Thank you.