RC delay trigger for 555

Hi,
I'm trying to design a delay circuit to trigger a 555.
The effect I want is similar to a switch-on delay circuit which I'd normally do with a RC series and a switch (normally closed) across the capacitor. This gives exactly the output I want:
- when the switch is first opened the capacitor begins to charge until it reaches the threshold voltage and pushes the trigger up/down to the required voltage, but
- if it's closed again before the threshold voltage is reached, it discharges the capacitor and the delay is reset [it's this behaviour that I really need].

The problem is that I need it to work with a normally open push button
- ie closing the switch triggers the 555 after a delay
- if the button is released before the delay time, do nothing, and reset the delay.
It's years since I've done any serious electronics and it's taxing my brain - it's the second part of the requirement that I'm struggling with.

I'm just using the 555 as a Schmitt trigger, btw.

Grateful for any help...

You don't necessarily need a 555.

Here is a suggestion that uses a CMOS Schmitt Trigger. CMOS requires much less current than a 555.

ljcox said:

You don't necessarily need a 555.

Here is a suggestion that uses a CMOS Schmitt Trigger. CMOS requires much less current than a 555.

Click to expand...

Thanks Len - that looks great, and thanks for the tip on using CMOS too. I'm setting it up on the breadboard right now.

I'm just getting back in to electronics again and this stuff is so much fun.

Thanks again.

electa176 said:

Thanks Len - that looks great, and thanks for the tip on using CMOS too. I'm setting it up on the breadboard right now.

I'm just getting back in to electronics again and this stuff is so much fun.

Thanks again.

Click to expand...

You're welcome. However, and forget to take electrostatic precautions with the CMOS Devices.