Simple 555 timer with delay after trigger release

[Dr Kingo]

Hello.
I am new to this chat, so I hope this is the right forum to post to.

I have the idea to use a 555 to build a small triggered timer with an 'after release' delay. I have been google-ing but can not find the right schematic/design that looks to work for me, so I will try this forum

Case is this:
I will be using this in my car. I have 2 batteries in the car and I have 2 individual voltmeters, 1 for each battery.
1. I want the voltmeters to turn on (double pole relay for that) when I start the car, and stay on for ~45 seconds after I turn off the car.
2. I also want to be able to check the battery status with a button (momentary on) without starting the car (same relay).
3. Plus I have a button (momentary on) for start-assist (intelligent battery combiner relay with 30 sec start-assist) so that this will also be a trigger for the voltmeters (30 sec start-assist + ~10-15 sec extra on time), (also same relay).
4. All other time the output of the 555 should be off.

As stated I will be using this in my car, so Vcc and input/trigger is 12v ~+.
I have the 3 different input triggers separated with diodes.
The circuit will be more or less always on, running of the extra battery.

I hope this is understandable and you get my point...

I can not figure out how to do this so that there is both the constant on time and a delay after the release of the input/trigger... maybe some combination between trigger, threshold, discharge and reset...? So here I need the help of some wiser people

Thanks a lot in advance,
Kingo

 

[ci139]

wiser people

forget about such -- it is mostly with what everything someones mind is in sync. with -- at the time of trying to find a solution to a specific problem -- so it's mostly where you are in time and less how you are labelled in/by your society -- e.g. if you can't solve your problem at one time then give it a try on another

 

[crutschow]

Here's a 555 circuit whose output goes high when the input goes high and the delay time doesn't start after the input goes back low.
The added transistor keeps the timing capacitor C1 at 0V until the input goes low.
The delay is approximately R1*C1.
If you want this to be triggered by more than the two inputs shown, just add more 50kΩ resistors in parallel at the Q2 transistor input, one resistor for each input.
Does that do what you want?

 

[Dr Kingo]

Thank you crutschow.

This looks like exactly what I was looking for, but could not find before. I have tested it and it does the job well. This forum was time well spent

Thank you again

 

[crutschow]

It's called a 555 retriggerable monostable multivibrator (or one-shot), of which there are several versions.