555 Monostable LED fading Circuit Question

Thread Starter

Jeremy McLellan 1

Joined Mar 26, 2017
5
Hi,

Sorry for starting another LED fading thread, I just couldn't find anything that was exactly what I was looking for. The circuit I have attached below is working just like I want it to. When the Door is grounded the LED's fade in and when it's un-grounded they fade out nicely as well.

Circuit.PNG

My question is, is there a way to add a Timeout feature to the circuit, say if the trigger is left low for an extended period of time it will time out, say 10 mins? I know I can do this with the resistor and Cap across pin 6 & 7 but I still want the circuit to turn off (fade out) when ever the trigger is set back to high...

Is this possible with just one 555 timer? Do would I have to set up another to reset the first?

Thanks in advance, I'm new at this and very much a noob so any advice is appreciated!
 

Dodgydave

Joined Jun 22, 2012
10,050
Yes it can be done with one 555 timer, note the circuit on the Right, it uses a Capacitor on the input trigger, so if the door is grounded the timer still times out and waits for the next trigger.

555mono.gif
 

Thread Starter

Jeremy McLellan 1

Joined Mar 26, 2017
5
Hi Dodgydave, thanks for the reply, I took your advice and modified my circuit to get the longer detail. As I mentioned before I need it to still recognize when the door is closed as well as opened. So I added a PNP transistor to toggle the reset pin opposite of the trigger so if the door is closed it will reset and fade the lights out. Does that look correct? I haven't bread boarded it but I have been playing with it on everycircuit website.

New Fader, 1000uF.PNG


The only issue I'm running into now is when the circuit is reset the light will fade out rather quickly, quicker than I would like. Is there a way to slow down the draining of the voltage? The only way could think of is by using a much bigger cap...I tried this below and it works but due to the new resistor in the circuit it lowers the voltage going to the base of the Mosfet not allowing the LED's to light up as bright...I think there is a solution for this I just can't wrap my head around it yet.

New Fader, 220uF.PNG

Thanks!
 

Thread Starter

Jeremy McLellan 1

Joined Mar 26, 2017
5
Ok, after putting on the bread board, I need to change some values but it seems to work...except it's constantly discharging through pin 7, I think it's due to the fact that when I put the input trigger low, I'm also putting the reset trigger high, which leaves it in a discharging state but the output still goes high and fades on the lights. If I disconnect the input it works as expected and light fade out.

Any suggestions?
 

Dodgydave

Joined Jun 22, 2012
10,050
The reset needs to be high for it to work, so tie it to pin 8,.

Are you saying that you have several doors in parallel and want to detect them when one goes to ground each time , even if one door is left open?
 

Thread Starter

Jeremy McLellan 1

Joined Mar 26, 2017
5
The reset needs to be high for it to work, so tie it to pin 8,.

Are you saying that you have several doors in parallel and want to detect them when one goes to ground each time , even if one door is left open?
I'm not sure if it's possible but I wanted to have if a door was opened (one or both doors) which creates a ground it would fade on the lights and if the door(s)are left open the Curcuit will timeout.

The twist is if the doors are both closed, opening that ground circuit I wanted the lights to fade back off instead of waiting for it to timeout... Hence why I had the PnP in there to try and reset when the doors closed.
 

Dodgydave

Joined Jun 22, 2012
10,050
I would replace R4 for a 1K resistor and put a led in series with it, that way when any door is opened, the led will stay lit until closed,....
 

Thread Starter

Jeremy McLellan 1

Joined Mar 26, 2017
5
I would replace R4 for a 1K resistor and put a led in series with it, that way when any door is opened, the led will stay lit until closed,....
Yes I think that would work but that light would just abruptly turn on / off, defeating the purpose of the fading on and off the lights?

I just need to create another (one shot) like you showed me on the trigger but on the reset pin...so when all the doors on closed (Door pins not grounded) it forces the reset low for a split second...just having trouble figuring it out, if possible.
 
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