help with switch timer

[blong1000]

I'd like to build a small 5 or 9 volt switch timer that will light an LED when a switch is closed for 15 seconds and keep it lit until reset. Does anyone have a schematic? I've seen several similar designs that might be adaptable but I'd also like to make it as small as possible and I'm horrible at redesigning existing circuits. Thanks

 

[Mehtab Ali Zahid]

is it necessary that the led will be turned on if switch is closed for 15s, or it should turn on with a voltage pulse.

 

[blong1000]

Mehtab; I need the switch to be held for 15 seconds. Students will be using this on a fire extinguisher trainer. I need the instructor to be able to retrieve the unit after it's been used by a student some time later and be able to tell if they discharged the extinguisher for the full time, ie 15s then reset it. I know this is fairly simple and I'm close to a solution but am having trouble latching the LED on and fine tuning the 15 seconds.

 

[colin55]

Show me the circuit you have so far.

You request is very vague.


Do you want to know if the student pushed the switch for 12 seconds, then released it, then pushed it for 4 seconds to get the full timing?
Do you want the timing to start again if the student pushed the switch for 14 seconds and if the whole operation is allowed to be 29 seconds, you will be able to determine if the switch was released prematurely.

 

[blong1000]

Collin; ya, that's a good question that I need to ask the actual instructor that will be working with the students. The way I have it now, with a single 555IC would, I believe act in the second way you descibed. But, after staring at your question for a while, I'm inclined to believe the first way would simulate the use of the extinguisher more closely(push, release, push again resulting in a total of 15 seconds or more, not requiring a continuous 15 second push) but, the preference of my customer may require a continuous push. I'll repost with the results of my visit with the customer and an image of my initial design in about 24hrs.