Simple transistor circuit problem

[rasklangus]

Hello, I am a novice experimenter with a basic understanding of electronics. I have made a circuit which uses a Darlington transistor to switch an LED on with a capacitor and variable resistor connected so the led will fade over an adjustable period of time. This bit of the circuit works great. I also want the option for the LED to stay on after the trigger is released until a reset button is pressed. I thought I could achieve this by joining the emitter and the base of the transistor so once triggered the base would be fed from the emitter and remain open. This doesn't work. I am obviously missing a vital bit of understanding as I can't see why. Could somebody more knowledgeable than myself (that's probably everybody reading this:) please explain why this doesn't work and possibly suggest what I might do to fix it? I have attached a picture of the circuit...

circuit diagram.gif (165.87 KiB) Viewed 6803 times

Thanks very much in advance for all your help.

David.

[piratepaul]

You are using digital terminology to discribe an anolog circuit.
I have not done the maths of the components, assuming you have the component valuse correct...
The reset function should be wired to the +5v line with a switch, the LED will stay on as long as the switch is closed, how ever the 15mohm resistor then will carry current if too much it will get hot and burn.
The latch bit has no function in this circuit.
A book on digital electronics would put you right for this sort of stuff.

[piratepaul]

Hi, I made a total pigs ear of this one, forget the above post... my mistake.

Move the latch to the collecor side of the transistor... thats it ... the rest is fine ( I have not done the calculations, I asume you got it right) The reset is fine, this is an anolog circuit but could be made from digital components.

Thanks for the input...

Tar&stuff.

[Pauldf]

Hi,
put basically the above latch wont work as the base voltage needs to be higher than the emitter voltage, usually somewhere around 0.7v (but dependant on the characteristics of the said transistors) for that transistor to conduct.

[beny]

@ rasklangus There can be so many different ways of doing this. The easiest by a mile is using a simple latch. Connect the 5v switch to the latch and just turn it on. It'll stay on until the latch is reset. Couldn't get much simpler than this. Sometimes, there're very simple solutions right in front of us and we just don't see it. I often ask others with fresh eyes to look over my project because with a fresh perspective it is easier to determine if there is something you could've done better or differently. Anyways, I hope this helped.

montaje smd

[remon7]

I think you are doing a fine work where you show your analog circuit into digital one. you may complete latches which is very easy process. because +5v switch turn into it. and hopefully you may complete it.