Help with a simple timer

Discussion in 'The Projects Forum' started by, Jul 19, 2011.


    Thread Starter Member

    Jul 12, 2011
    I have made up a very simple timer to enable an LED to continue to run on after the switch has been opened.
    I would include the diagram but don't know how to attach it.
    When the switch is closed a capacitor is charged through a low resistance, the current is also fed to the base of an NPN transistor which switches an LED.
    When the switch is released the capacitor continues to discharge into the base of the transistor keeping the LED running untill the capacitor is discharged.
    The only problem is the LED fades away. I want it to switch off cleanly, I expect this is possible using a 555.
    Could someone please post a circuit.
  2. wayneh


    Sep 9, 2010
    A comparator would also work. The LM339 quad comparator is widely available. You would put a reference voltage on one input, and the comparator output goes either full on (actually, open) or full off (completing a path to ground). Use a ~3k resistor to pull up the open, on output. The datasheet explains it. Anyway, you could control the timing by changing the reference voltage. If it's low, the cap must discharge to a lower level before the comparator switches. If it's high, a slight discharge will throw the trigger.
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    So ultimately you want to do this?

    Press a momentary switch and a LED turns on for a certain time period and then turns off.
    If this is what you are after, let us know what length of time you want the LED on and also what the LED's specs are, Vf (forward voltage) ans If (forward current).
  4. radiohead

    Distinguished Member

    May 28, 2009
    Here is a simple timer. Change the pot with a fixed value for a set time on. Play with the components to get what you want. The alarm clock buzzer input is the initiating trigger to turn the timing cycle on.