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Simple Circuit help

Discussion in 'General Electronics Discussion' started by jpinc, Feb 12, 2014.

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  1. jpinc

    jpinc

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    Oct 2, 2013
    I have only been working with electronics since this summer, and all the help I have received here has been great.


    Recently I built a simple circuit that has 3 total push button sensors.
    Two of the sensors will trip a green LED and the other will light a red LED. Only one of the two sensors controlling the green LED will depressed during each use. Also, the green sensor will always be pressed first, followed by the sensor lighting the red LED.

    That was fine but did not do exactly what i wanted.

    Instead, when a push button sensor is depressed, I want the LED to go on for a second (more or less) then go off, even is the sensor is still depressed, as conserving battery power is key. It is my guess that this is a lot more complicated than i originally thought. How would i go about doing this? Is there programming involved?

    Again, forgive me if i sound incompetent, but I am wiling to learn.

    And thank you for your time, in advance

    Joe
     
    Last edited by a moderator: Feb 13, 2014
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,365
    2,261
    Nov 17, 2011
    Use a monostable multivibrator, short monoflop. Google "555 monoflop" and you'll be offered hundreds if not thousands of circuits.
    Watch out: the circuit you want needs to be "edge triggered" (include these keywords in your search) otherwise it will not turn off if the button is still pressed.
     
  3. jpinc

    jpinc

    9
    0
    Oct 2, 2013
    Harold thank you so much for your quick response. That is exactly what I am looking to do.

    I searched and quickly found a simple circuit using a cmos 4098. This seems to be edge triggered. I continue to look and I am beginning to understand this a little more as I search for appropriate circuits. I now understand the RC coupled circuit will determine time of pulse.

    My next question is, because i have 3 push sensors that light up 2 different LEDs, will I need 2 or 3 individual 555 monoflops or is it possible to use one for the entire circuit? (Now that I think about it, my guess is that I can connect both push sensors lighting the green LED to same circuit imputs?)

    Related question,if in scenario 1, I want the LEDs to turn on for same pulse of time, will this differ from a second scenario where I would want the green LED to turn on for .5 seconds and the red LED to turn on for 1 second. (it is my guess that in scenario 2, two different 555 monoflops will be needed)

    I hope my question makes sense, thank you again for your time and help.
     
    Last edited: Feb 13, 2014
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,365
    2,261
    Nov 17, 2011
    You will need at least two monoflops. One for the red LED, one for the green LED. You can trigger the second monoflop by either of the two related buttons.
    Consider using three monoflps, one for each button. You can use simple diode OR circuit to make two monoflops light the same LED. This gives you maximum flexibility in chosing on times for each button.

    A single monoflop will not suffice unless you add additional logic like flipflops and gates.
     
  5. jpinc

    jpinc

    9
    0
    Oct 2, 2013
    Harold thanks again for all your help.

    http://en.f-alpha.net/electronics/basic-circuits/multivibrator/go-on/experiment-12-monoflop.html

    I came across this circuit and after some trial and error, yesterday I finally got it to work. It worked great but right away I noticed the LED had lost all its brightness. For my circuit, I am using 2 AA batteries and a luxdrive LED. This is the brightest LED I have found yet (if you know of others let me know,please) and when powered directly to the batteries, it was quite bright.

    Am I doing something wrong, or is there a way to regain the brightness of the LED with this circuit
    Or is there a different circuit I should be trying.

    Thanks for taking the time to read my post
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,365
    2,261
    Nov 17, 2011
    This circuit is designed to operate on 9V. The LED current will be approx. (9V-Vfled)/470Ohm=15mA (for Vfled=1.6V)
    If you operate it from 3V, the current will be only (3V-Vfled)/470Ohm=3mA.
    You need a lower resistance: R=(3V-Vfled)/Iled=100Ohm for approx. 15mA at 1.6V.
    Of course the value of R depends on the forward voltage Vfled of your LED.

    You should never do this with an LED. You need current limiting, read the tutorial.
     
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