555 timer lab help

Discussion in 'Homework Help' started by dave l, Feb 20, 2009.

  1. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    Hello,

    I am doing a lab using 2 555 timers. The first one produces a positive pulse for 25 seconds. The second one is used to flash an led. I have it working as such: When the logic switch A| is thrown, LED will flash for 25 seconds then go out. However, the final part of the lab is to have the LED stop flashing after 25 seconds and remain on constantly until the equipment is shut off using logic switch B|.

    Can someone please help? Thanks in advance!

    I've placed a picture of the diagram here: www.longworth.cc/lab3.jpg


    Dave L
     
    Last edited: Feb 20, 2009
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Connect the LED between Vcc and pin3 rather between ground and pin3. Don't forget the current limiting resistor.
     
  3. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Just a suggestion, you can control a 555 using pin 4 (which is an enable pin) instead of switching the power supply on/off.
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Connect a led on the output as I told you and not a logic probe.
     
  6. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    Hi,

    Thanks, the LED now stays on but I have another problem.

    Now the LED comes when unit is powered on. It does flash for 25 seconds then light on but the LED will not shut off.

    I've also hooked pin 4 of the first 555 timer to the reset switch.

    Dave L
     
    Last edited: Feb 21, 2009
  7. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    When you connect the led with the resistor from pin 3 to ground,
    you will get reversed action (so the led will stay on).

    Greetings,
    Bertus
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Use the circuit you posted, replace the BJT with a thyristor but switch the ground side of the second circuit rather its Vcc side.
     
  9. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    NPN and PNP transistors are the only ones available, Sorry.

    I've posted the latest schematic at: www.longworth.cc/lab3b.jpg

    Last week, in class, we talked about latches. Could some sort of latch be used for this function?

    Thanks!
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can make a thyristor by using a PNP and a NPN transistor if you want but you can also use a RS latch.

    Foe the latch solution, connect its set input (if it is active high) to the output of the first 555 and connect the led between the Q output and the output of the second 555.
     
  11. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    Here is the design I made from your advice: www.longworth.cc/lab3c.jpg

    Is it right?

    I'm working on a breadboard at home not using the multisim program for simulation so the actual work takes longer.

    Thanks Again!
     
  12. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Where is the link?
     
  13. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    Here is the design I made from your advice: www.longworth.cc/lab3c.jpg

    Is it right?

    I'm working on a breadboard at home not using the multisim program for simulation so the actual work takes longer.

    Thanks Again!
     
  14. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Connect a 10K resistor from R to ground, put power to the second 555 and use the output of the first 555 to control the reset of the second 555.
     
  15. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
  16. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Now it looks ok!
     
  17. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    I'm having trouble with the logic in my head.

    After the switch is thrown, the first 555 sends a 25 second positive pulse which sets the S of the latch to 1 and the R is tied to 0. Active high SR latch S1 R0 equals Q1. But when the 25 second time period is up and S goes back to 0...Isn't S0,R0 and invalid condition on the SR latch? How would the light stay on? And, where does the 2nd switch get connected to turn off the LED?

    Thanks for all your help. Sorry I'm not getting this.
     
  18. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A R=0 and S=0 is a valid combination for a NOR RS latch.

    With the second switch do you want to switch off the whole circuit or only the led?
     
  19. dave l

    Thread Starter Member

    Feb 20, 2009
    12
    0
    The lab instructions say 'After 25 seconds the LED should change and stay on constantly until the equipment is turned off by pushing and releasing logic switch B|. If at any time the equipment is turned off, your circuit must turn off the LED.

    What is Qs output for R=0 and S=0?
     
  20. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    For R=0 and S=0 the output will hold the previous output value.

    If you want to turn all the circuit off with a push button then use one which latches and unlatches mechanically when you press it.
     
Loading...