Trigger for 555Timer

Discussion in 'General Electronics Chat' started by jboulasii, Aug 22, 2009.

  1. jboulasii

    Thread Starter Member

    Aug 16, 2009
    11
    0
    I have a circuit in which i want the trigger for the 555 to be as soon as i apply power to the device. I was thinking of just taking the trigger directly yo ground. I want a one shot 5 second delay. With the trigger always grounded will I get my oneshot.
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    To trigger the 555 on power up connect a resistor from pin 2 to Vcc. Connect a capacitor from pin 2 to ground. Upon power up pin 2 has a negative trigger.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Something like this...

    [​IMG]
     
  4. jboulasii

    Thread Starter Member

    Aug 16, 2009
    11
    0
    Ive attached my full circuit. What im trying to do is change the bridge from full wave to 1/2 wave after some time peroid. I think that by turning off the two SCR's I should accomplish. I've isolated the gates using a pulse transformer with two secondaries. I'm hope is that on power up the 555 triggers and my count down starts say at 5 seconds. Once the 5 sconds is up I want the two SCR's to turn off.

    Questions,

    Do you think this conect will work?

    Do I need the pulse or is there a better way?

    Is i possible to use a straight RC newwork for the timming and not use the LM555? For me the fewer components the better.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A 555 is good, but it still needs a good power supply, not pulsating DC to power it up.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    How true.

    The common misconception is that 555 will keep its timing accuracy at whatever its supply voltage(within its rating) is.

    This is only partly true.

    The above statement should read: "The 555 will keep its timing accuracy at whatever STABLE supply voltage within its rating". This could mean voltage regulator for the supply voltage if one wants a stable timing output.

    Any fluctuation in Vcc during/within a timing cycle will add inaccuracy to the timing.
     
  7. jboulasii

    Thread Starter Member

    Aug 16, 2009
    11
    0
    Thanks for the input. does anyone have any ideas on how I can trigger this circuit and get my timing without the 555.
     
  8. Dragonblight

    Active Member

    Aug 19, 2009
    35
    0
    RC network, OPAMP oscillator, or a standard two transistor osciilator.

    Or you could just use a microcontroller so you can set up that tricky 5 second delay and play some NOP magic.
     
Loading...