pulse controlled circuit

Discussion in 'The Projects Forum' started by jblueink, Feb 7, 2009.

  1. jblueink

    Thread Starter Member

    May 27, 2008
    24
    0
    hi guys,am designing a circuit that i want to use to control two relays,in such a way that when one is on the other will be offand vice verse and each state will last for say 200s.
    i attached the picture of the circuit i have simulated only one of the relay is working.i tried NPN and PNP on the failing side but it did not work. can some body help pls.
     
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  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Try moving the relay on the PNP stage to its collector. Get rid of the collector resistor also.

    hgmjr
     
  3. jblueink

    Thread Starter Member

    May 27, 2008
    24
    0
    i have done that also but it will only come and remain on
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What is the timing of the simulated signal that you are using to drive this circuit?

    hgmjr
     
  5. jblueink

    Thread Starter Member

    May 27, 2008
    24
    0
    thanks guys let go and try it , i will give u the feedback as soon as am tru.
     
  6. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Just being curious her, but why on earth not just use a single relay with double throw contacts?
    I really can't see any reason for a second relay.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Alberto, below the PNP transistor you have the diode across the relay coil reversed. Cathode needs to be towards positive.

    Jblueink,
    Make the modifications that hgmjr suggested.
    Also, change the resistors on the bases of the transistors to 180 Ohms. This should be enough to put the transistors into saturation without excessive current.
    Change the 2N3906 transistor to a 2N2907 transistor. The 2N3906 is rated for up to 200mA collector current, but a more realistic limit is 100mA. The 2N2907 will go up to nearly 500mA collector current.
    Most importantly, check to make certain that your simulated signal generator is putting out a square wave that goes from a maximum of 0.5v to a minimum of 8.5v; eg: 0.5v to 0.5 for the low, and 8.5v to 9v for the high.

    I suspect that the output voltage from your signal generator is not going high enough to turn off the PNP transistor.

    If you want to switch the relay states at a lower input signal voltage and current, you will need to add another transistor.
     
    Last edited: Feb 7, 2009
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Use two NPN transistors instead of the current NPN/PNP configuration.

    Connects the base of the second NPN to collector of the first one via resistor and you're done.

    With NPN/PNP arrangement, there is the possibility that both relays will be ON if the input signal is disconnected unless steps is taken to avoid this condition.
     
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