need to invert the signal

Discussion in 'General Electronics Chat' started by engrdm, Jul 16, 2011.

  1. engrdm

    Thread Starter New Member

    Feb 19, 2010
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    hi .. i am having this circuit, and i want that the o/p of PNP BJT should now be inverted... i tried but failed , plz help me , how can i invert the signal so that if the relay was supposed to be off by the current schm now it should be On and viceversa...
     
  2. hspalm

    Active Member

    Feb 17, 2010
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    I wouldn't recommend changing the control circuit of a relay to change it's characteristics from NO to NC or vica versa, then you might end up with an (almost) always on current sink, which is not good. Or is this how it is now?

    You could switch the transistor for a NPN, connect it's collector to pin 2 of the relay, and it's emitter to ground. That connection on the schematic seems odd to me as a PNP transistor, like someone was mistaking it for a NPN.

    edit: which means you can directly replace the PNP with a NPN. by the way, it's hard to look at a transistor as if it had an output voltage, you would rather see it as a one-way switch for now to make the wiring easier to understand.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The relay has SPDT contacts, correct? Why not simply change which of the relay contacts you are using?

    What's the purpose of this circuit?

    You have D1 connected backwards. If T1 turns on, the diode and T1's gain are the only items that will limit current through the collector of T1.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    That may not work, as T2 is set up to sink current from the LED and T1's base. If T1 were replaced with an NPN, the LED and its' resistor would be the only current source for T1's base.
    That threw me for a moment, too. The transistor should be flipped around so the emitter is up, and the relay located above the transistor.
     
  5. hspalm

    Active Member

    Feb 17, 2010
    201
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    Yes, that's what you get for seeing the whole picture! Good thing you noticed.

    Something tells me that additional PNP at the end is OP's addition to the circuit to invert the signal, as he could just as likely put the relay on the existing T2.
     
  6. engrdm

    Thread Starter New Member

    Feb 19, 2010
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    sorry the diode placement is a mistake in the sch.
    the purpose of doing this is to make sure that the o/p of relay should be NC normally but it should open if there is power failure,

    i can use NC of the relay with some device to act as NC if there is power, but i thinkif i use a NPN as a switch connecteing base to vcc and let the C and NC of relay at Collector and emmiter, the problem i would face is that i should use GNd... but C can be from any other circuit.. is there another solution?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    What is the purpose of the circuit? A current flow monitor?
     
  8. engrdm

    Thread Starter New Member

    Feb 19, 2010
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    the purpose of the circuit is a vibration sensor ,,, will be used in alarm system when someone tries to break or open door/window forcefully... but i needed also a soltion to check if some how the wire has been cut .. or the power fails just to the sensor and not the alaram control pannel meaning someone has cut the supply so the relay should open the circuit which starts an alarm
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    So, are you constantly feeding the circuit a signal via L2?

    If not, it will be difficult to detect a cut wire when no signal was present to begin with.

    A power failure would be easy to detect with a separate relay that was energized by the power supply; if the supply failed the contacts would close.
     
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