I need help with my alarm setup

Discussion in 'The Projects Forum' started by soda, Jan 24, 2011.

  1. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    If you look at my circuit you'll see that when the PIR relay open, pin2 of NE555 will go low. This will then trigger the NE555 and the output will go high.

    My problem isn't in this part of the circuit, but it's on the output stage of the NE555. When i test the circuit with an LED everything is working fine, but the moment i connect my amplifier and alarm sounder to the output, Q3 blows.

    There's nothing wrong with my amplifier because when i connect it straight to the power supply, it's working correctly. The current the amplifier and alarm use is about 1,35Amp

    Can you please tell me what the problem might be on the output stage of the circuit
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    You need to limit the current through Q3 with a resistor between Q3 col and Q4 base. The way it is now it is shorted to +15V rail via Q4 base when Q3 is turned on.:eek:

    TIP35C seems to have a min Hfe of 20 so based on that its base current needs to be at least 1.35A/20=67mA. The resistor should be 15-Vbe(Q4)-Vce(sat of Q3)/Ic(Q3) = approximately 14V/70mA = 200Ω min.

    If you measure TIP35C gain to be more, then recalculate values.

    Ensure Q4 saturates when on (measure).

    Have fun,
    Ifixit
     
  3. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    You mean to say TIP32c or do i have to change Q4 to a "NPN TIP35c" Thought, if i use a npn at Q4 the output will be low when it's suppose to be high[​IMG] but i'm anyway going to do as you say with the TIP32c
     
    Last edited: Jan 25, 2011
  4. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    HI ifixit,

    It's fantastic. you gave me gr8 advise.thanks alot. I add a 270R resistor from Q3 col to the base of Q4 on the TIP32c and my circuit is working now.

    I have to admit that i blew 4 transistors which is sad. I don't know why i didn't thought of that, but anyway i'm going to make a note of what you showed me for next time. [​IMG]
     
  5. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Soda,

    Sorry about the typo... I ment to say TIP32C, which is a PNP and what you require, however, Q4's symbol is for an NPN (The emitter arrow should point toward the base for a PNP).

    Glad I could help,
    Ifixit
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Something else, basically a reliability issue. The base of Q1 is floating when the switch is open. You probably should add a 10KΩ-100KΩ resistor between the relay/R1 junction to ground, so the transistor is firmly turned off. Otherwise you are vulnerable to RF interference, as you have an open base floating (along with the PN junction), a potential AM radio receiver.
     
  7. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Ok thanks for that one Bill, i'll do as you just said. Thought, I never even think of that.

    I just want to add that i'm so happy to be a member of this forum. You know you people helped me alot and I've never before learned as much as i learned at this forum. [​IMG]
     
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