Two transistors from PIR

Discussion in 'The Projects Forum' started by allenpitts, Jul 1, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello AAC Forum,

    Working on a circuit to control two switches from one PIR.
    This the pictorial:
    [​IMG]
    This the schematic:
    [​IMG]
    I realize after drawing the schematic there is no difference, electronically, between
    signal 1 and signal 2. (I think.) But I thought that when the PIR signal stopped
    the voltage out from both sides would be zero because the transistors would
    no longer be saturated and there would be no voltage across the emitter and the collector.
    But when I connected the DMM to the signal connections and the ground
    not only did I get 6.29 volts before the PIR came on but the voltage dropped
    to a little over five volts when the PIR and the indicator LEDs came on.

    So the circuit indicator lights work as expected. That is when a hand is waved
    near the PIR sensor both of the LEDs come on and stay on for several seconds
    then go off and reset for another cycle. But the signal out from the circuit
    is at least five volts all the time.

    Thanks.

    Allen in Dallas
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606
    Can you add dots to show connections on your schematic? The way I think they are wired would indicate your 9 volt battery is seriously drained.

    Also do yourself a favor and use two resistors, not one where R1 is, one for each transistor. Both transistors turned on together mostly by luck.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    784
    You need R1a and R1b - manufacturing tolerances, doping level variations etc mean that one B/E junction will hog the current and starve the other.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606
    Today I see the connections on the wiring diagram. Your "signal out" lines connect to the 9 V power directly. Thus they are not seperate signals but the power lines, and your battery is drained.

    What signal levers are you aiming for on these lines? High and low voltage? When should it be high and when should it be low?

    BTW when a transistor comes out of saturation the voltage B-E is larger than when in sat, not zero.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    If the PIR is rated for 9V It looks OK like that, but I totally agree the TS should either put dots where wires that cross are connected, or "hump back bridges" where lines cross on the schematic but aren't connected.

    The PP3 style battery is among the lowest energy density you can pay a wad of money for - but as long as the PIR isn't a power hog, 2 LEDs shouldn't be a huge problem.

    Mention of using a base resistor for each transistor was in one of my earlier posts.
     
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