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siren circuit

Discussion in 'Electronic Basics' started by Andy, Dec 10, 2004.

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  1. Andy

    Andy Guest

    Is John Popelish here? I changed the labeling since
    my last post. I am still having difficulty to fully
    grasp this circuit. My next questions are as follows:

    I also removed the LED because we can see the simulation
    without it, and it does not (I think) effect the analysis.

    I have noticed that if we increase R2 from 47K to 2M the
    circuit will still oscillate, but any further increase and
    the circuit will not oscillate. I was wondering if I could
    obtain a formula to calculate the maximum value (which is
    2M by inspection of simulation results)

    following is the lt spice asc file as modified by me:


    Version 4
    SHEET 1 880 680
    WIRE 48 64 176 64
    WIRE 240 112 240 320
    WIRE 160 320 240 320
    WIRE 240 320 240 384
    WIRE 32 320 96 320
    WIRE -48 320 -96 320
    WIRE -96 320 -96 112
    WIRE -16 112 -96 112
    WIRE -96 112 -96 0
    WIRE -96 -80 -96 -144
    WIRE -96 -144 240 -144
    WIRE 528 -144 528 -128
    WIRE 240 -144 240 16
    WIRE 240 -144 528 -144
    FLAG 48 160 0
    FLAG 240 464 0
    FLAG 528 -48 0
    SYMBOL res 224 368 R0
    SYMATTR InstName R3
    SYMATTR Value 50
    SYMBOL cap 160 304 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C
    SYMATTR Value .47µ
    SYMBOL pnp 176 112 M180
    SYMATTR InstName Q2
    SYMATTR Value 2N3906
    SYMBOL res 48 304 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R2
    SYMATTR Value 47k
    SYMBOL npn -16 64 R0
    SYMATTR InstName Q1
    SYMATTR Value 2N3904
    SYMBOL res -112 -96 R0
    SYMATTR InstName R1
    SYMATTR Value 4e6
    SYMBOL voltage 528 -144 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value PULSE(0 9 0)
    TEXT -114 506 Left 0 !.tran 10s
     
  2. Agreed. minor effect.

    Did you also lose the diode that connected between the base of Q1 and
    ground? That has a big effect on frequency. Without that diode, R1
    mostly affects the off time and R2 mostly affects the on time.
    The exact value is dependent on the gain of the transistors and the
    amount of bias current from R1. The resistor has to provide enough
    positive feedback from the linear biased operating point to produce
    closed loop gain greater than 1.
     
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