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siren circuit

A

Andy

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
Andy said:
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.

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.
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)

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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