Re: An actual circuit

On 5/24/24 17:59, Edward Rawde wrote:

"john larkin" <jl@650pot.com> wrote in message
news:bk815jh3skuecf1tap8o41rpgdh5kkq8o5@4ax.com...

On Thu, 23 May 2024 13:06:46 -0700, john larkin <jl@650pot.com> wrote:
>

On Thu, 23 May 2024 15:35:00 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
>

I was having a conversation with a younger person who seemed to be of the
view that to make an LED flash you would need something to decide when it
should be on or off. So that would be some kind of software or digital
system.

...

>

>
The classic NPN astable circuit can hang up, with both transistors
saturated. I wonder if he jfet circuit can hang too, with Idss
grounding both drains and not enough gain to oscillate out of that
state.
>
Even when they have a hang state, luck usually kicks them off into
oscillation. Your source resistors and  asymmetric drain resistors
help it start up. Try making both drain resistors 3.3K.
>
If you make the source resistors lower, it will hang up.
>

 Yes I noticed both points when I was designing it.
I wanted to have it start up by itself, preferably without a kickstart
capacitor.
So I had a complicated circuit with two more diodes and a transistor in the
hope that I could detect the hang state and force it off balance.
I couldn't get that to work
Then I accidentally made R2 3,3k and R6 3.3k and I didn't see how it could
start so quickly with no other help.
Eventually I noticed 3,3k which maybe LTSpice takes as 3k.
 If R2 and R6 are both 3.3k then LTSpice says it slowly drifts into operation
after 40 seconds.
But why does it go one way and not the other?
Is that an artefact of asymmetry in the simulation?
Or is there some hidden asymmetry in the circuit I'm not seeing when R2 is
3.3k?

Below are a pair of astable circuits. The left one is like yours,
with a hangup state. I start it by specifying an initial condition.
The right one will start all by itself.
Jeroen Belleman
=========================================
Version 4
SHEET 1 1192 680
WIRE -80 -96 -272 -96
WIRE 96 -96 -80 -96
WIRE 240 -96 96 -96
WIRE 416 -96 240 -96
WIRE 864 -96 416 -96
WIRE 1088 -96 864 -96
WIRE -80 -64 -80 -96
WIRE 96 -64 96 -96
WIRE 240 -64 240 -96
WIRE 416 -64 416 -96
WIRE 864 -64 864 -96
WIRE 1088 -64 1088 -96
WIRE -272 32 -272 -96
WIRE 96 48 96 16
WIRE 112 48 96 48
WIRE 128 48 112 48
WIRE 240 48 240 16
WIRE 240 48 192 48
WIRE 768 64 704 64
WIRE 864 64 864 16
WIRE 864 64 848 64
WIRE 704 112 704 64
WIRE 944 112 704 112
WIRE 1040 112 1008 112
WIRE 1088 112 1088 16
WIRE 1088 112 1040 112
WIRE -80 128 -80 16
WIRE 128 128 -80 128
WIRE 384 128 192 128
WIRE 416 128 416 16
WIRE 416 128 384 128
WIRE 96 160 96 48
WIRE 416 160 416 128
WIRE 864 160 864 64
WIRE 992 160 864 160
WIRE 864 176 864 160
WIRE 1088 176 1088 112
WIRE -80 240 -80 128
WIRE -32 240 -80 240
WIRE 0 240 -32 240
WIRE 48 240 0 240
WIRE 240 240 240 48
WIRE 320 240 240 240
WIRE 368 240 320 240
WIRE 704 256 704 112
WIRE 752 256 704 256
WIRE 816 256 752 256
WIRE 992 256 992 160
WIRE 1040 256 992 256
WIRE -272 288 -272 112
WIRE 96 288 96 256
WIRE 416 288 416 256
WIRE 864 320 864 272
WIRE 1088 320 1088 272
WIRE 0 336 0 304
WIRE 0 336 -272 336
WIRE 320 336 320 304
WIRE 320 336 0 336
WIRE -272 352 -272 336
WIRE -272 448 -272 432
FLAG 96 288 0
FLAG -272 288 0
FLAG 416 288 0
FLAG -32 240 G1
FLAG 320 240 G2
FLAG 112 48 D1
FLAG 384 128 D2
FLAG -272 448 0
FLAG 1088 320 0
FLAG 864 320 0
FLAG 1040 112 D4
FLAG 752 256 G3
SYMBOL nmos 48 160 R0
SYMATTR InstName M1
SYMATTR Value 2N7002
SYMBOL res 80 -80 R0
SYMATTR InstName R1
SYMATTR Value 1k
SYMBOL res -96 -80 R0
SYMATTR InstName R2
SYMATTR Value 100k
SYMBOL voltage -272 16 R0
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL nmos 368 160 R0
SYMATTR InstName M2
SYMATTR Value 2N7002
SYMBOL res 400 -80 R0
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL res 224 -80 R0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap 192 112 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL cap 192 32 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 100n
SYMBOL voltage -272 336 R0
SYMATTR InstName V2
SYMATTR Value 3
SYMBOL diode 304 240 R0
WINDOW 0 -33 66 Left 2
SYMATTR InstName D2
SYMBOL diode -16 240 R0
WINDOW 0 -40 61 Left 2
SYMATTR InstName D3
SYMBOL nmos 1040 176 R0
SYMATTR InstName M3
SYMATTR Value 2N7002
SYMBOL nmos 816 176 R0
SYMATTR InstName M4
SYMATTR Value 2N7002
SYMBOL res 1072 -80 R0
SYMATTR InstName R5
SYMATTR Value 1k
SYMBOL res 848 -80 R0
SYMATTR InstName R6
SYMATTR Value 1k
SYMBOL res 864 80 M270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R7
SYMATTR Value 100k
SYMBOL cap 1008 96 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 33n
TEXT 88 376 Left 2 !.ic v(G1)=0
TEXT 48 -136 Left 2 !.tran 100m