Re: An actual circuit

On Fri, 24 May 2024 16:22:23 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:

"john larkin" <jl@650pot.com> wrote in message
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On Fri, 24 May 2024 14:50:19 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
>

"Jeroen Belleman" <jeroen@nospam.please> wrote in message
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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.
>

>
Thanks for that. I guess I'm biased towards components I could easily get
in
the 70s. And also through hole components I can easily build a real
circuit
with.
>
>

>
I like to use surface-mount parts on a Dremeled PCB.
>
https://www.dropbox.com/s/7bihbjbaojvta0z/Z382_1.JPG?raw=1
>
https://www.dropbox.com/s/zaftysxtgclxj82/Z412_Proto.JPG?raw=1
>
You can do fast, 50-ohm picosecond stuff this way too.
>

>
Thanks I probably do have some copper clad board I could do that with.

 

>

Use a round-end carbide dental burr. They are cheap on ebay.

It's cool, kind of an art form. Takes some practice.

I have a few square feet of gold-plated copperclad FR4, which is even
nicer. Regular copperclad looks grungy after a few months.