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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: An actual circuit
Date: Fri, 24 May 2024 16:22:23 -0400
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"john larkin" <jl@650pot.com> wrote in message 
news:uts15jlh0oo1hin58uu4a574kg5q3j9q5b@4ax.com...
> On Fri, 24 May 2024 14:50:19 -0400, "Edward Rawde"
> <invalid@invalid.invalid> wrote:
>
>>"Jeroen Belleman" <jeroen@nospam.please> wrote in message
>>news:v2qmeq$2eknc$1@dont-email.me...
>>> 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.
>