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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: An actual circuit
Date: Fri, 24 May 2024 16:59:58 -0700
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On Fri, 24 May 2024 22:35:57 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Edward Rawde <invalid@invalid.invalid> wrote:
>> "Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message 
>> news:v2r325$2h00c$1@dont-email.me...
>>> john larkin <jl@650pot.com> wrote:
>>>> On Fri, 24 May 2024 16:22:23 -0400, "Edward Rawde"
>>>> <invalid@invalid.invalid> wrote:
>>>> 
>>>>> "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.
>>>>>> 
>>>>> 
>>>> 
>>>> 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.
>>>> 
>>>> 
>>> 
>>> I'm not nearly as posh as you are
>> 
>> Not sure why that made me laugh.
>> 
>>> , so for my purposes a spritz of Krylon
>>> clear acrylic on a clean board is a ticket.
>>> 
>>> You can solder right through it, and it keeps the copper looking nice for 
>>> a
>>> long time.
>> 
>> I seem to remember, a long long time ago using copper clad board, some clear 
>> sticky tape and a very sharp knife.
>> Followed by Ferric Chloride.
>> An etch resist pen could also be used if the tape removal didn't go exactly 
>> as planned.
>> 
>> But yes it's true that after removing the etch resist you might want 
>> something to keep the copper looking nice and shiny.

Soft Scrub. It's basically an optical polish.

>>
>
>Oh, I haven’t etched a PCB in forty-odd years. This is strictly dead-bug
>stuff. 
>

Insect cruelty. Plus you have to count their tiny feets backwards.



>Cheers 
>
>Phil Hobbs