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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: An actual circuit
Date: Fri, 24 May 2024 22:35:57 -0000 (UTC)
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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.
>

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

Cheers 

Phil Hobbs 


-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics