Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: An actual circuit Date: Fri, 24 May 2024 22:35:57 -0000 (UTC) Organization: A noiseless patient Spider Lines: 140 Message-ID: References: <1cv15jdls60tjfcndqe9qm7bgjdtchv9u9@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Sat, 25 May 2024 00:35:58 +0200 (CEST) Injection-Info: dont-email.me; posting-host="0f741b375bb12ae64014156dc1b373b1"; logging-data="2663961"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18T8NIqxjzIdB+cLDElsfTC" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:iFL1zaFZ0PjJHTLOwlFK2q3IGpI= sha1:75HArdG1bFLd3PEz5Wvcovh4WZM= Bytes: 6524 Edward Rawde wrote: > "Phil Hobbs" wrote in message > news:v2r325$2h00c$1@dont-email.me... >> john larkin wrote: >>> On Fri, 24 May 2024 16:22:23 -0400, "Edward Rawde" >>> wrote: >>> >>>> "john larkin" wrote in message >>>> news:uts15jlh0oo1hin58uu4a574kg5q3j9q5b@4ax.com... >>>>> On Fri, 24 May 2024 14:50:19 -0400, "Edward Rawde" >>>>> wrote: >>>>> >>>>>> "Jeroen Belleman" wrote in message >>>>>> news:v2qmeq$2eknc$1@dont-email.me... >>>>>>> On 5/24/24 17:59, Edward Rawde wrote: >>>>>>>> "john larkin" wrote in message >>>>>>>> news:bk815jh3skuecf1tap8o41rpgdh5kkq8o5@4ax.com... >>>>>>>>> On Thu, 23 May 2024 13:06:46 -0700, john larkin >>>>>>>>> wrote: >>>>>>>>> >>>>>>>>>> On Thu, 23 May 2024 15:35:00 -0400, "Edward Rawde" >>>>>>>>>> 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