I think TRIG (pin 2 of the 555) is supposed to be wired to the junction of the
100K resistor and collector of the phototransistor in the opto isolator. The
equivalent of a "typo" in a schematic.
From: email@example.com [mailto:firstname.lastname@example.org] On
Behalf Of Michael Tope
Sent: Monday, February 13, 2012 9:15 PM
Subject: Re: [Amps] Grid current protection HELP
I don't see how the opto-isolator triggers anything. According to the schematic
it switches 100K to ground on a node fed by the output of a
7815 regulator. That doesn't make any sense to me. I think there are some
errors in the schematic. There is probably a pull-up between pin 2 of the 555
and the 7815 output instead of just a wire, I think that creates other
problems, so without staring at it for more time than I have tonight, I can't
tell you exactly how it is supposed to be configured. I just can't see how it
could possibly work the way the schematic shows it.
73, Mike W4EF..............
On 2/13/2012 4:59 PM, rick darwicki wrote:
> Hi all,
> A while back K6XX published an article that can be found at
> It is a simple opticoupler that triggers a 555 timer base on the voltage drop
> across a resistor.
> I built the circuit but only connected pin 2 to the 100k resistor from the
> It has different levels of working with different connections I have tried.
> Grounding the relay jack will key Q1 if the output is low, but it starts
> high. At one point I could vary the voltage in to the opticoupler and trigger
> the 555. Also, the time delay worked too. Now I can't get it to do much hi hi
> I need to start over.
> I'm a tube and voltage guy and almost nothing I've built SS except for
> a 160 meter surface mount SDR and my copy of the Alpha 78 QSK board
> has worked first time out hi hi
> Can someone look at the schematic and let me know what the problem is, IE
> correct it?
> I can see the trigger tied to B+ and do not think that is correct.
> I assumed pin 4 needed to go to B+, maybe not.
> I made a PCB and it is a pain makeing changes and cutting foil.
> Please, any help from you solid state guys? My 8874s need you. K6XX does not
> Rick, N6PE
> Talk is cheap... except when Congress does it.
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