Unwanted triggering by mains

Terry,

You pin 12 is a floating antenna when the N.O. pushbutton is open.

A quick fix: R1 100k 12 ___ //----/\/\---+------\ \ 11 | 13 | >O----// C1 --- .--/___ / 1nF --- | | '------------- // ===

You could increase R1 if 100k isn't aggressive enough.

An alternate would be to connect your existing 10nF input cap to GND, and have your 100k pull-down and 1meg pull-up swap places. Then you'd have 10nF to GND to filter RFI,

1 meg keeping that 10nF discharged, and 100k to charge the 10nF rapidly when the pushbutton is pressed.

John Larkin's MOSFET timer is decent too if you don't mind gutting and starting over.

Cheers, James Arthur

Reply to
dagmargoodboat
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I like mechanical twist timers. They are pretty EMI hard.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Oh, I assumed the switch was on the tin. Those wires will be very sensitive to RFI. Best to put the switch on the metal enclosure keeping all wires of the CMOS circuit inside.

Rick C.

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Reply to
gnuarm.deletethisbit

Most "tin" that you encounter in the wild is actually tin plated steel, it's a common misnomer like "lead" in pencils.

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  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

The so-called 'tobacco tin' is magnetic, so clearly not actually tin!

Reply to
Terry Pinnell

Connect the ground terminal to the tin can, put the switch ON the tin can. Tie a small capacitor from the can to the 13 V right where it enters the can. Make sure the 120 V wires from the relay are nowhere near the other circuitry. Possibly put a smaller capacitor, like 100 -

1000 pF across the bottom 100K resistor. As long as this added cap is less than the .01uF cap, it should still trigger. Also, there are two more gates in the package, you need to tie those inputs to either ground or +13 V. Never leave CMOS inputs floating.

If that doesn't fix it, you may have to put a dedicated 12 V supply in the can, and use a line filter box to bring the 120 V power to it.

Having long wires running around is essentially connecting an antenna to your device, and always causes problems.

Jon

Reply to
Jon Elson

If he ACTUALLY has a real TIN box, it is a museum piece, and should not be used for such projects, as it is close to a century old. I'm 68, and have NEVER encountered a pure tin anything, except maybe at a museum.

Jon

Reply to
Jon Elson

Sorted ;-)

Once again I'd not built with maintenance in mind, so had some work to get it back onto the bench. Next time...

And as I expected, even after virtually wrapping it with the mains cable input and output, to a load, I could not get the existing circuit to trigger when overhead lights were switched on or off.

But I then made the easiest-to-implement suggestion, your 330 pf to ground from 4013 pin 12, and re-installed it. A dozen tests and no spurious triggering. Thanks a bunch Rick!

P.S: Re your puzzlement about "the switch": I think you must have missed or misunderstood something in my original post. The only 'switch' on the circuit case is the N/O button. " - Circuit in 2 oz tobacco tin mounted near mains cables and mains switch supplying fluorescent lights."

Terry, East Grinstead, UK

Reply to
Terry Pinnell

Thanks Rick, that fixed it! (See my wrongly indented reply a moment ago.)

Terry, East Grinstead, UK

Reply to
Terry Pinnell

Your problem is not magnetic, it's voltage spikes entering the box through wiring.

One quick and ugly fix would be to put a cap from ic pin 10 to ground. Maybe 1 uF. And increase the .01 uF to same.

Actually, you don't need that input differentiator. Switch to V+ and a resistor to ground would do it. Or SPDT between V+ and gnd.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

g

or

=1

e steel? Replace with steel box. Your junction impedances at IC inputs are really good receptors of magnetic field induced voltages.

That means the interference is not radiated, it has to be conducted, and is most likely coming in the 13V supply connection, possibly ground bounce, p ossibly something being picked up by the power supply leads being too close to a mains line. Dunno about UK, but in U.S. NEC, low voltage and mains vo ltage have to be physically separate, cannot be in same box or conduit or w rapped together.

Reply to
bloggs.fredbloggs.fred

Try 1nF across the 100K. You have a high impedance (100K) node going outside the box.

--Spehro Pefhany

--Spehro Pefhany

Reply to
speff

Voltage spikes are everywhere. Plug a banana lead antenna into a scope input and watch.

Terry mentioned his box triggering when other lights where switched. That's not magnetic coupling.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Thanks, but fixed as reported in my two posts up-thread.

Terry, East Grinstead, UK

Reply to
Terry Pinnell

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