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I need help with a way to light up my SWR meter with internal led which wou ld be really close to RF passing thru the SWR meter. That way it would only light up the meter when I transmit. if anyone could point me to a circuit that would do this, probably need two leds one on each side of the meter, would be really grateful. I know there are ways using 12V but I want to sta y away from batteries or having to plug/unplug cables. My email is k4wi@ear thlink.net. Thanks, Cort K4WI

Reply to
my2004vette
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Do you have the drawings for your SWR meter?
Reply to
John Fields

You could pick off the RF with a small capacitor or a gimmick.

Then go to an RF choke to ground. From the junction, a schottky diode (1N5711 maybe) drives the LED anode, with the led cathode grounded. Tweak the cap or the gimmick to tune the brightness.

Most LEDs have a lot of capacitance, so may not work directly from the RF.

A lot depends on your frequency and RF power level.

A good LED is bright at 1 mA, especially green ones.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
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Reply to
John Larkin

Just 1 mA? You talking about surface-mount LEDs?

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Reply to
Mr. Man-wai Chang

LEDs have gotten more efficient, which is how there are now the superbright.

Forty years ago, you'd put 15mA into an LED and get a bit of light, good for display but not for lighting anything up.

Now you can put that current through some "average" LED and get a lot more light.

He also factored in the sensitivity of the eye, apparently more sensitive in the green spectrum so you can see it better than another color, thus lower output is needed to be a bare minimum.

You drop the current through an LED that has great output at 15mA, and you get a decent indicator at much lower current.

MIchael

Reply to
Michael Black

They all have a chip inside.

I have some Avago high-efficiency green LEDs, in T1-3/4 packages. Greens are not just efficient, they coincide with the eye's sensitivity curve.

These are bright a 1 mA and visible in room light at 1 uA. I tried to determine the minimum current that would produce discernable light up close, in total darkness, dark-adapted. It was about 800 picoamps.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

Do you have the picture and datasheet of that 1mA LED? Could I read them?

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Reply to
Mr. Man-wai Chang

Just get a high efficiency LED and run about 1mA and see! - everyone else are not going to do all your work for you

Reply to
David Eather

They were a gift from James Arthur, and I don't know the part number. Just browse the Avago web site, or Digikey, for high-output green LEDs.

I was surprsded by the sub-nA light output. At roughly 100 mV/decade current, I'd have guessed that the voltage drop would be so low that it wouldn't have enough energy to make photons.

It would be fun to use a PMT and some signal averaging to see how little current would still make some light.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

I think he's still trying to wrap his mind around the idea that putting more current through an LED won't automatically increase the light output.

If only, all those Poly-Pak type LEDs forty years ago, that were dim with

15mA going through them, would have been fine.

Michael

Reply to
Michael Black

It does, up until the LED dies.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

"But does it short out, or open up?"

Michael

Reply to
Michael Black

To get to 1mA means I would need to buy another resistor. That's money. :)

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Reply to
Mr. Man-wai Chang

  1. Would a 3V resistor short itself when connected to a 3V DC input?
  2. If answer to (1) was "no", then why do I need a current-limiting resistor connecting 2 3V LEDs in series to a 5V DC input?
  3. If answer to (1) was "yes", then what current-limiting resistor should I use in (1)?
--
   @~@   Remain silent. Nothing from soldiers and magicians is real! 
  / v \  Simplicity is Beauty! May the Force and farces be with you! 
/( _ )\ (Fedora 19 i686)  Linux 3.10.3-300.fc19.i686 
   ^ ^   16:48:02 up 19:20 0 users load average: 0.00 0.01 0.05 
???! ???! ???! ???! ???! ???! ????? (CSSA): 
http://www.swd.gov.hk/tc/index/site_pubsvc/page_socsecu/sub_addressesa
Reply to
Mr. Man-wai Chang

Are you two talking about 3mm/5mm LEDs or hi-power LEDs here?

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Reply to
Mr. Man-wai Chang

If it's a military grade LED, then I was NOT supposed to be able to find anything about it! :)

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   @~@   Remain silent. Nothing from soldiers and magicians is real! 
  / v \  Simplicity is Beauty! May the Force and farces be with you! 
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   ^ ^   16:48:02 up 19:20 0 users load average: 0.00 0.01 0.05 
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Reply to
Mr. Man-wai Chang 

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So is time, but you don't seem to have a problem wasting everybody 
else's.
Reply to
John Fields

WTF is a 3V resistor?

Reply to
JW

For that matter, WTF is a 3V DC input?

Reply to
JW

I repost the questions. Thanks!

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Reply to
Mr. Man-wai Chang

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