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If you put too little voltage across an LED, nothing happens, as I would expect. However, I've seen a demo where a red LED is powered with a 1.5V cell, and it lights. I assumee that red LEDs are 1.9v or so, and was wondering whether there is a range in which they can glow a little bit, and if so, what accounts for it.

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I'm betting what you saw was a joule thief.

However, small amounts of current do flow below the forward voltage (see image) of an LED so perhaps it could very dimly light before hitting Vf.

enter image description here

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    \$\begingroup\$ That Joule Thief link wasn't very useful, I found thejoulethief.com/joule.html (Schematic images here and here, Eagle files also on that page) much more helpful. There's a Wikipedia article, but that isn't particularly good. \$\endgroup\$
    – Kevin Vermeer
    Jul 25, 2011 at 20:23
  • \$\begingroup\$ @Kevin It was the most applicable one for a 1.5V AA battery I thought... perhaps not the device as a whole \$\endgroup\$
    – NickHalden
    Jul 25, 2011 at 20:44
  • \$\begingroup\$ @Kevin Funny how the kit has two \$2.2k \Omega\$ resistors and one \$1k \Omega\$, but the schematic has the opposite... \$\endgroup\$
    – Adam Lawrence
    Jul 26, 2011 at 17:00
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    \$\begingroup\$ No, I meant you could see a AA battery wired to an LED, no other circuitry involved, but I like the Joule thief so much I'm giving this answer the credit \$\endgroup\$
    – Dov
    Apr 2, 2012 at 15:03
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    \$\begingroup\$ Some LEDs do very definitely glow quite noticeably in dim environments at voltages well below Vf; the real "culprit" is the dynamic range of the human eye. The amount of light required for a glowing object to be visble in a dark room is often well below 1/10,000 of the amount required to be noticeable in bright sunlight, so it doesn't take much current at all to make things glow when they shouldn't. \$\endgroup\$
    – supercat
    Feb 11, 2016 at 17:36
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Some LEDs have a forward voltage of 1.5V or less at reasonable currents. Look at the datasheet of some red LEDs for example. The forward voltage will also be lower at higher temperature.

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  • \$\begingroup\$ This is almost certainly what was happening to me, but the Joule thief posting was highly useful! \$\endgroup\$
    – Dov
    Apr 2, 2012 at 15:02
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The reason, that you can use diode below saturation voltage (knee voltage) is that the current value depends on voltage by Shockley diode equation. See wiki

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