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The photograph shows the printed circuit board from under the cover of this 240V AC indicator lamp. The page description says that the device is a neon indicator lamp, a statement confirmed by the associated technical data sheet. As one can see from the photo, the board contains a simple series connected circuit going from one pin of the 240 volt AC supply, through two 75 kilohm resistors, then through the lamp, and then through the component labelled S731001. The lamp component measures 3 mm x 2 mm and the light-radiating portion is a mere 1 mm square.

Questions: Is the lamp truly a neon lamp of some kind, and if so, what? I ask because I can't find any information on the web about even the existence of surface mounted neon lights (I would actually like to find the SMD equivalent of a neon NE-2 bulb) and I'm also puzzled by the S731001 component, which I think might be a diode. And if it is a diode, why would one be needed in a neon bulb circuit?!

enter image description here

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    \$\begingroup\$ Not really an answer to your question ... so just a comment here ... this device might replace a neon lamp as an indicator. But is not a complete replacement. For instance, this book contains logic circuits using neon lamps that I doubt the above device is capable of replicating. \$\endgroup\$
    – st2000
    Commented Aug 6 at 12:21
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    \$\begingroup\$ the page you linked says that it is an LED \$\endgroup\$
    – jsotola
    Commented Aug 6 at 18:14
  • \$\begingroup\$ LOL! I missed that. \$\endgroup\$
    – CrimsonDark
    Commented Aug 7 at 4:23

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That’s an LED lamp. It’s a replacement for a neon lamp. There’s a diode rectifier at the top of the board because the LED needs DC, and two 75k resistors in series for current limiting at the bottom.

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    \$\begingroup\$ And here I was hoping you'd ferret out a nice smd neon bulb for me! ;) My first use of ne-2 bulbs was as a long series chain to act as a voltage regulator for a Geiger counter that need 1.4 kV. Tweaking by selecting them, as they varied a bit in voltage drop from 70 to about 90 V. \$\endgroup\$
    – periblepsis
    Commented Aug 6 at 11:18
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No, it even says on "Product Information" tab that it's a LED indicator.

It is simply a module which kind of looks like a neon lamp indicator and can directly be connected to mains like a similar module which has a neon indicator (and the much needed series resistor).

The circuit itself seems to be pretty naive, as there is only a LED with resistors and a rectifying diode in series.

It means that the LED will only light up during positive or negative mains cycle but not on both like a neon indicator.

And because LEDs are sensitive to reverse voltage and it will cause them to degrade and damage, the rectifying diode in series is not the best idea to prevent reverse bias.

Diodes will leak a bit, so it might leak enough to reverse bias the LED somewhat, but unknown how much. It might have made more sense to put the diode in anti-parallel over the LED to protect and limit the reverse bias to safe 0.7V level.

And while doing so, it will conduct anyway on reverse cycle so it could have been replaced with a LED so the two anti-parallel LEDs would protect each other and one LED would light up on positive and the other LED would light up on the negative mains cycle.

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    \$\begingroup\$ that would, however, easily have increased product cost by 0.1ct, just to make a better product! \$\endgroup\$
    – Marcus Müller
    Commented Aug 6 at 13:04
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    \$\begingroup\$ Good catch on the reverse bias with only a series diode +1. I've seen some premature failures that appear to be due to this cause. An additional LL4148 across the LED handles it. Could also buy an LED with two dice in inverse parallel. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Aug 7 at 3:13
  • \$\begingroup\$ I somehow missed the statement about it being an LED ... but in any case, the comments and observations are most appreciated. \$\endgroup\$
    – CrimsonDark
    Commented Aug 7 at 4:23
  • \$\begingroup\$ @CrimsonDark Also worth noting that one end of the indicator is marked with +, which probably wouldn't have been done for a neon. \$\endgroup\$
    – Mark Morgan Lloyd
    Commented Aug 7 at 6:56

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