Vactrol information

$l t_{rise}$ is time to 63% of final conductance @ $l I_F$ = 16 mA, in ms.
$l t_{fall}$ is time to 100kΩ (VTL5C1, VTL5C2, VTL5C3, VTL5C3/2, VTL5C4, VTL5C9) or 1MΩ (VTL5C4/2, VTL5C6, VTL5C10) after removal of input @ $l I_F$ = 16 mA, in ms.

* Dual vactrol.

Advanced Photonix

These are available from SynthCube, Digi-Key, and Mouser. The datasheets have some strange numbers which may be typos. Note the much lower on resistances are measured at much higher $l I_F$ than for Xvive.

* Dual vactrol.
** Datasheet says 400MΩ but I assume this is a typo for 400kΩ.

$l t_{rise}$ is:

• NSL-32, NSL-37V5C3/2: time to 63% of final conductance, in ms.
• NSL-32SR2, NSL-32SR3: time to 63% of final conductance @ $l I_F$ = 5 mA, in ms.
• NSL-32SR2 sorted: time to 63% of final conductance @ $l I_F$ = 16 mA, in ms.
• NSL-33-007: time to 63% of final conductance @ $l I_F$ = 4.6 mA, in ms.

$l t_{fall}$ is:

• NSL-32, NSL-32SR2 sorted: Time to reach 100kΩ after removal of $l I_F$ = 16 mA, in ms.
• NSL-32SR2, NSL-32SR3: time to 100kΩ after removal of input @ $l I_F$ = 5 mA, in ms.
• NSL-33-007: time to 37% of final conductance after removal of $l I_F$ = 4.6 mA, in ms.
• NSL-37V5C3/2: time to 37% of final value 100kΩ [sic] after removal of the light source.

Photoresistors for vactrols

If one is making one’s own vactrols, one may want to use LED and LDR combinations that mimic the behavior of commercial vactrols. This is harder than it looks.

For example, note that for the LDRs listed here, $l R_{on}$ is measured with an illuminance of 1 or 2 foot-candles or 10 lux. 1 foot-candle is roughly the same as 10 lux, and indoor office lighting is typically around 350 to 500 lux. Even at 0.5 mA current, a green LED at zero distance is likely to be much brighter than 10 lux and will presumably drive a photoresistor to much smaller resistances. Hence these values for $l R_{on}$ cannot be compared to the $l R_{on}$ values in the vactrol specifications.

As for $l t_{fall}$, for vactrols these are shown as larger, sometimes by orders of magnitude, than $l t_{rise}$. But for photoresistors, $l t_{fall}$ is the same order of magnitude and often smaller than $l t_{rise}$, so generally very much smaller than for vactrols. I believe the explanation is in the way $l t_{fall}$ is defined and measured.

Quoting from the PerkinElmer catalog:

When light is suddenly applied, the photocells resistance drops very fast, typically reaching 63% (1-1/e conductance) of its final values in under 10 msec.

When the light is removed, the resistance increases initially at an exponential rate, approximately tripling in a few milliseconds. The resistance then increases linearly with time.

For vactrols $l t_{fall}$ is described as the time it takes to reach either 100k or 1M (one or the other is used for different vactrols) after turn-off. (Except for NSL-33-007 and maybe NSL-37V5C3/2.) But if you’re going by the time constant for the initial exponential change, that is far shorter. The GL55, NTE, and Advanced Photonix photocell datasheets quote fall times with no definition and no measurement description, so who knows. But the from the PerkinElmer catalog, photocell $l t_{rise}$ is the time to reach $l 1-1/e$ and $l t_{fall}$ is time to reach $l 1/e$, and $l t_{rise} > t_{fall}$. A similar definition is used for the NSL-33-007 vactrol and maybe the NSL-37V5C3/2. The much smaller values for these make sense, fast initial response to turn-off followed by very slow return to 100k or 1M. I think that explains the apparent disconnect between vactrol and photocell fall times.

The verdict is that one might be able to use the photoresistor specs to guide selection, but their actual performance in DIY vactrols compared to commercial vactrols can only be determined by experimentation.

GL55 series

GL55 series photoresistors/LDRs are commonly used for making vactrols. LDRs with this designation are available from vendors on AliExpress, eBay, Amazon Marketplace, et cetera. I assume they are made by multiple manufacturers and not necessarily to the same specifications. I do not know what manufacturer originated them. Juried Engineering has a listing that says they make them, or at least brand them, and to “Be mindful of the many, inexpensive, counterfeits available on the market” and theirs certainly are not inexpensive ($5.49 each). But they provide no datasheet. The only datasheet I can find seems to be anonymous.

[Dia = diameter in mm; $l R_{on}$ = light resistance in kΩ; $l R_{off}$ = dark resistance in MΩ; $l t_{rise}$, $l t_{fall}$ = rise time, fall time in ms; spectral peak in nm]

NTE Electronics

NTE Electronics has a line of LDRs for which there is a datasheet . Digi-Key sells them. Following are LDRs with diameter under 5.1 mm.

Advanced Photonix

Digi-Key and Mouser both sell LDRs from Advanced Photonix . Unfortunately they seem to have only individual datasheets for each product, with varying test conditions, making comparison tedious. Following are LDRs with diameter under 5.1 mm.

Waitrony

These are sold by Tayda.