R.G. wrote:Yep, they do recommend that. Got that same app note.
What they don't tell you is how to turn on and off a 100W incandescent bulb in 'way less than a millisecond so you can avoid polluting the measurement of the LDR rise/fall times with the rise/fall of the emittance and color temp of the bulb as it comes on and off. I went with LEDs, even in the face of the incandescent being more standard in some ways because I could start and stop the light much, much faster than the LDR could respond, so I could ignore issues of startup/tail off of the light source. I could never come up with better than some kind of d@mned fast mechanical shutter mechanism to to on/off faster than the LDR, so I went with the LED.
Yeah, that one has me a little stumped, too. I've seen LCD shutters that are pretty fast - up to a couple of milliseconds, but they cost more than I'm willing to spend at the moment. I've got some thermal inertia charts from an old Sylvania incandescent light manual. I might just be able to just flip a switch on the bulb and drop the data from the TDS1000B into a spreadsheet containing the numbers from the Sylvania manual and see if it corrects my readings enough to duplicate the charts published by Perkin Elmer. If that doesn't work then I might consider getting the LCD shutter. I really want to avoid having to go with LED's, if at all possible. Yes, they do switch on nearly instantly, but they aren't the same spectrum as incandescent lights, which will mean my charts won't match PE's charts, or probably anyone elses.
The rise and fall times for the LDR are much faster at high light intensities, and this is where I'm likely to have the greatest amount of error in my readings. Fortunately, this isn't the range that the LDR's in the Uni-Vibe light tank operate at.
You wouldn't happen to know any test engineers who used to work at Perkin Elmer's optoelectronics division before they shuttered, would you?
R.G. wrote:
Good luck to you in your search. I went through that for much the same reasons back when I was messing most actively with the 'vibe. I finally determined that an LDR you can actually get is worth ten of the perfect ones that may only exist in 10th century Spain or on one of Jupiter's moons. What that rapidly telescopes down to is to build it with LDRs you can actually get, and if you aren't happy with the sound, keep looking, meanwhile playing the ones you have. Love the ones you're with, but keep your eyes open, if you will.
Yeah, understood. I just feel better having that perfect match described exactly on a sheet of white paper. I trust specs a lot more than I trust my ears. Sign of age, I guess.
R.G. wrote:
In reality, the LDR is a dying technology. The RoHS legislations that are being proposed and put in place all over the world will phase out all cadmium bearing anything, and the "CD" in CdS stands for Cadmium. It's ironic that the folks who tell us what we may do because it's good for us to do only what they tell us to are also phasing out the incandescent light bulb. Better buy a stock of those 100W incandescent reference bulbs.
Yes, I'm already considering alternatives. I'm planning on the light tank and driver being a plug-in module. I'll make something that approximates the same functionality using strictly solid state technology for use in the EU. I can make a kit version of the LDR light tank module for sale in the EU, since CdS cells are permitted in electronic kits and for hobby use. You just can't sell finished products that use them.
Personally, I think the RoHS regulations are a little extreme. The amount of cadmium in a CdS cell is really pretty low, but the RoHS regulations forbid anything over 100ppm in any homogenous material. Even if the CdS film were microscopically small it would still violate the RoHS regulations.
There are also other semiconductor materials that can be used to make LDR's. Cadmium just happens to be the least expensive for visible wavelengths. Selenium is probably one of the earliest materials used. Germanium can also be used. In fact, GeCu sensors are commonly used in infrared astronomy. I don't know if it would be possible to fabricate a GeCu cell that would work adequately with visible light. If I managed to come up with one I'm sure Marshall and Fender would be in line to buy them for optoisolators.
I'm not planning on needing this test box any longer than it takes to produce my charts, which is one of the reasons I don't want to spend too much money on it. Maybe I'll stock up on 100W "soft white" bulbs, and in another 20 years I'll make a little cash selling NOS light bulbs for vintage Easy Bake ovens.