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Old 19th July 2010, 11:01 AM   #11
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If you use a linear 100kohm pot to control the leds in my Lightspeed Attenuator circuit the volume comes on too fast for comfort, a log 100kohm is much smoother and progressive and really starts to ramp up the gain around 2o'clock.
Cheers George
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Old 19th July 2010, 11:39 AM   #12
Ipanema is offline Ipanema  Malaysia
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Hi wapo54001, I'm trying to understand your concept. Are you measuring and applying compensation to the LDR in discreet step e.g using a lookup table?

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Old 20th July 2010, 01:42 AM   #13
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Originally Posted by Ipanema View Post
Hi wapo54001, I'm trying to understand your concept. Are you measuring and applying compensation to the LDR in discreet step e.g using a lookup table?

Regards.
Still trying to work that out, but definitely space constrained. Probably will wind up applying compensation, since for a T attenuator, you have to deal with three resistors per channel, total six for a two channel board. May have to go to a single channel board, or two PICs per stereo board.
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Old 20th July 2010, 02:22 AM   #14
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Originally Posted by georgehifi View Post
If you use a linear 100kohm pot to control the leds in my Lightspeed Attenuator circuit the volume comes on too fast for comfort, a log 100kohm is much smoother and progressive and really starts to ramp up the gain around 2o'clock.
Cheers George
George, I finished the first draft of my two channel board and have just finished modifying that "playback" design to individually measure the resistance-vs-LED-current of each of the four devices on each carrier board.

When I measure the first half dozen or more and chart them in Excel I'm hoping I'll have a composite picture of the "average" Silonex device, and from there I'll be in a good position to understand what is going on and why a log pot works to control what appears to be itself a log device. It seems like it should turn into a log-log overall response. Not sure why it doesn't -- either the Silonex is not actually a log device, or else there is something in your implementation that affects the overall response or, one other possibility is that the overall response of the device is logarithmic, but the operationally useful part of the curve (at the low resistance end) is more linear than the entire curve from 20ma to full OFF. I'm guessing that will turn out to be the case.

When I find out, I'll publish the result here. First step is to get a decent full picture of the curve of the Silonex. I don't think anyone has done one yet.
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Old 24th July 2010, 04:20 AM   #15
gootee is offline gootee  United States
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It's pretty close to log, I think, if you're looking at R vs I.

I thought (assumed maybe?) that Lightspeed was using linear pots.

But, anyway, the actual OVERALL RESPONSE to the control (or pot) is a whole different animal, because of the way the pot is wired, and because there are two LDRs (series and shunt). For the Lightspeed, the attenuation factor itself starts out sort of level near zero then curves upward and increases its slope, goes through an inflection point in the middle, and then curves back to more level near one.
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Old 28th July 2010, 05:05 PM   #16
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I haven't done any calibrated tests yet, but in testing my control system I find that the Silonex resistor response is very, very flat into the hundreds of ohms (I can control to the 1/10 ohm level) and then turns a very sharp corner near 1000 ohms and sensitivity increases rapidly. Above about 20K ohms it becomes extreme and difficult to manage. At zero current the resistance is on the order of 30 mega ohms on the two samples I've tested so far, and the change in current from 30K ohms to 30M ohms is not much.

I can't believe that this response is logarithmic. Taken in its entirety, it's more like a mirrored "L" shape.

My T attenuator plans call for two attenuation settings (.5 & 1dB) of approximately 150K and 300K. This will be very hard to accomplish.
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Old 31st July 2010, 02:42 AM   #17
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Here is a rather accurate graph of the Silonex LDR response.

Current is divided into 460 equal steps along one axis, and the Silonex resistance is charted in relation to the current delivered. Resistance range shown is 40 ohms to 85K ohms, approximate. Pretty spectacular corner.

Second chart is LOG scale
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File Type: jpg ScreenShot002.jpg (34.8 KB, 1432 views)

Last edited by wapo54001; 31st July 2010 at 02:46 AM.
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Old 31st July 2010, 03:46 AM   #18
Blues is offline Blues  United States
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I have the same response plotted on Excel...keep it up...I'm following your Thread. Like you I've branched out on a different path of controlling these LDRs albeit a simpler way.
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Old 31st July 2010, 01:14 PM   #19
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Here's the image from Silonex website -- fwiw, I found that the devices followed the "control law" pretty acurately.
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Old 31st July 2010, 01:19 PM   #20
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I knew I had the Excel file somewhere -- here's the Silonex vs the control law:
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