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Old 2nd June 2011, 03:07 AM   #1
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Default Improved LDR Volume Control

Hi All. I have been following the thread on the “Light Speed” and other LDR controls. There are a few issues of note.
1/ Source impedance is relatively low especially around the mid range.
2/ Output impedance is too low for some high impedance input amplifiers.
3/ Despite the “No Pot’ in the signal path, there are at least two in the LED illumination circuit.
The following design has no Pots at all, minimum source impedance of 27K and output of around 21K which may still be too low in some cases.
I’ m calling it a Shutter light for want of a better name. A shutter is moved across four 25mm long tubes housing the LDRs via a 270 deg cam. The four LEDs are not in line with the LDRs but offset toward the centre point. If they were in line, the source impedance would drop into the 100’s of ohms area in the centre of travel (135 deg). At the centre point both shunt and series LDRs are relatively shaded and both have 13K or so of resistance. As the shutter moves from the centre; move light falls on one and less on the other increasing the source impedance.
Alignment and channel matching is done simply by shoving the LED’s one way or another. Response can also be altered by moving them closer to the centre, or just the series one to create a log response. I’m still messing about with this and have yet to do any listening test. You thoughts?
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File Type: jpg shutterlight3.jpg (48.2 KB, 713 views)
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Old 2nd June 2011, 03:12 AM   #2
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Default Some graphs

Attached images are of the wiper transfer. A 5 volt supply connected across the LDR's and the control moved in 10 deg steps.
The R curve is the sourse impedance across the LDR's
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File Type: jpg sourseimpedence.jpg (17.1 KB, 685 views)
File Type: jpg whiperresponse.jpg (16.8 KB, 665 views)
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Old 1st July 2011, 06:18 PM   #3
mcspack is offline mcspack  United States
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Interesting, how is this project coming? Any listing impressions yet compared to standard lightspeed? I'd love to see some more pics of the insides of this device as well as a schematic?

thanks

MC
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Old 2nd July 2011, 08:01 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I am confused with the way you describe the impedances.

The Source impedance is the output impedance of the Source as seen by the Receiver.
The output impedance is that seen by the Receiver.


The Source has an output impedance
The receiver has an input impedance
The LDRs have a source impedance and an input impedance.
The Source sees the input impedance of the LDRs.
The Receiver sees the output impedance of the LDRs.
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Old 12th July 2011, 02:51 AM   #5
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Hi Andrew. To avoid confusion just consider the LDR network as a standard pot. The plot on the left is the resistance through 270 deg of rotation Across the pot. i.e. top to bottom (the connection to the source, CD etc)
The plot on the right is of the resistance through 270 degs of the wiper to the bottom or earth of the pot ( connects to the amplifier)
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Old 12th July 2011, 02:54 AM   #6
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Hi MC. Sorry for the delay. Have been away working and haven't got around to doing some serious listening as yet. Will post some more images of the unit apart to give you an idea. I also have CAD drawings I used to construct it, if you are interested.
Cheers,
Simon
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Old 12th July 2011, 03:09 AM   #7
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Default Internal Pic

Here is a pic of the unit. The left half has the LEDS and 270 deg log taper. The right is the shutter and LDR light shafts.
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Old 12th July 2011, 04:21 AM   #8
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I understand the concept of your design, but if I'm seeing it correctly, it looks like both LDRs are operating exactly the same way, that is it appears they increase and decrease resistance together, whereas one should increase awhile the other decreases. Am I seeing it right?

Mike
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Old 12th July 2011, 04:48 AM   #9
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Hi Mike. In the photo you are seeing the bottom half of both channel LDR tunnels. The other two are hidden under the shutter. Would a pic of the shutter half way help. If so I can post one.
Cheers,
Simon
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Old 12th July 2011, 05:00 AM   #10
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Default Image for Mike

Mike. Here is a pic of the shutter at the half way point and of the light tunnels with the LDRs in the ends.
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File Type: jpg half open.jpg (76.0 KB, 455 views)
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