Improved LDR Volume Control

[blessgin]

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?

[blessgin]

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

[mcspack]

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

[AndrewT]

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.

[blessgin]

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)

[blessgin]

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

[blessgin]

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.

[Michael Bean]

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

[blessgin]

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

[blessgin]

Mike. Here is a pic of the shutter at the half way point and of the light tunnels with the LDRs in the ends.