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Old 18th March 2012, 01:31 AM   #1
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Smile Light Dependant Resistor Current Control

A Light Dependant Resistor LDR is: is a resistor whose resistance decreases with increasing incident light intensity; in other words, it exhibits photoconductivity. It can also be referred to as a photoconductor or CdS device, from "cadmium sulfide," which is the material from which the device is made and that actually exhibits the variation in resistance with light level

A popular circuit for LDR's is to connect them as a L pad attenuator, however all of the Pad forms are readily available to suit LDR's including balanced audio use.

This thread discusses how to go about current control of the anode and cathode terminals of a LDR, a typical device, ( but not limited to ) is the popular Silonex NSL32SR2

The ideal LDR circuit attenuator (a very popular way of using LDR's), would have focus on current being maintained hence controlled whilst a voltage is being varied, as the properties of LDR's ideally suit current control, not voltage control.

A typical attenuator circuit using fixed voltages into potentiometers or any resistance thereafter is a variable ohmic device and therefore has preference altering current straight away before voltage, Such circuits dismiss or abandon current control. This thread will attempt a difficult subject, circuits that do the opposite. That is to have preference or some ability at least, of current control of the anode and cathode terminals.

Last edited by Chris Daly; 18th March 2012 at 02:11 AM.
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Old 18th March 2012, 02:06 AM   #2
gootee is offline gootee  United States
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To be more clear, LDR devices have four terminals and include an LED encapsulated with the CdS cell and the current through the LED, varying the LED's brightness, is what causes the resistance of the CdS photocell to change.

Are you speaking of discussing better types of voltage-controlled current sources, for the LDRs' LEDs?

If so, the Howland or Improved Howland topologies should probably be considered. Here is a link to a good application note about them, by the late Bob Pease:

http://www.ti.com/lit/an/snoa474/snoa474.pdf

There are some simple precision current source and current sink circuits, such as those in Figures 17 and 18, in:

http://www.ti.com/lit/an/snoa621b/snoa621b.pdf

which also appear in:

http://www.ti.com/lit/an/snla140a/snla140a.pdf

I'm not really sure what the benefit might be, from using explicit current-control, instead of just using a potentiometer.

What you might be talking about is wrapping a feedback loop around the whole thing, so that the current setpoint is maintained even if other things vary (e.g. voltage noise in a resistor-based current supply). Is that what you meant? If so, an active voltage-controlled current source might be useful, or even necessary.

Cheers,

Tom Gootee

Last edited by gootee; 18th March 2012 at 02:35 AM.
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Old 18th March 2012, 02:39 AM   #3
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Quote:
Originally Posted by gootee View Post
Are you speaking of discussing better types of voltage-controlled current sources, for the LDRs' LEDs?
Yes I am referring to voltage controlled current sources,

Here is a LDR attenuator I designed that performs extremely well. It uses and exploits the usually dismissed property of single rail op amps, giving up voltage at their outputs relieving additional circuitry to supply V+ .It contains voltage followed by current regulation within the feedback path, it manages to contain the LDR devices as inputs to the inverting op amp paths, and buffers the wiper to assist current delivery.

The schematic shows a link across the wiper to the upper and lower op amps - this is a schematic error, this point is joined. i will re publish to clarify.

The transistors that are saturated act as band gaps to lower the voltage to the LDR's and improve performance. The combination of 3 on the UAR and two on the LAR provides a reasonable attenuator response, Also 2 series band gaps to each upper arm LDR works equally as well.

Other possibilities are inviting resistance approx 18k across the UAR path to gnd, however band gaps in my opinion provide better aural presentation.

Cheers / Chris
Attached Files
File Type: pdf LDR Stereo Attenuator.pdf (15.2 KB, 340 views)

Last edited by Chris Daly; 18th March 2012 at 03:02 AM.
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Old 18th March 2012, 02:44 AM   #4
gootee is offline gootee  United States
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I'm still not sure of the need for such a control system, because it seems that in the end it would depend on having a very stable reference voltage with which to set the setpoint and if you had that then you could just use it with the usual potentiometer scheme. What am I missing?

Edit: You posted while I was writing. I'll take a look.

Interesting. I'll have to try simulating your circuit.

Last edited by gootee; 18th March 2012 at 02:54 AM.
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Old 18th March 2012, 03:10 AM   #5
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Republish of schematic correcting wiper connection joined to UAR and LAR op amps
Attached Files
File Type: pdf LDR Stereo Attenuator.pdf (15.2 KB, 271 views)

Last edited by Chris Daly; 18th March 2012 at 03:12 AM.
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Old 18th March 2012, 05:16 AM   #6
BFNY is offline BFNY  United States
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Quote:
Originally Posted by gootee View Post
I'm still not sure of the need for such a control system, because it seems that in the end it would depend on having a very stable reference voltage with which to set the setpoint and if you had that then you could just use it with the usual potentiometer scheme. What am I missing?
Look at a standard LED - at the "knee", or "hockey stick" part of the I-V response. At that key point, a small change in voltage gives a large change in current.

It would be better if you could arrange things in a linear fashion. So a 10% change in control voltage *always* = a 10% change in current, even at the "knee". Of course, it could be any linear I-V relationship, that's just an example. The standard LED I-V curve is far from linear.

Then you use your pot to give the control voltage.
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Old 18th March 2012, 09:15 AM   #7
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The measurements of the schematic above, provide: voltage at the UAR LDR's anode to cathode, ranges from 1.35v to 1.66v, and LAR anode to cathode from 1.46 to 1.7v, current is regulated to 24ma , 1.25/26+1 (UAR comprising two LDR's ) and current measured at the anode of UAR LDR (1) ranges between 16.9ua and 24.1ma, and is shared evenly to the other UAR LDR(2) also 16.9ua to 24.1ma.

I consider the design is very close to achieving best possible audio performance from the NSL32SR2, it certainly sounds wonderful. I consider there may be better current control, for instance by lowering the potentiometer resistance to 10k.. which may enable the available current from the LM317 to more fully access the NSL32SR2's... balancing this against the circuit also performing as a attenuator. The circuit has surprised me in the past with earlier revisions, by often doing the opposite of what you consider it might first do.

Cheers / Chris

Last edited by Chris Daly; 18th March 2012 at 09:17 AM.
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Old 19th March 2012, 12:11 PM   #8
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AndrewT has asked me in another thread:

"....voltage control works Current control is an alternative to voltage control.
If current control can be made to work well, then that does not "prove" that voltage control is wrong nor that is is ineffective nor that it is obsolete and must be replaced ".

If we do not want to make progress with LDR's, then yes stay with voltage.

Silonex have provided research into this http://www.silonex.com/audiohm/levelcontrol.html ( see attenuator configurations ) and state:

"It is better to drive the coupler LED from a constant current source, to minimize the effects of variations in LED forward voltage from device to device and temperature"


Could this be why Voltage control based LDR's require extreme matching ?

My own research using a audio system comprising pioneer pds 801 audio synthesis dsm,Quad 909 into Martin Logan quest z ( a friends audio system ) immediately shows the audio differences of voltage control and current control. current control providing much better resolution detail and stereo channel identity.

The philosophy of progress is a fairly sure and pleasant objective for humans....I believe.

Cheers / Chris

Last edited by Chris Daly; 19th March 2012 at 12:36 PM.
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Old 20th March 2012, 05:18 AM   #9
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With reference to my schematic, and noting additions of transistors as bandgaps. Here are measurements when the potentiometer is paralleled with 2x 22k resistors in effect 50k/44K = 23.4K, so as to increase current across the LDR relative to a 50k potentiometer.
25k looks to be a sensible value of potentiometer overall.

To still achieve a log attenuator response, its necessary to add 2x more series bandgaps to UAR
The approach of increasing voltage across the pot ( remember current is already upper limit regulated by LM317 ) and decreasing it with bandgaps is beneficial to LDR's, providing lower voltage at the device.

Why all these bandgaps ? This is a result of LM7805 setting voltage in the feedback path, then lowered as much as is allowed for Vin to LM317, also within the feedback path, that has drop out as a current regulator if voltage gets too low across its input. Adj terminal measures 4.05v

note: voltage measured at the anode of LDR, current measured at inverting input UAR and LAR, and anode of LDR

UAR
.990v to 1.510v .

.054ma to 21.2ma

LAR

1.531v to 1.785v

.126ma to 15.26ma


Also note at potentiometer end travel there is still voltage and current across LDR.

Another charming feature is that the audio provides silence at beginning of pot travel.

Cheers / Chris

Last edited by Chris Daly; 20th March 2012 at 05:45 AM.
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Old 20th March 2012, 03:13 PM   #10
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I did some sound tests tonight with Model 7 with the 23.4 k pot, comparing it to an earlier model with 50k. Whilst Model 7 is very good, the earlier model is just so much better in sound presentation.... it was referred to as the bench mark.

Its differences are in how voltage is regulated and it keeps the bandgaps with two groups of 3, 1 leading in for each, and 2 each for UAR and LAR . I am updating schematic, but need to research the real reasons why it performs so well.

Is anyone finding this design interesting ?

Cheers / Chris

Last edited by Chris Daly; 20th March 2012 at 03:18 PM.
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