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Old 18th June 2010, 04:45 PM   #3301
AndrewT is offline AndrewT  Scotland
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another simple solution is to insert a quiet/loud switch.
The switch bypasses a pair of 360k series resistors that reduce the current flowing to the series LEDs. Completely out of the audio signal stream.
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Old 18th June 2010, 05:44 PM   #3302
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another simple solution is to insert a quiet/loud switch.
The switch bypasses a pair of 360k series resistors that reduce the current flowing to the series LEDs. Completely out of the audio signal stream.
Cool idea! I need to think about this one. Surely though, it changes the Zo by quite a lot. How does the downstream amplifier like this?

Karl
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Old 18th June 2010, 06:51 PM   #3303
AndrewT is offline AndrewT  Scotland
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leaving the 360k in circuit is specifically for low level listening.
The output impedance of the signal side (when listening to low level signal) is largely determined by the Shunt LDR. The high value of the Series LDR hardly affects the impedance.
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Old 18th June 2010, 11:46 PM   #3304
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leaving the 360k in circuit is specifically for low level listening.
The output impedance of the signal side (when listening to low level signal) is largely determined by the Shunt LDR. The high value of the Series LDR hardly affects the impedance.
The attenuation for the T attenuator at Zo=6k tops out at 88dB when R2=0. I need to rewrite the spreadsheet to allow different resistor values that would result in higher attenuation, even if the price is a higher Zo. But I'm guessing that 88dB is practically silent, isn't it?

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Old 21st June 2010, 04:07 PM   #3305
maxw is offline maxw  United Kingdom
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What difference does the value of the pot make? Everyone seems to use 100K but what if I use 50k or 10k?
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Old 21st June 2010, 04:14 PM   #3306
AndrewT is offline AndrewT  Scotland
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a low value pot can only put a low value resistance in the circuit formed by the regulated voltage source and the LED.
That results in a higher value of minimum current through the LEDs.
That reduces the range of attenuation available from the LDR/LED combination.
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Old 22nd June 2010, 02:18 PM   #3307
maxw is offline maxw  United Kingdom
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Originally Posted by AndrewT View Post
a low value pot can only put a low value resistance in the circuit formed by the regulated voltage source and the LED.
That results in a higher value of minimum current through the LEDs.
That reduces the range of attenuation available from the LDR/LED combination.
Thanks for the explaination.


I want to use a digital pot to control the LDR but can't get a log one that is the right value so would there be any problem with using a 100k linear digital pot with a resistor across the input like this?

Last edited by maxw; 22nd June 2010 at 02:20 PM.
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Old 22nd June 2010, 03:34 PM   #3308
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Thanks for the explaination.


I want to use a digital pot to control the LDR but can't get a log one that is the right value so would there be any problem with using a 100k linear digital pot with a resistor across the input like this?
Hmm, what would the ultimate curve look like after you combine the S-curve with the non-linear curve of the LDR module? Have you tried graphing the composite?

Karl
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Old 22nd June 2010, 03:50 PM   #3309
maxw is offline maxw  United Kingdom
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Hmm, what would the ultimate curve look like after you combine the S-curve with the non-linear curve of the LDR module? Have you tried graphing the composite?

Karl
Not sure, only ordered my LDRs yesterday! I'll have a play when they arrive. With a 256 step digital pot there would be room to adjust the resulting curve in software. I'll be controlling the digital pot via I2C from an ardunio.
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Old 22nd June 2010, 05:04 PM   #3310
udailey is offline udailey  United States
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That will work but you will want to make sure your wiper can handle the power.
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