volume attenuator relay

MisterBill2

Joined Jan 23, 2018
9,010
since you insist with the LDR I would like to know how you would do the circuit.
Using an LDR, it will take two of them, the first one in series with the signal line and then a second one shunting the signal output from the attenuator, which would be the same as an "L-pad", functionally a voltage divider. The particular scheme for controlling the light intensity on each LDR will depend on the control intention required. No mention has been made of how the control command is initiated, nor has any mention of how many steps are desired. This might be an application with only two levels of attenuation.
We do not have any real description of the desired operation aside from it being an attenuator. And why are only step reductions requested? One system that does allow remote control with only resistive devices would be a motor driven "T-Pad", which was included in one high-end audio device that I had worked with. Not cheap, but noise free and no power consumed except while changing positions.
And as for the non-linearity of the opto-isolation device described in the Farnell link, that is the transfer function of the package, it is not the current/voltage characteristic of the resistor portion of the device. LED operation is very non-linear, which is the way diodes work. But only the resistor would be in the audio portion of the circuit.
 
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Thread Starter

arivel

Joined Mar 4, 2018
42
I don't understand what other information you need.
maybe one thing is not very clear, with six relays you can do 64 attenuation steps using the following configuration:
https://en.wikipedia.org/wiki/Logarithmic_resistor_ladder
you say that in an LDR it is not the resistor that is not linear but the led.
since the LDRs are composed of LEDs and resistor that must work together, the mere fact that the resistor taken alone is linear is of no use to me because it makes no sense to make a volume attenuator that has only two attenuation steps.
in the circuit you would make, you talk about the arrangement of the LDRs which is that of a voltage divider, ok, but I'm interested in the control circuit.
 

MisterBill2

Joined Jan 23, 2018
9,010
I don't understand what other information you need.
maybe one thing is not very clear, with six relays you can do 64 attenuation steps using the following configuration:
https://en.wikipedia.org/wiki/Logarithmic_resistor_ladder
you say that in an LDR it is not the resistor that is not linear but the led.
since the LDRs are composed of LEDs and resistor that must work together, the mere fact that the resistor taken alone is linear is of no use to me because it makes no sense to make a volume attenuator that has only two attenuation steps.
in the circuit you would make, you talk about the arrangement of the LDRs which is that of a voltage divider, ok, but I'm interested in the control circuit.
I did not see any information about the number of loudness levels desired, nor information about how the selection would be made.
The concern was about non-linear devices in the audio stream, not in the control stream, since relays are as non-linear as can be. The LED function is to alter the resistance of the LDR.
Now since you already have decided what you want, why bother asking here???
 

AnalogKid

Joined Aug 1, 2013
9,287
maybe one thing is not very clear, with six relays you can do 64 attenuation steps using the following configuration:
https://en.wikipedia.org/wiki/Logarithmic_resistor_ladder
With a resistor network on the outputs of the counter, you can make a single LDR attenuator circuit step through 16 or 64 or however many steps of different volume levels. BUT, the steps will not be linearly or logarithmically spaced, because the relationship between LED current and LDR resistance is more complex than that. The volume will go up and down, but not with precision such as each step is 1 dB. And the volume will not track among multiple channels. It sounds like you want more precise control than that.

ak
 

AnalogKid

Joined Aug 1, 2013
9,287
since the LDRs are composed of LEDs and resistor that must work together, the mere fact that the resistor taken alone is linear is of no use to me because it makes no sense to make a volume attenuator that has only two attenuation steps.
Ah, but a relay also has only two attenuation levels, full on and full off. I think he is suggesting an LDR circuit as a replacement for each relay. This could work, but I think the relays will be smaller, cheaper, more predictable, and work better driving a resistor ladder. Also, the LDR's finite values for both the on and off resistances will introduce a cyclic non-linearity into the attenuator string.

ak
 

Thread Starter

arivel

Joined Mar 4, 2018
42
since relays are as non-linear as can be
??
as you can see from the image of the attached file the signal flows in the resistors which are connected to the relay contacts.
in case you don't remember well, the question I asked was only about the opamp. it was you who rambled on the LDRs
 

MisterBill2

Joined Jan 23, 2018
9,010
in order to understand better, is this the LDR configuration you mean?
https://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/
That is the equal of what I was suggesting. The difference being that I would suggest not having such a complex arrangement of multi-turn devices to regulate the LED intensity.
And it was not a rambling on my part. The original intent was to "have no semiconductors that dirty the signal ". The LDR achieves that goal very well.
 

Thread Starter

arivel

Joined Mar 4, 2018
42
this attenuator has other problems as well.
it is not possible to use a remote control unless you want to do it motorized but it seems to me a complexity that is not worth it.
the LDRs must be coupled because each LDR has a different curve from the other and there is a risk of having a different attenuation between one channel and another.
I repeat it again, I asked a question about the opamp and nothing else.
 

AnalogKid

Joined Aug 1, 2013
9,287
Here is a rework of the schematic in post #1. The opamp is acting as a voltage follower, not a comparator, so it can handle all three required output states. Also, the two output transistors are emitter followers, not saturated switches, so the relay coil will see the slow turn-off edge of the R1-R2-C1 time constant. The circuit should produce an output pulse of approx. 0.5 s.

Note: The LM358 output stage is not symmetrical. When it is saturated low, it is within 0.5 V of the negative rail. But when it is saturated high, it is around 2 V away from the positive rail. This means that the positive and negative pulses going to the relay will not be the same amplitude.

ak

UPDATE - Schematic deleted. See post #54.
 
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Thread Starter

arivel

Joined Mar 4, 2018
42
if I'm not mistaken, by eye, when this opamp doesn't work its output is always around 6 volts.
what are the reasons for choosing LM358D?
 

AnalogKid

Joined Aug 1, 2013
9,287
Too many schematics, too late at night. Let's try this again.

This is a re-work of the post #20 schematic. I like it better than the post #1 schematic because the logic part of the circuit is not using Vee as its GND potential. The #1 schematic shows a CMOS counter, so the output signal voltage range should be enough to drive the 358 into both positive and negative saturation.

The schematic comments from post #51 still apply. R2-R3 give the opamp circuit a gain of two. This keeps the output saturated long enough for the relay to respond. The effective pulse width at the relay coil, which is a bit longer than the opamp saturation period, should be approx. R1 x C1. Note that neither this circuit nor the relay will respond to rapidly-changing signals.

The LM358 is a good-enough part for this application, very robust, and very cheap. The LM324 is a quad version of the same part. Since you are using six of these circuits per audio channel, it seemed like a good place to start. Depending on the relay coil current requirements, you might be able to go with a more rail-to-rail type part, for more equal positive and negative pulse amplitudes.

So, what did I miss this time - ?

ak

Relay-Bidir-Pulse-1-c.gif
 
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AnalogKid

Joined Aug 1, 2013
9,287
https://media.digikey.com/pdf/Data Sheets/Panasonic Electric Works PDFs/TX_Series_Rev2010.pdf

This part has a coil current of only 11 mA at 9 Vdc. This is low enough to be driven by the opamp directly, eliminating Q1 and Q2.

Also, it is DPDT. The second set of contacts can be used to load the counter with the previous value on power up. <post #40> It also has low-current, bifurcated, gold contacts - perfect for audio switching. The maximum operate time is only 4 ms, so C1 could be reduced for better response to fast-changing inputs.

Not cheap, but it does everything you want.

ak
 
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AnalogKid

Joined Aug 1, 2013
9,287
Here is an updated schematic based on the Panasonic relay. If the power supplies are set to +/-10 V, I think this is a good solution for a 9 V relay coil. Note - the peak input signal voltage cannot exceed Vcc.

The two power supply decoupling capacitors (C2, C3) are relatively large because the opamp is driving the relay coil directly. Again, C1 can be reduced for a faster response time.

ak
Relay-Bidir-Pulse-2-c.gif
 

AnalogKid

Joined Aug 1, 2013
9,287
Good point. For some combination of 12 relays for two channels, the static power would be enough to matter in a battery-operated system, not so much for AC power. With a high-sensitivity type, you could drive the relay directly from the counter output; more expensive.

And you lose the ability to retain the volume setting when power is off.

Interesting - the relay coil current for the non-latching version is higher than the latching version.

ak
 

MisterBill2

Joined Jan 23, 2018
9,010
A motorized dual volume control like was used in some stereo receivers for remote control would be smaller and simpler to control and allow a continuously variable setting, with the added advantage of also working for local control with a knob. That motor/geartrain was just a small bit larger than the control and added about an inch and a half to the length. And that receiver was unique in havil both a digital frequency display AND a slide-rule readouot. And the tuning was also remoted. Older technology, I am sure.
 
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