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Automatic Television Volume Limiter

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My original thread is here.

I found this circuit on the interwebs:


According to the description:
There's one by Terk (Audiovox) for about $25 on line and $40 in stores. It's gotten mixed reviews. Besides, I need three, so I built this one and it works fine. Now we have constant volume on all the channels.

How does it work? It's inserted between the TV and the DTV or HD box or DVD box using standard RCA audio cables. It takes the signal from the box and amplifies it to a point where it doesn't amplify any further, resulting in constant volume.

It uses a light dependent resistor (LDR) that shunts the input to ground as the input volume increases. The circuit is based on one in EG&G Vactec's application notes. I couldn't get their circuit working right, so I made some changes and added an input amp and output buffer to get the levels right.
This seems like exactly what I want. However, I have a couple questions:
- how would I modify the circuit to accept speaker-level inputs and outputs, vs line-level (RCA) in/out?
- I only have TL072 and TL074 chips - could I use one of them in place of the TL082?
- I wont be able to get the LDR/LED chip in the time space Id like to have it done by, so can I use a LED and LDR coupled together with some heat shrink?
- If so, what colour LED should I use?

According to the datasheet of the LDR/LED chip they used, the LED has a forward voltage of 2.0v so this makes me think its a red LED. Also, the LDR has an on resistance of 40 ohms @ 20mA and 140 ohms @ 1mA, and an off resistance of 1-5 Mohms. Is this typical for an LDR?

To make it easier on whoever helps me, here are the datasheets I am referencing:
TL07x
TL082
LDR/LED Chip
Original schematic page
 

RCinFLA

Well-Known Member
Hope you are not talking about driving a speaker. You will need a power amplifer to do that.

As to tapping from amp driving speakers, put 15 ohm resistor across output and find which lead is ground to connect to your ground.

Many TV's have a stereo headphone jack that will give you what you want. I use it for mid-size LCD TV's with poor internal speaker audio acoustics. Buy a $25 computer speaker set with bass speaker and left/right channel speakers. Sound is good, low cost, and simple to setup.
 

vsmGuy

New Member
I had a quick look and I am shocked to see that they have dsigned an AGC using LDR + LED setup!

Do they use that anymore after optoisolators were invented?

The sensetive LDR itself will cost 100 times more than any opto..

Maybe this is sooo old that they figured - oh wel.. let's give away this deign for free.
 
By speaker I mean something along the lines of a few watts; the little tiny ones that come in crappy 13" tv's. Nothing too major. I was thinking of something like this to drive it:


However, back to my original question - can I drop a TL072/74 in place of the 82?
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
I had a quick look and I am shocked to see that they have dsigned an AGC using LDR + LED setup!

Do they use that anymore after optoisolators were invented?
Yes, because an LED/LDR combination works, an opto-isolator doesn't.

There have been a very few LDR/LED components actually commercially built, but they were very hard to locate, and very expensive.

The whole point of it is to give a variable resistance - which an LDR does, and an optoisolator doesn't.
 

marcbarker

New Member
LDR makes an excellent volume limiter.

If it was me I wouldn't bother with all that circuitry posted though, I'd wire a light bulb across the TV speaker (via a variable rheostat) and place the LDR across the appropriate place inside the TV audio section.

I don't know if it's still the case today, but nearly every cordless telephone had an LDR used as a ring indicator, based on an opto-coupler that uses an LDR/neon combination.
 

jbeng

Member
Nigel Goodwin said:
There have been a very few LDR/LED components actually commercially built, but they were very hard to locate, and very expensive.
Do a google search for "Vactrol" from Perkin-Elmer or look here. Like Nigel said, they're not cheap.

Jeff
 

vsmGuy

New Member
Yes, because an LED/LDR combination works, an opto-isolator doesn't.

There have been a very few LDR/LED components actually commercially built, but they were very hard to locate, and very expensive.

The whole point of it is to give a variable resistance - which an LDR does, and an optoisolator doesn't.
Nigel,

I am cure you would agree tath we are interested in modulating the current for the audio signal than changing some resistances.

I agree that not all optos have that good response to audio, but if you leave out those solely meant for digital islation, you can use the opto to modulate the signal current. Instead of the LDR you bias the LED and PhotoTransistor such that it puts the PhotoTransistor in the active region and thus changes inbrightness of tyhe LED leads to change in base signal current.

I hope I made sense. I am sure someone knows about this method?
 

vsmGuy

New Member
However, back to my original question - can I drop a TL072/74 in place of the 82?
No worries mate.

The TL07x is a MUCH better opamp than TL08x.

I don't know why they named it that way though .. higher should be better is what the industry teahces us eh?

Any ideas guys?
 

crutschow

Well-Known Member
Most Helpful Member
Nigel,

I am cure you would agree tath we are interested in modulating the current for the audio signal than changing some resistances.

I agree that not all optos have that good response to audio, but if you leave out those solely meant for digital islation, you can use the opto to modulate the signal current. Instead of the LDR you bias the LED and PhotoTransistor such that it puts the PhotoTransistor in the active region and thus changes inbrightness of tyhe LED leads to change in base signal current.

I hope I made sense. I am sure someone knows about this method?
A photo transistor is a very non-linear, unipolar, current-device, and does not act like a resistor. It doesn't just have poor response for this application, it has an unusable response.

An LDR acts like a normal bipolar linear resistor (but variable). That's why an LDR is used for attenuating audio and an opto coupler isn't.

It is possible to use a MOSFET transistor in such an application for low signals, since a MOSFET can work as a bipolar resistor (although still somewhat non-linear) at very small signal levels.
 

marcbarker

New Member
it puts the PhotoTransistor in the active region
Majority of cassette tape recorders over the last 30 years have used ALC circuits, often using a reverse-connected BJT as a shunt in the signal path.
A JugFet is better.
 
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Alright, glad to hear I can drop a TL072 in place of the 82. Now, what would I use for an LDR and LED? Will a red LED and a standard radioshack CdS-type LDR work? Im not sure of the value of the ones I have.
 

kchriste

New Member
Forum Supporter
Sometimes a green LED is better because some LDRs usually are more sensitive to green light. See this document for more info. Page 24 has info on matching LEDs to various types of LDRs. Also, seal it up so no ambient light hits the LDR.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Nigel,

I am cure you would agree tath we are interested in modulating the current for the audio signal than changing some resistances.
No, because we aren't - it's variable resistance that's required.

I agree that not all optos have that good response to audio, but if you leave out those solely meant for digital islation, you can use the opto to modulate the signal current. Instead of the LDR you bias the LED and PhotoTransistor such that it puts the PhotoTransistor in the active region and thus changes inbrightness of tyhe LED leads to change in base signal current.

I hope I made sense. I am sure someone knows about this method?
No, an LDR is a variable resistance, a photo-transistor isn't, and won't work in the same way - it's not a question of the response to audio, some opto's are more than acceptable for audio use, but this isn't audio use anyway.
 

vsmGuy

New Member
Thanks Nigel for the insight!

I was arrogant because it made kinda "sense" to me that optos be used and I remember seeing a few designs that used exactly that but maybe those wer just prototypes that never made it to production.

I learn something new and prune out some wrong information from my mind every day :)
 
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