SparkFun Forums

[SpikedCola]

My parents mentioned that they would like something like this, so I figured while Ive still got the chance to use my schools lab before exams are done (and we go on a 1.5 month break) I would get it built.

Basically, the volume difference between commercials and tv programming is very large, enough that they have to turn the volume down during commercials and back up during regular programming.

I did some googling and found this site:
http://www.redcircuits.com/Page67.htm

I modified it a bit to remove the "control-flat" switch, as I want it to always be in auto-volume-control mode. Other than that, Ive kept the circuit the same. I plan on hooking it up inside the TV, splicing into the wires going to the speakers. The only other change I made to the circuit was putting a connector footprint in place of the gain pots, so I could re-locate the pots (hence the pots missing on the schematic).

Attached is the schematic (WIP - Power supply section is being worked on right now); once the schematic is good I will create the PCB layout and update this post.

I had one question about the project, though - do I NEED to use polyester caps? Remember this is going to be in a 6+ year old TV, and used for antenna broadcasts. Im sure they will be able to live with less-than-perfect sound quality.

Please look over the schematic (click for a bigger version) and tell me if I need to make any changes (or if Ive broken any basic schematic-drawing rules). My last day at school is Thursday, so I must have this done by then. Thanks!



EDIT PSU section added. JP5 goes to a 15-0-15 transformer

[riden]

I don't think that circuit does what you think it does. It sounds like you want a circuit that will keep the volume on the TV from increasing during certain commercials, an automatic level control (ALC) or automatic gain control (AGC). The circuit you describe is a loudness control, which adjusts that frequency response based on the output level. On my stereo, the Loudness switch boosts low level signals when the volume is below a preset level. Try a Google on "automatic level control" and see if you find anything that might help you.

[SpikedCola]

Bloody hell. There goes my good mood for the night.

Searching automatic gain control circuits gives me lots of definitions and papers on them but not much in the way of practical diagrams. Any ideas?

[riden]

What is needed is an amplifier with its gain controlled by a feedback circuit such that the gain is reduced after a certain level is reached. There has to be some adjustments to attack and delay so when the limiting cuts in it isn't noticeable by the listener. Maxim/Dallas had a chip, the MAX9756, that might have worked. It seems to have been discontinued. However, Analog Devices has the Analog SSM2166 (available via sampling and Digikey) that looks promising. The packaging is 14 pin SOIC, so you might want a couple of Sparkfun breakout boards to facilitate construction on a breadboard.

[SpikedCola]

The only problem with ordering a chip is it wont be here in the time frame I would like it to be completed in. I have access to almost every common chip and part, either here or at school, so even if the circuit is a bit bigger than a single-chip solution, Id prefer that over ordering another chip.

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

[riden]

Sorry, missed this post earlier. Here is my take on your questions:

1) Take your existing audio output, terminate it by connecting it to an appropriate load, and use a voltage divider made out of resistors to attenuate the output to an acceptable level. Then you will have to add an audio amplifier on the output of your device and connect it to the original speakers. If the TV isn't one of those mega audio booming ones, you could get by with a pair of LM386s or equivalent.

2) Your TL07x parts should work just fine. They may even work better because of the lower noise specs (although I haven't researched this).

3) LDR range is totally dependent on the characteristics of the given LDR. For instance, I have a drawer of CdS photo-resistors and each one has slightly different characteristics. At least the author told you what his/her LDR produced and you can adjust accordingly. An LED and Photocell encapsulated with shrink wrap could work. You will have to experiment (try different color LEDs, distance between LED and photoresistor, etc.) until you are able to get an output range similar to the one that the author used.