VU meter

Can I suggest that if you want to do something like that there are much much better and easier ways to do it ?

Graham

The text explains it... its a log amp... piecewise linear approx of log... dBs are 20*log(vout/vin). Use a $1 tiny avr

When that method originated there were no AVRs of course.

Graham

Points A & B connect to points A & B in the LED chain.

Is it switch though? what is so special about the 6th and 7th leds, if it is a switch, is A always one led under B or can they move up and down independently? Where are the five stages? we have 10 leds so does this mean every stage has two leds.

Build the damn thing and maybe you will SEE how it works.

Somehow, I doubt even seeing it will make you one bit wiser.

....... Phil

You're in SEB Phil, please try to be nice.

Is that your professional analysis of the circuit?

No, the A and B are just indications that the arrows marked A and B should be connected to the points A and B on the right --- this makes the schematic diagram a little less cluttered.

As for the "5-stage feedback": I think BobG has the right interpretation. A VU meter is supposed to have a logarithmic response, but all the amplifiers in this circuit have constant gain (linear response). As the diodes in the diode string light up, they switch some extra resistors into the third opamp's input, which changes the circuit's overall gain, and presumably this ends up approximating a logarithmic response. I don't see why it's described as "5-stage" though. It looks like it only has 3 or 4 different gain settings.

These days, you can buy VU-meter chips with a logarithmic amplifier and a bunch of diode drivers. Or you can do it in firmware in a cheap microcontroller.

I am not consered with the logaritmic scale. I wanted to ask about the leds and th driver transistors, how this works, and if A and B signal switch, as otherwise, I see no reason why someone would start using A B C etc to denote nets.

Ok, hope that's clear Thanks

Believe me, it's far better to do that than have traces going from one side of the schematic to the other merely to connect those points (as opposed to a line that is being used throughout the circuit, such as the power supply line). You either have to have those long connections riding over all kinds of things, or have them go far out of the way just to keep it clear of the rest of the schematic.

It's really clear, once you know this is happening (and I admit that in this case, it wasn't so clear because of the way the schematic wsa drawn). The way I saw it when I was a kid was to have a triangle on it's side (so the point faces left or right) and then the other point has a similar triangle pointing towards the first one. It was then pretty obvious that they were two points to connect together, because they were special symbols for the purpose.

You can even do it so there is one source, and multiple "receivers", which can be useful for showing where the power supply line goes, again so you don't have long traces all over the schematic. In essence, that's just a variant on the ground symbol. You know those all connect together.

The thing about schematics is that they should be drawn so you can easily interpret them by looking. THis is not the same thing as circuit layout. If you drew them the way you'd wire a circuit, it would usually be a real mess to follow, yet the schematic properly drawn spreads the circuit out so two points that might on a circuit board be next to each other are at opposite ends of the schematic.

MIchael

I find Phil's restraint admirable.

Graham

All BEWARE

Phil Allison and Eeyore are well known intelectual property theives actively targetting this newsgroup.

The way they operate is this:

first they would give you some sort of irrelevant answer and say that you should do it their way (but not revealing their way)

The next step they will start becoming dismissive of you and try to lead you to believe that you are an idiot and that they are the oracle of knowledge

After that, they will slide into insults and mud throwing

Just what the board needs........ Another idiot troll.

When they do slip up and post something with apparent technical content, they are often wrong, thereby displaying their profound ignorance.

It is more likely that the basic/beginner poster does not understand the answers they are given, and will not ask for clarification. I noted several recent posters who received valid replies, but became belligerent over them.

I don't think you can trust the descriptive text. He begins with the statement that the audio amp runs on 0v-18v power rails but the schematic clearly shows +18v and -18v from the amp.

You should probably look for a better circuit such as one based on this chip: