Want to make a standalone VU meter module, could do with advice

[OwenM]

With only 2-wire 15VAC, you won't be able to create +/- supplies with a bridge. You can only produce half-wave rectifier based bipolar supply, as one lead will have to be ground. With the full-wave bridge, you have no ground.

Ah ok, thanks, learning point noted, I had assumed (hoped) the 2 wire's would correspond to either side of a normal transformer secondary, but it makes sense that it's one line of 15VAC and a ground, thinking about it. I think I'll find a way to use a +V and ground op amp like the LM324. I assume I'll only be able to get 7v ish from my 15v ac supply when half-wave rectified...

[Dave H]

I'd use the last circuit from the the link in post 10. It uses a single dual op-amp (TL072) and it doesn't need a +/- supply. It will run off a single 9V battery so you could use a regulated DC wall wart.

[Wal_zz]

Owen,

The smoothing capacitor will charge to the peak value of the 15v AC, which is around 21v without any load. With a load applied the DC voltage will drop to a value dependent on the current drawn by the Op Amp. A better idea would be to use a full wave bridge and use the spare op amp in the TL072 as a rail splitter which generates +/- voltages from the single ended supply. Unfortunately, as I am new here I don't yet have privileges to post images or other files to clarify what I'm saying.

The circuit shown in figure 8 of the article is almost exactly what you need. Instead of using a 9v battery supply feed the circuit from 15v AC > Full wave bridge > 1000uF Smoothing capacitor. The net result is you will have a +/- 10v supply for the Op Amp. I hope this is clear for you, if not feel free to ask more questions... and have fun!

Walter

PS: Here is some bedtime reading for you about rail splitting and virtual grounds.

[OwenM]

Owen,

The smoothing capacitor will charge to the peak value of the 15v AC, which is around 21v without any load. With a load applied the DC voltage will drop to a value dependent on the current drawn by the Op Amp. A better idea would be to use a full wave bridge and use the spare op amp in the TL072 as a rail splitter which generates +/- voltages from the single ended supply. Unfortunately, as I am new here I don't yet have privileges to post images or other files to clarify what I'm saying.

The circuit shown in figure 8 of the article is almost exactly what you need. Instead of using a 9v battery supply feed the circuit from 15v AC > Full wave bridge > 1000uF Smoothing capacitor. The net result is you will have a +/- 10v supply for the Op Amp. I hope this is clear for you, if not feel free to ask more questions... and have fun!

Walter

PS: Here is some bedtime reading for you about rail splitting and virtual grounds.

Thanks Walter and Dave! Great info all round.. Ok, I think I'm getting my head around it. The final circuit (fig 8) of the article I posted seems to be the way to go. I think throughout today, pondering it, I've been down several dead ends working it out and now I think I'm there but I'm a bit confused on the terminology!

So as I only have a 15VAC supply, NOT a transformer with a 15v secondary, I can't use the standard bridge rectifier found on amps to get my + and - . But I don't need it for my proposed circuit (fig 8), as the second half of the op amp is set in such a way that, I can see, is doing some sort of half-way voltage dividing to the source voltage to make a virtual ground of some sort. I'm not sure I get exactly how, yet! But I guess from it's feedback to the signal input and its perhaps floating the circuit midway and causing the op amps to behave as though they are halfway in between the single positive voltage and ground, giving a + - situation as far as the op amps see it... I may be way off, but doesn't really matter for now as I'll learn more of that as I build it and it might be a bit beyond my comprehension right now. I'll read the rail splitting and virtual grounds link you posted Walter! Either way, the main point is the circuit can be fed with +9v ish and Ground.

So I might just get a 9v DC, or dig one out of the shoebox of spares and build this circuit more or less as is. I'll probably remove the meter voltage-range selector switching and, I imagine, I will need to omit the meter rectifiers, as the meters I am probably buying have one in. (would the meters have the cap C1 in them too, if not what's the way in which it is usually accommodated?)

I think I understand the principle of using my 15vAC supply and rectifying it, but I'm confused at the term full range bridge when I only have the single ended, one half of what you would normally have with a 15v transformer secondary, supply. You mention that I could use a full wave bridge, which to my understanding needs either end of the transformer supply and always creates a positive and negative voltage, but then say that it will be single ended however, and an op amp could be used to create the + and - supplies (which I gather is exactly what's happening in the proposed circuit). So I think I'm missing something here, sorry! But that's all good, I don't have a variac or bench DC power supply yet so I'm going to take the opportunity to build a few bridge rectifiers at a nice safe 15v and get to know them a bit better.

thanks for all your time everyone! I'm really excited to start getting my head around this!

[Wal_zz]

So as I only have a 15VAC supply, NOT a transformer with a 15v secondary, I can't use the standard bridge rectifier found on amps to get my + and - . But I don't need it for my proposed circuit (fig 8), as the second half of the op amp is set in such a way that, I can see, is doing some sort of half-way voltage dividing to the source voltage to make a virtual ground of some sort. I'm not sure I get exactly how, yet! But I guess from it's feedback to the signal input and its perhaps floating the circuit midway and causing the op amps to behave as though they are halfway in between the single positive voltage and ground, giving a + - situation as far as the op amps see it... I may be way off, but doesn't really matter for now as I'll learn more of that as I build it and it might be a bit beyond my comprehension right now. I'll read the rail splitting and virtual grounds link you posted Walter!

So I might just get a 9v DC, or dig one out of the shoebox of spares and build this circuit more or less as is. I'll probably remove the meter voltage-range selector switching and, I imagine, I will need to omit the meter rectifiers, as the meters I am probably buying have one in. (would the meters have the cap C1 in them too, if not what's the way in which it is usually accommodated?)

I think I understand the principle of using my 15vAC supply and rectifying it, but I'm confused at the term full range bridge when I only have the single ended, one half of what you would normally have with a 15v transformer secondary, supply. You mention that I could use a full wave bridge, which to my understanding needs either end of the transformer supply and always creates a positive and negative voltage, but then say that it will be single ended however, and an op amp could be used to create the + and - supplies. So I think I'm missing something here, sorry! But that's all good, I don't have a variac or bench DC power supply yet so I'm going to take the opportunity to build a few bridge rectifiers at a nice safe 15v and get to know them a bit better.

Clarifying a few points in the order you gave..

- The wallwart supply is simply a 230v to 15v transformer in a box that provides 15v AC out with no other internal components.
- You can use any standard bridge rectifier or you can build your own using discrete parts.. your choice.
- The second half of the TL072 is generating a virtual ground, effectively dividing the incoming DC of approximately 20v into a +/- 10v supply for the Op Amp.
- Running your project from a 9v battery to start with is fine and you can switch to Mains power down the track.
- You can remove redundant parts that won't be required for your application. R3, R4, S1, D1, D2.
- Leave the meter rectifier (D3-D6) in place. That way you can use a meter with or without an internal rectifier. Capacitor C1 controls meter ballistics (the speed at which the needle moves) You may find that 100uF is slightly too large and the needle is sluggish on music signals when you finally do a real world test.
- Don't forget to use a smoothing capacitor after the bridge when building an experimental supply. (A 1000uF 25v would be suitable) This won't be required in the final circuit as C2 and C3 will perform the smoothing function.
- That tutorial on rail splitting is obviously beyond you at the moment so revisit it later after you get your head around the foregoing stuff.

[OwenM]

Clarifying a few points in the order you gave..

Awesome thanks, I think, if my assumption that the wall-wart is NOT a normal transformer is wrong, then I understand it all more or less (at least for my level) and everything is set for me to go forward once I get the meters themselves. Thanks so much for your time! I think I didn't understand post #15 and started thinking of my AC supply as something more complex than a transformer.

All other points I'm clear on, cheers, and I'll nibble away at that rail splitting tutorial once I'm really clear on the stuff on my plate at present!

[Wal_zz]

Good to hear!

History of the VU meter

[OwenM]

I knocked up a schematic in LTspice, as I figured I may as well learn a few more skills on the way! Does this look kind of right? I had a question...

R7 and R14 (replacing R2 R3 and R4 in the original) were in the original circuit to achieve a varying input sensitivity from 500mV to 50V, my signal is probably going to be 1v RMS at most. Am I right that as the R2, R3, R4 resistance increases in the original then that shifts circuit sensitivity to a higher voltage range? If so should I have a slightly higher resistance for my R7 and R14 (left and right), to achieve a shift from 500mV max to about 1v max, or will that be easily adjustable via my R5 and R12 trim pots? I'm still grappling with how the rail splitting OpAmps U1-1 and U2-1 (my circuit) are operating and not sure if that's something to worry about!

The 7500 resistance on the meters is just from what the article I posted quoted as a nominal impedance at 0Vu, I don't expect it's particularly accurate for every meter and it's just a rough figure!

I only have a 2200uF cap available for C1, I simulated on a rectifier app and seemed to be ok, but am I causing a problem using a higher than rated filter cap?

All the best in these odd times!

[OwenM]

Thanks guys, I made a....nice looking level meter that may or may not be relatively close to some kind of VU standard! It works great, I realised the error in the schematic I posted above, removed those diodes but that should have included their wire too. I spent an hour trying to see why that would work until I got rid of it and it all made sense, ha!


VU Meter Schematic opamps ps sorted.pdf

I know I didn't need the smoothing cap(s) on the power supply, but made it an exercise in making a simple power supply as well as the buffer/meter circuit.. Only had 470uf caps up to voltage hence two! Tested it plenty and learned a load, about opamps in particular, which has been a sticking point for me...

cheers for the help!

[Wal_zz]

Pleased to see the finished project Owen, you've done an excellent job!
Now that many weeks have passed I'm curious if it is now in use, what the reactions of the boys were, and how it is helping keep audio levels under control.

[OwenM]

Pleased to see the finished project Owen, you've done an excellent job!
Now that many weeks have passed I'm curious if it is now in use, what the reactions of the boys were, and how it is helping keep audio levels under control.

Thanks, and thanks very much for your help! You were very patient with me and thanks to you and others I feel I fully understand the circuit, at least, on the 'advancing beginner' level! I had a lot of fun making a nice box for it and practicing my quasi French polishing on it too



Unfortunately we didn't get to test it in anger as our end-of-term was focused on frantically getting international kids home (it's a boarding school) and since then we haven't been able to have anyone back. But I talked about the project with the kids most involved with doing the parties and they are keen to make a similar thing with me when we get back to school, so I'll keep this one for my studio setup and they can learn about putting together the same thing and learn a bit about the principles involved. I'm currently teaching a beginner electronics theory course with them online (they are very much at the begging, we are just looking at ohms law and common components, I have no ideas above my station of teaching beyond this stage!) and this will serve as a nice little project when they get back to school, along with the little veroboard guitar pedal layouts I've been playing with. I'll probably try and do the version with the 9v battery supply, or at least build the board with them and then finish it and add the power myself, but they will get a lot from watching it being built I think and may get them more into the subject, hopefully!

Don't worry, I'm a music teacher and this is just a side lesson while we are all locked down and bored, I wouldn't let anyone touch anything with the potential to even give a mild shock. They think just seeing an op amp chip is the coolest thing and telling them how you can use it to make something happen has fired their enthusiasm!

If nothing else it's been great just using it as a guide for my home setup, even my partner has gotten used to looking over to the meter and then turning up the TV, rather than just cranking the amp straight away (giving me a shock when I go to play music from my computer later), and she's the most technophobe person I know of