LED meter

I've spent the last few days looking into different designs of LED VU (or Peak) meters.

Most of them are based on the 3915/16 with a full-wave rectifier before them. Some just on 339s and many others on uCs.

What I can't find though is something that would fit my idea of having a meter with 4-5 LEDs. Ideally, I'm looking for:

1) 4 - 5 (or even more but not necessarily 10) LEDs that would cover a range of -20 dB to +28dB or thereabouts
2) Full wave rectified
3) Single supply voltage of around 24V
4) NO uC 

There are tons of designs out there that cover one or more of the above requirements but unfortunately I can't find anything that would work in my case.

Any ideas/ suggestions are much appreciated! 

warpie said:

Most of them are based on the 3915/16 with a full-wave rectifier before them. Some just on 339s and many others on uCs.

What I can't find though is something that would fit my idea of having a meter with 4-5 LEDs. Ideally, I'm looking for:

1) 4 - 5 (or even more but not necessarily 10) LEDs that would cover a range of -20 dB to +28dB or thereabouts
2) Full wave rectified
3) Single supply voltage of around 24V
4) NO uC 

There are tons of designs out there that cover one or more of the above requirements but unfortunately I can't find anything that would work in my case.

Click to expand...

Why wouldn't an active rectifier followed by an integrator feeding a bank of comparators work for you?

Hi Andy,

Can I do this with a 20-24V single rail supply?  I haven't found any example no matter how much I searched for.

warpie said:

Hi Andy,

Can I do this with a 20-24V single rail supply?  I haven't found any example no matter how much I searched for.

Click to expand...

LM139 has a 36 V supply capability like all of your favorite op-amps and such.

You need a Sanyo  led driver chip.  One of the following:
LB1403N
LN1414N
LB1423N
LN1433N
LB1443N

Andy Peters said:

Why wouldn't an active rectifier followed by an integrator feeding a bank of comparators work for you?

Click to expand...

This should work, for 4-5 LEDs its probably the best option. BTW you dont mention but do you need RMS display? if so you can use something like the THAT2252

warpie said:

Hi Andy,

Can I do this with a 20-24V single rail supply?  I haven't found any example no matter how much I searched for.

Click to expand...

This strikes me as an ideal opportunity to design your own meter...

There are surely tens of similar designs floating around the WWW and even on this forum.

cut and patch together a piece from here and a piece from there..

full wave rectifiers are well explored, simple comparator meters are likewise around,

Just assemble your own recipe from common condiments (that is all circuit design is... assembling bits and pieces together).

JR

PS: You might learn something in the process. We already know how to design a meter.

It would be really appreciated if you could guide me through the process of designing what I need.

For starters, would this work as a single rail (+24V) full wave rectification? Simulation seems to work but obviously I'm open to any improvements before moving on to the next stage (lm399).

One thing I noticed is that a coupling capacitor in the input will result in a non rectified (but "trimmed" at the bottom) waveform at the output.

KA2284 LED Level Indicator:

https://www.banggood.com/10Pcs-KA2284-LED-Level-Indicator-Module-Audio-Level-Indicator-Kit-Electronic-Production-Kit-p-1231002.html

Jakob E.

Thanks Jakob but I kind of wanna learn how to do it  ;D

Also, I believe that the KA2284 doesn't cover any of the requirements.  Saying that,  it looks interesting.

warpie said:

It would be really appreciated if you could guide me through the process of designing what I need.

Click to expand...

That's the spirit...

I may be repeating myself but there are multiple workable solutions for each circuit block that you need to mix and match.

1- perhaps a differential receiver (need for this depends on where /how meter is used). If built inside another product you can probably just grab a signal... a differential stage (op amp and 4 resistors) will still insure good signal integrity perhaps useful for low level signals.

2- power supply- see 1, if building inside something you can grab power from that.

3- rectifier... Many meters work from half wave but full wave is not (very) heavy lifting. As I've shared before the rectification gets harder for low level signals.  The rectifier in my TS-1 only delivered a 20kHz -3dB bandwidth above -50dBu and that was with additional circuitry. Most simple meters are not that demanding.

4- smoothing- meters often use a peak or average dynamic characteristic with specific attack and release rates. Again many meters are not that critical.

5- actual meter... there are canned solutions like LM391x remarkably still sold. The simple voltage comparators (like lm339) are very popular and easy to implement.

For starters, would this work as a single rail (+24V) full wave rectification? Simulation seems to work but obviously I'm open to any improvements before moving on to the next stage (lm399).

One thing I noticed is that a coupling capacitor in the input will result in a non rectified (but "trimmed" at the bottom) waveform at the output.

Click to expand...

In general to perform AC functions (like rectification) from a single rail voltage establishes a usable v/2 (12V). +/- 12v will handle most audio signals but perhaps incorporate -6dB in the input stage to prevent clipping.  Back in the day I recall some grounded FW rectifiers using trick op amps that would sink down to 0V input and tolerate input voltages below supply but I wouldn't trust LM358 to do that.

Step 1 research multiple typical circuits and after you understand them, figure which is good for you.

JR

warpie said:

It would be really appreciated if you could guide me through the process of designing what I need.

For starters, would this work as a single rail (+24V) full wave rectification?

Click to expand...

Should work on any supply voltage. All you need is a pad at the input to drop the +28dBu level to within the scope of the power rail.

Cheers

Ian

> would this work as a single rail (+24V) full wave rectification?

Does the '358 do the right thing when inputs are pulled below the negative rail? Does its output go to dead-zero (or close enuff)? (I would not trust the simulator on either point.)

> ...single rail ...full wave rectification?

Let Me Google That For You.

Last time I looked, there were many dozen schemes, many of them useful. Most have shortcomings, and some authors don't fully cover them.

Some of the "cleverest" rely on an opamp which stays sane when an input is pulled below the negative rail. Invert the signal. Also take the straight signal. Mix the two together. Done right, the result is near perfect. Use the wrong chip, it's crap. The input and output impedances are high and varying-- if you are promised "one opamp", you are sure to need more to buffer in/out for most practical cases (not a sig-gen and a 'scope).

The first hit today expects a -0.23V rail! Cleverly (for TI), they know a chip (they sell it). (Actually it makes the cap-makers rich: _8_ caps just to buzz-down to -0.23V.)

A 2005 EDN Design Idea has no diodes! It uses specific chips that do a right thing when driven past the rail.

You can always cheat. Run a low-power speaker chip into a step-up transformer, and diode that. You have NOT specified your dynamic range. But 60V peak signals and a little ADC trim gets 30+dB with non-precision rectifier.

The cleverest I know is Neve's 192 PPM. It gains-up. The FW rectifier is too-too-too clever, and hits a peak catcher. It also semi-LOGs the output to cover the PPM range to BBC specs. Single 24V supply(!).

Is full-wave really needed? How much of your material is significantly asymmetric? In tracking, male speech and a few instruments are. In mixed program, asymmetry is unlikely. How exact does it have to be? Meet FCC modulation limits? Or just tell you the signal is "hot"?

BTW: the last time I googled this, I had many good ideas, and then found a better plan from China on eBay. I wanted line-voltage. Not really precision but WTH. Transformer for power, transformer to bring line voltage down to chip-size, chips, resistors, caps, needle-meter... then I saw these modules which LCD display both 0-250V and 0-100 Amps, with backlight. For less than a DIY approach. (I don't presently have a pile of parts.) Some wiring and a box....

If the lowest indication (LED) is -30dB or -20dB, do I really should worry whether the rectifier is very accurate when its output goes near 0V?

Some of the "cleverest" rely on an opamp which stays sane when an input is pulled below the negative rail.

Click to expand...

Like?

I really like reading your posts but I have to admit that sometimes I'm just getting more confused.

FWIW, the reason I want a single rail is because I want to keep the LEDs separate from the  audio rails. But, maybe there is a way to use the audio rails for everything and avoiding injecting any noise into the audio signal...

warpie said:

If the lowest indication (LED) is -30dB or -20dB, do I really should worry whether the rectifier is very accurate when its output goes near 0V?
Like?

I really like reading your posts but I have to admit that sometimes I'm just getting more confused.

Click to expand...

Sorry, that's a classic WWW TMI problem....  I shared about the obscure "trick" rectifier that depends on a unique op amp to reinforce the need to operate a conventional single rail rectifier from a virtual ground at V/2.

FWIW, the reason I want a single rail is because I want to keep the LEDs separate from the  audio rails. But, maybe there is a way to use the audio rails for everything and avoiding injecting any noise into the audio signal...

Click to expand...

In consoles is is not unusual to string LEDs between + and - rails so no current gets dumped into grounds, while this is manageable too.

JR

> If the lowest indication (LED) is -30dB or -20dB, do I really should worry whether the rectifier is very accurate when its output goes near 0V?

Define the problem. Do math.

If "-30dB" is 30dB below 0.775 or 1.0 Volts, it is near 0.030 Volts. A 0.6V diode drop looms large.

You mentioned +28dB. Now 30dB down is -2dB or around 0.5V. A 0.6V diode is still a large drop, but in-sight of the signal level.

It will be incredibly hard to beat the Neve 192 plan for cost and parts-count. Load the output with 1K, you get 0-1V for over 40dB of input range. Trim your '339 thresholds for the steps you like.

warpie said:

If the lowest indication (LED) is -30dB or -20dB, do I really should worry whether the rectifier is very accurate when its output goes near 0V?
Like?

I really like reading your posts but I have to admit that sometimes I'm just getting more confused.

FWIW, the reason I want a single rail is because I want to keep the LEDs separate from the  audio rails. But, maybe there is a way to use the audio rails for everything and avoiding injecting any noise into the audio signal...

Click to expand...

Look there, page 2
https://groupdiy.com/index.php?topic=49634.msg762968#msg762968
Almost textbook application, except R2 and R20.

For a 5 led meter this is what you want. KA2284/KA2285; it is made by several manufacturers, National among them. I had to search to find a data sheet in English here:    <www.electrow.com/vu%20meter>

The IC's can be found on ebay rather inexpensively.  <www.eleccircuit.com/easy-stereo-vu-meter-by>
If it does not come up on the meter circuit just look in the index.  You can always build a one off on a piece of perf board.

Best of luck
Bill Wilson

abbey road d enfer said:

Look there, page 2
https://groupdiy.com/index.php?topic=49634.msg762968#msg762968
Almost textbook application, except R2 and R20.

Click to expand...

Would you please check your link?
Thanks!
Best,
Bruno2000