Signal level indicator with a VFD display

I have a real soft spot for gadgets with vacuum fluorescent displays. When I saw something like this in a comment, I got excited about buying one for myself. The only difference was that the one I saw had some extra “bells and whistles,” like a clock and something else, but I just wanted an indicator—and I found one :)
A short review of a large indicator with a “warm” display.


Since this device is very simple, I’ll try not to drag the review out. But honestly, you can just skip to the end and watch the video demo—it’s worth a thousand words.

For anyone interested in general information about both the seller and the product, here’s a review in the usual format, too.

Two things surprised me while placing the order:
1. The seller doesn’t charge the standard Taobao fee for shipping within China, so the price is $16.88 plus the agent’s fee.
2. This kit weighs 309 g, which is quite a lot.

The reason is simple: the packaging gets five stars. The indicator is wrapped in a decent layer of bubble wrap, and everything is packed in a large box without rattling around inside.
There’s also a little extra room in the box. I think that’s for orders that include the case. I ordered just the display, power supply, and cable, and now I really regret it—I’ll have to order the case another time.


So, the kit includes:
1. Level indicator
2. Power supply
3. Cable for connecting to the signal source.


The most annoying thing is that I don’t need the cable or power supply at all, but I had to pay for them anyway, which is frustrating. The seller doesn’t have a “display only” listing.


I didn’t even unpack the cable. It’s a standard 3.5 mm jack at both ends. I’ve got plenty of those at home from monitors, even without the seller’s help. But I’ll say a few words about the power supply—and they won’t be kind...

The power supply is rated at 12 V, 1 A, but it arrived “half-disassembled”: the clips on one side of the case had come undone, so I took a look inside. If the clips hadn’t been open, I would have thrown it away without opening it.


Inside, it’s all pain and tears. This power supply is rated at 12 W, but it’s complete junk.


And here’s the indicator itself.
The seller offers three options:
1. Standard size, green glow
2. Standard size, blue glow, Samsung display.
3. Larger size, green glow.


I chose option two. I didn’t want the green one, but I tried to find the dimensions of the larger indicator and couldn’t.


Another reason I chose the blue indicator was that the gaps between its segments looked smaller to me.


The circuit board measures 180 (181) × 47 mm and is 19–20 mm thick. The illuminated area itself measures 123 × 21 mm.

I can say that the display really is large—not like those times when you look at a product page and it seems big, then it arrives and turns out to be tiny.


1. The left side of the board has the trimmer potentiometers, operating mode switch, and signal input.
2. The right side has only the power input.
3. Each channel has its own resistor. They’re set to the middle position by default, which is more than enough for proper operation with a standard line input. You can increase or decrease the sensitivity; the adjustment range is wide, so it should work with most signal sources.
4. The switch lets you independently select Mono/Stereo mode and turn the peak detector on or off.



All the components are mounted under the display, with a rubber spacer between them.
I didn’t remove the display, let alone bend it back, because the leads are very stiff and there are a lot of them. I think you should only do this as a last resort, since it’s very awkward and you could damage the display.
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The voltage converters and regulators are located near the power connector. Judging by the traces, one supplies the display’s filament (with DC); in the photo, it’s the large PWM controller on the right. The other generates the high voltage for the display (you can see its inductor on the left).
Yes, the filament is powered with DC, unfortunately.

The supply voltage is 12 V. Current draw is 316 mA with all segments off and 266 mA with the display fully lit.


An NE5532 op-amp is installed next to the signal input. The input signal passes through a capacitor to a 20 kΩ trimmer, then through another capacitor to the op-amp input. I didn’t look into what happens to it after that.
The Mono/Stereo switch sends a signal to the microcontroller and has nothing to do with the input signal.


The whole thing is controlled by a GD32F103CBT6 microcontroller made by GigaDevice Semiconductor Inc.
It’s a 32-bit, 108 MHz controller with 128K Flash and 20K SRAM, based on an ARM® Cortex®-M3 core - link.

There are also several 74HC595s and ULN2003s under the display.


The underside of the board is completely bare... Most of the board is clean, but some components are installed crooked, which in this case clearly points to hand assembly.


Unlike the IN-series displays, this type really is a “warm tube,” since it’s an electronic tube and contains all the necessary elements: a cathode, an anode, and a grid.


The housing consists of six glass panels, and the display is thin enough that you can even photograph its insides through the edge.


But for the amplifier I’m slowly putting together, I’ll probably use neon indicators after all. I’m planning to implement fully “dual mono” circuitry, and this model won’t work because it has a common ground at the input.


As I mentioned above, I ordered the display without a housing, but only later realized I didn’t have a light filter that size—or even any film that could serve that purpose.
As a result, when I shine a flash on it, you can’t see anything on the screen at all.


If you light up most of the segments, they are visible under the flash (on the left side of the display, there’s some interference from my fingers), but a light filter is essential. It makes this type of display much easier to read. In this case, you need either a blue or a neutral filter.


It’s much easier to see without the flash, but I wouldn’t say the display is very bright, so it’s best to choose a filter that isn’t too dark.

When powered on, the display lights up faintly, then all the segments light up for about 3–4 seconds. The display operates in 13+12 or 25-bar mode—yes, that’s an odd arrangement, which is why mono mode looks better. The display’s total resolution is 100×17 pixels, plus the frame.


Six to seven seconds after power is applied and all the segments light up, the display switches to operating mode. With no signal, only the frame at the bottom and on the left and right is lit.


There are pads for two jumpers on the underside of the circuit board:
1. Bright — slightly reduces the display brightness, but the reduction is small and the display isn’t very bright to begin with, so there’s no point.
2. Frame — turns off the frame, but it looks worse without it.

All the signals—Mono/Stereo, Peak Detector On/Off, Brightness, and Frame—are active-low, so it’s easy to control them from another microcontroller.


Video version of the review.



And as I’ve already said, it’s better to see it for yourself. I filmed all four operating modes and used music from different genres—at least, tracks I like and that can be used on YouTube.
1. Stereo + peak detector — Shine Bright - Digital Camel
2. Mono + peak detector — Stay Near - Luwaks
3. Mono without peak detector — Time To Wake Up (Ahlstrom Remix) - Cacti
4. Stereo without peak detector — Turn The Page (SLCT Remix) - Matt Sierra



And now for the verdict.
First, about what exactly is shown on the display. The description called it a spectrum analyzer, which is often mistakenly called an “equalizer,” even though those are completely different devices.
I tried feeding it sine waves at different frequencies, and its response changed, but there were hardly any obvious peaks. I also tested it with music and changed the equalizer settings, but didn’t notice much difference. I’d say it’s just a pretty animation that moves to the music, but while filming the video (probably about an hour and a half in total), I noticed that the display looked an awful lot like Winamp’s visualizer. I wouldn’t call it a spectrum analyzer, but it works perfectly well as a nice little gadget to install in your amplifier.
And of course, I really like that it has no clock or other nonsense—it’s just a level meter with nothing unnecessary.

This review was sponsored by the purchasing agent yoybuy.com, which covered the shipping costs. Affiliate link to sign up. New users get a 10-off-50 coupon; maybe I’ll get something out of it too.
The kit costs $16.88 including shipping to the agent. Shipping from the agent depends on various factors. The kit weighs 309 grams, according to the order page on the agent’s site.

On AliExpress, this meter costs almost twice as much—link, another one, and one more interesting option.