Trying to get a VU meter and driver working

[Esppse]

Hello guys,

I have installed this VU meter in my circuit:
https://www.aliexpress.us/item/3256801088051448.html?spm=a2g0o.order_list.order_list_main.17.25671802jzyLE7&gatewayAdapt=glo2usa&_randl_shipto=US

It is meant to read the Input guitar signal. I have tested only with a guitar signal. (Distortion circuit disconnected). Does it have to be connected to the pedal circuit to test the VU at all? Or will a guitar signal alone should move the needle?

I did not want to use the VU driver that came with it because the board was too big, so I went with this schematic here:
https://mas-effects.com/vu.pdf

This is my stripboard layout:



I couldn't get the needle to move at all. The Meter - to the left, Meter + to the right. (looking from the read of VU meter)

Does anyone know why this wasn't working at all?

Thanks

[duck_arse]

we can't see what you have built. we don't have any of your voltage measures. you need to show and tell.

[GibsonGM]

You COULD try connecting it with THEIR circuitry and be sure it all works....

[tonyharker]

The guitar output is too small to get the meter to work. You will need a preamplifier to boost the guitar output. Probably to about 1-2 volts.

[GibsonGM]

Yes...but OP says they're trying to use an 'aftermarket driver', and I'm not sure we know enough about it to know if it will drive it.  Not all meters are created equal.  So using the board that came with it might show something...it would be educational      If working, one could measure the voltage output to the meter and know what it expects to see to operate.  Then rebuild a smaller sized driver to preamp the signal...   

[Rob Strand]

Does anyone know why this wasn't working at all?

Did the meters ever work with the supplied meter driver?

Have you wired the meter with the correct polarity?

From the advert the meters are spec,
DC internal resistance: 650 ohm + -15%
DC current; 500uA +-15%

You can check the meter is working and get the polarity right as follows:

+9V ---- 33k resistor --- Meter +      ,  Meter - --- gnd

It should deflect a little over half way.

If you don't see any deflection try reversing the meter connection  - maybe they are marked wrong!

[mark2]

That circuit you're using is mine (but not the layout). It works well with all the meters I've tried.... provided they respond to a < 9V range.

Definitely test the meter directly as Rob recommends.

The guitar output is too small to get the meter to work. You will need a preamplifier to boost the guitar output. Probably to about 1-2 volts.

This circuit gives you a variable boost and can definitely do a couple volts.

[Esppse]

Does anyone know why this wasn't working at all?

Did the meters ever work with the supplied meter driver?

Have you wired the meter with the correct polarity?

From the advert the meters are spec,
DC internal resistance: 650 ohm + -15%
DC current; 500uA +-15%

You can check the meter is working and get the polarity right as follows:

+9V ---- 33k resistor --- Meter +      ,  Meter - --- gnd

It should deflect a little over half way.

If you don't see any deflection try reversing the meter connection  - maybe they are marked wrong!

Hey I did as suggested. The meter does deflect:
https://youtube.com/shorts/u7gPWNmf81M?feature=share


These are the voltages of the TL071, and the M+ M-

1: 6.85
2: 0.25
3: 3.09
4: 0.00

5: 0.04
6: 0.20
7: 9.21
8: 8.17

Also reading 4.5v at the divider.

M+: 0.00
M-: 0.00

I tried to get Needle movement with the trimpot cranked both ways, and set in the middle. Trimpot measures proper resistance to ground when measured.

Here are some other shots of the stripboard circuit:

















Did I layout the stripboard right? I checked it a dozen times

[Rob Strand]

Hey I did as suggested. The meter does deflect:

Looks good.

These are the voltages of the TL071, and the M+ M-

1: 6.85
2: 0.25
3: 3.09
4: 0.00

5: 0.04
6: 0.20
7: 9.21
8: 8.17

So upfront we expect the measured voltages to be lower than the true circuit voltages.  That's because the DMM will load down parts of the circuit with high valued resistors.

The measurements that stand out as errors is the low pin 2 voltage and the low pin 6 voltage.   If the 47nF cap (C2) has a short it will pull down pin 2.   However that would cause pin 6 to go high, so cap short doesn't fit what we are seeing.   So the other angle is pin 6 is low and that is causing pin 2 to go low.

One reason for pin 6 low would be a short on pin 6, another is a faulty IC.

I haven't yet looked at your layout but I'd be looking for a short on pin 6.

[Esppse]

Hey I did as suggested. The meter does deflect:

Looks good.

These are the voltages of the TL071, and the M+ M-

1: 6.85
2: 0.25
3: 3.09
4: 0.00

5: 0.04
6: 0.20
7: 9.21
8: 8.17

So upfront we expect the measured voltages to be lower than the true circuit voltages.  That's because the DMM will load down parts of the circuit with high valued resistors.

The measurements that stand out as errors is the low pin 2 voltage and the low pin 6 voltage.   If the 47nF cap (C2) has a short it will pull down pin 2.   However that would cause pin 6 to go high, so cap short doesn't fit what we are seeing.   So the other angle is pin 6 is low and that is causing pin 2 to go low.

One reason for pin 6 low would be a short on pin 6, another is a faulty IC.

I haven't yet looked at your layout but I'd be looking for a short on pin 6.

I just tried replacing the chip, the 47k resistor, and the 10uF cap, no luck. I check the continuity of everything to Pin6, there are no shorts I found. I checked it both with the chip installed and removed. Are those supposed to be 3.3K resistors from the diodes to ground? In the circuit, they read 1.73k to ground, which is probably right? I am curious because we used a 33k before to probe the meter by itself and that moved the needle.

[Rob Strand]

I just tried replacing the chip, the 47k resistor, and the 10uF cap, no luck. I check the continuity of everything to Pin6, there are no shorts I found. I checked it both with the chip installed and removed. Are those supposed to be 3.3K resistors from the diodes to ground? In the circuit, they read 1.73k to ground, which is probably right? I am curious because we used a 33k before to probe the meter by itself and that moved the needle.

Yes, the 1.73k looks correct.  Pretty much 2x3k3 in parallel but through the meter which makes the resistance a little over 3k3/2 = 1.65k.

I checked quite a bit of the layout and it looks pretty good.   

After checking the internal circuit of the opamp, the voltage on pin 1 looks off.  I expect it to be a low voltage like pin 5.   If something is connecting to pin 1 it will screw up pin 6 (and that screws up pin 2).   However, pin 1 looks clean.  Maybe have a close look by eye.   The list of obvious things is certainly narrowing down.

[antonis]

<off topic ON>

There is no need for such a high values of R7, R8 & R1 resistors..

<off topic OFF>

[duck_arse]

a resistor marked "3K3" is suspcious to me. are you sure it measures k and not ohms? it looks like a foreign language decimal place replacement, you know, so no one gets confused.

[deadastronaut]

if its the same meter as this below, then i put together this circuit for it.

and works like a charm after tweaking a couple of values.  i attached the board to the back of the meter ( in a 19'' rack)

and run 2 wires to the lamp from the 9v on board...with a resistor on + wire.

not tiny, but it works.

[Rob Strand]

The issue isn't so much the circuit but why the DC voltages aren't correct.   DC wise it is a pretty simple circuit.  If you wanted to test an opamp on your breadboard then you would build something quite similar!

As for Rob's ckt vs mark2's they both should work.   With *DC* meters marked with VU scales Rob's current drive circuit will have a more accurate scale.  By the same token at say -20 VU mark2's circuit is only 3dB off and at 0VU they can be calibrated to be the same.   It depends if you are going for general indication or a precision instrument.
(The VU spec actually enforces a whole lot of stuff about response to transients, so it shows correct levels with transient signals, which means tweaking RC values around the meter part of the ckt.
https://en.wikipedia.org/wiki/VU_meter
)

[Esppse]

Thanks for the help all. In the end, I ultimately went with a different and much smaller driver. I built about 7 of these and they all worked:




You may have to shift a couple components to different holes for fit, but this was the smallest I could get to drive a VU. This was based off some schematic (not mine) I lost a while back. I laid it out in the smallest dimensions possible.