GSSL - LED METER QUESTION

[anythingcansoundgood]

Hello!

I initially posted this in an old thread (here: https://groupdiy.com/threads/my-gssl-variable-scale-meter-board.30057/#post-1171342), but since I didn’t get any replies, I thought I’d try again here. Please let me know if this isn’t the right place for this question!

I’m working on building a simple LED-based GR meter for a GSSL compressor. I want the scale to be fixed at 20 dB.

I came across information suggesting that the current output for the meter is negative. Is that correct?

If that’s true, I assume I’ll need an inverting op-amp configuration. My question is: with the same inverting buffer circuit, could I amplify the signal by 2.5x to bring it to a 0–5V range? If so, would I then be able to use the circuit from page 2 of the LM3914 datasheet (https://www.ti.com/lit/ds/symlink/lm3914.pdf) directly?

Here’s a schematic to clarify what I mean (the IC shown is the LM3914):



As my V+ and VLed source, I would be using the +12V output of the 78L12 voltage regulator (already in the GSSL's circuit).

Thanks!

 

[Hobbyist]

Hi ! I don't have experience with TL072 in this circuit. Tested it with TL071 based on the original thread and works perfect in my unit with 3 different settings. Trimmers are needed to tweak ! Some pictures of the pcb and the process.
Good Luck !

 

[anythingcansoundgood]

Hi ! I don't have experience with TL072 in this circuit. Tested it with TL071 based on the original thread and works perfect in my unit with 3 different settings. Trimmers are needed to tweak ! Some pictures of the pcb and the process.
Good Luck !

I'm currently searching for a schematic I could easily reproduce in Kicad, because it's not easy to understand where the traces are on the layout. But at this point I think I'll simply breadboard mine and we'll see I'll also implement a trimmer for the gain, thanks for this tip !

 

[Harpo]

Hello!

I initially posted this in an old thread (here: https://groupdiy.com/threads/my-gssl-variable-scale-meter-board.30057/#post-1171342), but since I didn’t get any replies, I thought I’d try again here. Please let me know if this isn’t the right place for this question!

I’m working on building a simple LED-based GR meter for a GSSL compressor. I want the scale to be fixed at 20 dB.

I came across information suggesting that the current output for the meter is negative. Is that correct?

If that’s true, I assume I’ll need an inverting op-amp configuration. My question is: with the same inverting buffer circuit, could I amplify the signal by 2.5x to bring it to a 0–5V range? If so, would I then be able to use the circuit from page 2 of the LM3914 datasheet (https://www.ti.com/lit/ds/symlink/lm3914.pdf) directly?

Here’s a schematic to clarify what I mean (the IC shown is the LM3914):

View attachment 138419

As my V+ and VLed source, I would be using the +12V output of the 78L12 voltage regulator (already in the GSSL's circuit).

Thanks!

maybe some remarks...
increase R75 to at least 2K in order to not upset the driving TL074 pin8 with other resistors in parallel connecting to a virtual ground node.
R100 same value as R75 with 5K trimmer in series for a variable gain range 1 ... 3.5 might come handy.
U17a will be a single opamp, not a dual opamp, maybe TL071. Opamp output is pin 6 instead of pin 1.
Supply this opamp pins 4/7 from GSSL-pcb +/- 15VDC rails.
With R95 1K21 (setting LED current to 10mA) and pin 9 of LM3914 connected to U14-supply for bar-mode, don't connect this chip to the 78L12 voltage rail. Use the optional 7812 vreg. rail instead for this current demand far exceeding the 78L12 current drive ability.
As always YMMV.

 

[anythingcansoundgood]

maybe some remarks...
increase R75 to at least 2K in order to not upset the driving TL074 pin8 with other resistors in parallel connecting to a virtual ground node.
R100 same value as R75 with 5K trimmer in series for a variable gain range 1 ... 3.5 might come handy.
U17a will be a single opamp, not a dual opamp, maybe TL071. Opamp output is pin 6 instead of pin 1.
Supply this opamp pins 4/7 from GSSL-pcb +/- 15VDC rails.
With R95 1K21 (setting LED current to 10mA) and pin 9 of LM3914 connected to U14-supply for bar-mode, don't connect this chip to the 78L12 voltage rail. Use the optional 7812 vreg. rail instead for this current demand far exceeding the 78L12 current drive ability.
As always YMMV.

Thanks! I just realized I never clarified what is my goal with all this. I want to create my own SSL comp in a 500 series format, and my circuit is heavily inspired by the GSSL Gyraf diy project.

Since it's 500 series, I'll can simply use the +/- 16V from my rack for opamp and LM3914.. I'll breadboard that tomorrow.

Anyway, thanks for the tips on the resistances values!

 

[anythingcansoundgood]

maybe some remarks...
increase R75 to at least 2K in order to not upset the driving TL074 pin8 with other resistors in parallel connecting to a virtual ground node.
R100 same value as R75 with 5K trimmer in series for a variable gain range 1 ... 3.5 might come handy.
U17a will be a single opamp, not a dual opamp, maybe TL071. Opamp output is pin 6 instead of pin 1.
Supply this opamp pins 4/7 from GSSL-pcb +/- 15VDC rails.
With R95 1K21 (setting LED current to 10mA) and pin 9 of LM3914 connected to U14-supply for bar-mode, don't connect this chip to the 78L12 voltage rail. Use the optional 7812 vreg. rail instead for this current demand far exceeding the 78L12 current drive ability.
As always YMMV.

Hello!
I breadboarded this, and I found that this circuit worked when the non-inverting input of the op-amp wasn't tied to ground. Any explanation for this?
Thanks!