Tune-O-Matic tuner

[fredrik]

@telec16

The input resistance (R3 on my schematic) should be fairly low, as someone (I think it was @fredrik ?) pointed out before

Yeah, datasheets for both AVR and PIC recommend 10k or below (at least with the default samping intervals):

1145 VC LFO Module

I’d say maybe – worth trying but may not work great. As you say, they’re voltage dividers, but assuming the ADC inputs on the PIC are similar to the one’s on the AVR (Arduino), they aren’t the usual high impedance inputs, but instead they use a sample and hold capacitor that needs a certain current to charge fast enough. AVR recommends keeping the source impedance at 10k or below for reliable operations (and I should go read the PIC data sheets I guess but have actual work work to do). EDIT: Na…

The built-in clamping diodes are supposedly good for around 1 mA, so if they’re good enough on their own depends on your worst case scenarios

[Jos]

The schematic looks good to me (apart from the missing draft/untested text).

I used 220 Ohm resistors for the display (I used a display I had laying around for years so the LEDs are not as bright as you will find in more recent displays) and 10k for the red and 56 k for the green LEDs because I do not like LEDs that light up my studio. So these are personal adjustments I made. The values you use are fine.

[fredrik]

@Bas

Any apparant flaws or improvement possibilities?

If you’re using an external 5 V regulator, you should connect it to +5V, not VIN, to avoid unnecessary voltage drops. See here:

Design glitches: Arduino Nano supply voltage

(thought I’d collect my nano supply notes in one place to make it easier to link to) A common design glitch in circuits that use an Arduino Nano is to connect regulated 5 V to the VIN pin instead of the +5V pin. This results in the Arduino running at a lower internal supply voltage, often 4 V or below, which may be a problem if the design depends on analog inputs or digital output levels. Voltages below 4.5 V are also out of spec for the 16 MHz clock used by the Nano. Details below, but TL;DR …

[Bas]

Thanks Fredrik. I will make sure to adjust this accordingly.

[telec16]

I think you can omit the 100ohm resistor (R4) on the feedback of your opamp, TL072 are not Current Feedback Amplifier, so it shouldn’t be an issue without it… Or I’m missing something?
But you should definitely put a 1k resistor on the output, to protect it against shorts (even if the TL072 is already protected, it won’t hurt).

[analogoutput]

@fredrik

The built-in clamping diodes are supposedly good for around 1 mA, so if they’re good enough on their own depends on your worst case scenarios

I like to add the diodes on the grounds that they’re easier to replace than the on-board ones, if it ever comes to that.

[analogoutput]

@telec16

I would use an external 7805, the one on the Arduino is pretty small and can get hot easily…

An L78L05C (TO-92) will go up to 100 mA depending on heat sinking; the L7805C (TO-220) (to 1.5A) strikes me as overkill, but that depends on what your LEDs draw and how many you have on at once. If that’s a concern you could add driver circuitry for the LEDs, but that would complicate things.

[Bas]

I started doing it this way after a conversation with Tom (ThreeTomModular). I would have to look back in my chat history what the exact reasoning was. I have used it since.

[analogoutput]

@telec16

But you should definitely put a 1k resistor on the output, to protect it against shorts (even if the TL072 is already protected, it won’t hurt).

Per the datasheet, the TL07x output can be shorted to ground or to either rail indefinitely. Current limiting resistors on op amp outputs are something people tend to add reflexively, and as you say, it won’t hurt. But it’s really not necessary.

[telec16]

It will limit the output current, which is good for the opamp (it prevent overheating), and can also protect whatever you plug into. And if you put the feedback input after the resistor, you will have no distortion even under load (up to a certain point obviously).

[analogoutput]

“Can be shorted indefinitely” I think means by definition it’s protected against excessive output current. And whatever it’s plugged into is presumably what’s shorting the output and doesn’t deserve protection . But yeah, I’m not against current limiting resistors (inside the feedback, as you say, especially if it’s supposed to be a 1V/oct signal), just saying it’s acceptable not to have them.

[fredrik]

Well, it doesn’t do anything (the inputs have crazy high impedance, so there’s hardly any current going through it and the voltage drop is probably picovolts), but on the other hand since it doesn’t do anything you can of course leave it in (And use any resistor, if you run out of 100 ohm.)

As for the output, as noted above the TL07x has a high enough internal resistance that they can deal with a short – output voltage drops rapidly as the output impedance drops below a few hundred ohms:

I think a series resistor is still a good design practice, though, since it gives you a bit of extra protection (by limiting dissipation) and also provides a bit of isolation if you have unusually capacitive loads (i.e. long cables).

[Jos]

This subject has been discussed before and I know it will come around again. Maybe it is a good idea for someone (@fredrik, your reasonings seems to be accepted by most) to summarize this and put it on a ‘best practices’ or ‘suggested practices’ page of sorts, leaving room for other opinions or idiosyncratic use of components?

[Bas]

Sorry for the hijacking your thread Jos.

I have added some of the feedback in the design. I will share my findings once the PCB arrive in ~14 days.

[Bas]

Counting the days waiting for the pcb to arrive, got me thinking…

As I intend to be sharing this design, that’s derived from the efforts of others, i was wondering about the licencing.

• Jos’s software is published under GNU GENERAL PUBLIC LICENSE (which is very free)
• Sam’s work shared here and on his page does not state any obvious licence? Did I overlook this?

Me personally, I’m not too fussed what anyone does with my stuff. But of course I want to respect the work of both of them. What license would be best suited for my repository?

[Jos]

I think I based the license on the source of the code which is the source Sam refers to. I’m not sure though. Let me check this.

[Edit]: In the code I took from Sam’s website it says:

/*

• This program is free software; you can redistribute it and/or modify
• it under the terms of the GNU General Public License as published by
• the Free Software Foundation; either version 3 of the License, or
• (at your option) any later version.

*/

So yes, I based the license on what I found in the code. I did not check the origin of these comments but I stuck to that same license.

[Bitnik]

I hope Sam will clarify the licensing of his original work so people can use part of it without fear of damaging his business model.

[Bas]

PCB’s arrived today. I should have all the parts to get one ready for testing tomorrow:

[ChristianBloch]

Nice! Still planning on sharing the Gerbers once tested?

[heckseven]

I’m hoping it’ll be something like the GPL or MIT licenses.