A little help with a schematic for the Audio Shield 4.x Rev. D?

[TonyAme]

Hello,

I am making a simple pcb for the Teensy Audio Shield 4.x Rev. D. I am using EasyEDA and a "footprint" I found for the shield. I just have a few pots and connections for audio in and out. (And also have a 5V regulated power supply).

New to using the Teensy/Shield. Does that "footprint" look correct? Any advice, help would be sincerely appreciated.

TonyAme

 

[PaulStoffregen]

Connect the 4 pots to 3.3V, not to 5V. And connect the 4 capacitors between the signal and GND, not between signal and 5V.

 

[TonyAme]

Thanks very much for the help on this.

TonyAme

 

[PaulStoffregen]

Also on the 2 output connectors, J6 and J5 need to have GND to OUTGND on the audio shield. Looks like their GND pins are connected only to each other but not the audio shield GND.

 

[MarkT]

The pots have ground and supply switched too I think, caps normally to ground.

 

[TonyAme]

Thanks for catching the reversed power/ground I had for the pots. Here is an updated schem.
Any suggestions/ comments welcomed.

Thanks,
TonyAme

The pots have ground and supply switched too I think, caps normally to ground.

Yes, I've updated my schem. Thanks for checking it over, I appreciate it.

 

[TonyAme]

Also on the 2 output connectors, J6 and J5 need to have GND to OUTGND on the audio shield. Looks like their GND pins are connected only to each other but not the audio shield GND.

I noticed that on the Audio Shield diagram REV D (for Teensy 4.0), that all of the audio input and output (line level) gnd's are connected together?
I've also updated my shem, see attached image. (I might still be "missing the boat" on the audio ground, please let me know).
Sincere Thanks.

 

[TonyAme]

Updated schematic for Teensy Audio board. If anyone could use this schematic for their own project, please feel free. (Also, would be great if anyone could point out any potential problems, or make suggestions).

I have RCA jacks, which will connect CdS photo-resistive sensors to the analog input ports, adding their resistive values to that of the pots. CdS sensors are activated via spdt switches.

Thanks again,
TonyAme

 

[PaulStoffregen]

I don't understand the purpose of J9,J10,J11,J12 on this new schematic. But if you intend to plug external cables with lengthy wires into those jacks, you might consider whether connecting Teensy's I/O pins directly might be asking for trouble? Teensy can be easily damaged if any pin is driven hard (relatively low impedance) below GND or above 3.3V. Normally for connecting external cables that might get a variety of possible (wrong) things plugged in, some form of protection circuitry is usually desired.

 

[TonyAme]

The J9-J12 will be used for plugging in a photocell sensor that is wired to plug into female RCA jack. I do this kind of thing with Arduino Nano's, but never before with a Teensy. It adds a fun, unexpected element to the sound. Yup, I should find out what kind of protection circuit to add to the analog input. Thanks for pointing that out. (Any ideas or suggestions for the protective circuit would be welcomed).

Sincere Thanks,
TonyAme

 

[TonyAme]

In case anyone else should find this thread to be of value I have been researching ways to protect the analog input pins of the Teensy from going below GND or above 3.3V if using a resistive sensor as in my circuit above.

I've been finding that the use of diodes, either Schottky or all purpose silicon diode, sometimes used along with a current limiting resistor. However, both of these things will effect your analog readings. Here's a few references.
Diode as input protection, by devxplained.eu
And from mosiac-industries

And a simplified schematic to illustrate.

 

[TonyAme]

Working on a possible way to protect an analog input pin (pin 17) from over or under voltages. Just need to determine the values of the Schottky diodes to use. (The jack J11 will receive a CdS resistive photosensor whose resistive value would be added to that of the pot VR3, (when switched on by S3). C3 is a ceramic 0.1uf cap.

 

[MarkT]

Everyone and their dog uses a BAT54S (dual Schottky's in series) for such protection of a single line (usually the SOT23 version). If protecting multiple lines you can get the QSBT40 or similar (that one's a quad protection circuit, ie 8 diodes).

 

[PaulStoffregen]

I would move the 470 ohm resistor.

Consider the scenario where the pot is turned fully clockwise. The 3.3V power line gets effectively connected directly to the wire that goes to the outside world. If the line gets driven hard with a voltage over 3.3V, it could quick destroy Teensy if it's able to source more current than everything is currently consuming from the 3.3V rail.

 

[PaulStoffregen]

You might also consider physical placement of C3, perhaps with a comment on the schematic, or just keep in mind when you create the PCB layout. As drawn, the diagram kinda implies C3 will be located physically close to the pot. For best effect, you really want to place it physically close to Teensy. But don't overthink and stress to much about a few millimeters. Just try to get it reasonably close and minimize the length of wire between the capacitor and AGND / GND pin.

 

[TonyAme]

You might also consider physical placement of C3, perhaps with a comment on the schematic, or just keep in mind when you create the PCB layout. As drawn, the diagram kinda implies C3 will be located physically close to the pot. For best effect, you really want to place it physically close to Teensy. But don't overthink and stress to much about a few millimeters. Just try to get it reasonably close and minimize the length of wire between the capacitor and AGND / GND pin.

Understood. And will will move R3 to the signal leg of the photoresistor jack J11. Move C3 closer to Teensy.
Thank you MarkT for; "BAT54S (dual Schottky's in series) for such protection of a single line..."
I think I got it now.
Sincere thanks for the info on this.

Appreciated,
TonyAme