help protecting an output

I’m interfacing an SAMD21G to a music synthesizer.

I’m using the timers to generate pulse waveforms to be used as triggers on the synthesizer.

The synth has a +12V & -12V power supply. My board has only +12V power. Synths generally deal in signals of +/-10V, and in this case I need triggers that rest at 0, and go somewhere between 7V and 10V.

I have this all working with the following circuit:Falstad circuit, image below.

BUT - If the user accidentally plugs my trigger output into a synthesizer output (which could be anywhere between +12V & -12V…) - I want my board to survive… but instead, it fried my processor… :’(

I would have thought the transistor (2N2222) and 100kΩ would have been enough…

Looking for help on how to beef up the protection here?

I might add: 30+ years in software, but this is my first circuit design... so I probably need a little ELI5 hand holding here!

I would have thought the transistor (2N2222) and 100kΩ would have been enough...

Yes, that should have been safe. (That's assuming your real circuit matches your schematic. ;) )

An opto-isolator could give more protection and even if an opto-isolator gets fried it usually stays isolated.

Well, if it helps- here’s the full schematic. The board was made from this, so I’m pretty sure the live circuit matched this. As I said, it did function fine when the outputs were connected to inputs.

The output marked “PAD_BEAT” on page 2 was the one that was accidentally connected to an output (a signal slowly varying between 0 and +10V).

Jumpers JP1 and JP3 were both installed. The processor unit on the first sheet is an Adafruit Feather M0 Express with an onboard regulator, so feeding 5V into the “USB” power bit will regulate a 3V3 rail. The USB jack was not connected to anything when the event happened.

Pulsar Buddy rev A mod.pdf (25.7 KB)

Hi,
Welcome to the forum.
It is easier if you post diagrams as jpg or png, this will make it easier for members using "smarty" phones and other weird platforms to view your circuit.
OPs circuit.


Tom... :slight_smile:

To be sure I understand, you required:

1) Arduino digital output (3.3 / 0) to trigger a synth input which must be below 0 for "low" and about 7 to 10 for a "high"

2) plugging the above into a Synth output cannot damage the Sam21 Circuit.

Am I correct?

John

Gotcha - didn't realize the forum only inlines JPGs....

Here's the second sheet of the schematic, which includes the output circuit that received the unfortunate voltage:

@JohnRob - pretty close, I'd rephrase as:

  1. Arduino digital output (3.3 / 0) to trigger a synth input which must be below 1V for "low" and somewhere between 7V and 10V for "high"
  2. Plugging the above into a synth out, +12V to -12V, should not damage the circuit or the SAM21

mzero:
BUT - If the user accidentally plugs my trigger output into a synthesizer output (which could be anywhere between +12V & -12V…) - I want my board to survive… but instead, it fried my processor… :’(

I would have thought the transistor (2N2222) and 100kΩ would have been enough…

Absolutely not going to survive -12V - the easiest way to blow up a transistor is reverse-bias the thing.
If you pull an NPN transistor’s collector voltage below the emitter voltage the base-collector junction conducts
and drags the base voltage down with it. At about 5V below the emitter voltage the base-emitter junction goes into zener breakdown and pulls lots of current and the chip melts. Anything connected to the base is also at risk.

Protection requires something to limit current, then something to clamp the voltage. A resistor and a diode to ground would do the trick here. Diode with anode to ground, cathode to the collector, and 1k resistor on the output to limit current through the diode from the external source.

So... like this: falstad circuit

right?

mzero: Gotcha - didn't realize the forum only inlines JPGs....

Nothing to do with the forum.

PDFs are not part of HTML.

They are not images as such. Programs frequently put an image in a PDF so that the PDF contains an image file which is not at all useful. :roll_eyes:

In your case, it is a pure PDF which constructs the diagram from its various pieces. This means it can actually be edited and perhaps more importantly, you can actually extract useful data from it such as by cut-and-paste. You can also magnify it. :grinning:

TomGeorge has reduced it to a degraded image in order to inline it. :astonished: