EV2 Expression/Volume Pedal notes

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I have a PI 3b+ in a MIDI foot controller and part of the IO is a pair of expression pedal sockets. After, ahem, getting the tip and sleeve round the wrong way, they are working correctly now. But during testing I disconnected one while the system was powered up and it shorted out the PI, which is now dead. The circuit feeds 3v3 to the ring, which is connected to the pedal potentiometer, and measures the voltage off the tip (connected to the pot’s wiper). When the pedal is disconnected the plug temporarily shorts the ring (3V3) to ground. So I added a 150ohm resistor on the 3v3 supply wire. But I don’t know if that’s enough and I’m not game enough to try it without being fairly certain I won’t destroy another PI.

Is a current limiting resistor enough? If so what value is “enough”?

Or do I need something more complicated to protect the PI?

I don’t know exactly how long the short lasts for, but it’s well less than a second in practice.

Note: The circuit has an switch in the socket so I can add a little bit of extra resistance when the jack is plugged in. That way I can detect the “unplugged” state (ADC reads full 3V3) and max plugged voltage 3V1 as separate states.

  • I see the EE guys use 330R. I sometimes use 470R, and if I want to play safe 520R. – tlfong01 4 hours ago
  • @tlfong01 Ok sounds like a need a slightly higher value. Still would like to know what the “proper” solution to something like this is. – fret 2 hours ago
  • OK, so we have agreed on the current limiting resistor value. Now let us brainstorm to get a “proper” solution. First thing first, let us look at the (1) schematic: imgur.com/gallery/aRTIokz, (2) QUESTION: electronics.stackexchange.com/questions/480171/….. / to continue, … – tlfong01 2 hours ago
  • Let us agree what hardware and software we are talking about: (1) Pedal EV-2, (2) ADC ADS1115: (1) EV-2 Expression/Volume Pedal – Fractal Audio Systems $120 shop.fractalaudio.com/ev-2-expression-volume-pedal (2) ADS1115 16-Bit ADC – 4 Channel with Programmable Gain Amplifier – AdaFruit $15 adafruit.com/product/1085 – tlfong01 2 hours ago
  • Now let us agree on how to connect the treadle pot to Rpi: (1) Positive side of pot to B, (2) Middle of pot to A, (3) Pot ground to C. I am not using names such as Vcc, tip, sleeve, ring etc to avoid confusion. You might like to comment on my naming, or make counter suggestion. – tlfong01 2 hours ago
  • Now I am drafting an answer. You might like to let me know how you are going to connect the treadle to ADC ADS1115, and Rpi GPIO power pins (5V0 or 3V3) and Rpi GPIO signal input pins (input range between 0V and 3V3), input current preferably less than 2mA.. – tlfong01 1 hour ago
  • Question: The treadle schematic shows only terminal A, B, and C. How come you inserted the extra terminal that causes all the trouble? – tlfong01 1 hour ago
  • Question 2: Why are you suing ADS1115 I2C 16 bit ADC? This ADC si not that user friendly, comparing the much more popular SPI 10 bit/12bit MCP3008/Mcp3208. If you are not following as tutorial/instructable, I would suggest you to start with MCP3008, and if everything goes well, you can upgrade to ADS1115. MCP3008 10bit can do 2 ** 10 = 1,024 steps, and MCP3208 12 bit can do 2 ** 12 = 4,096 steps (no proof read dodgy calculation) which I think is more precise and sensitive than my stupid human foot! – tlfong01 1 hour ago

2 Answers


If you short circuit the 3.3V rail to Gnd you will instantly kill the Pi.

Frankly anyone who connects a power rail to external circuitry (from the Pi or any other device) is just asking for trouble!

You can use the power rails to provide power to external circuitry with suitable care – which excludes a TRS plug which is practically guaranteed to cause a short – they are designed for audio (or occasionally video) signals. You should NEVER connect power on a live system – devices which are designed for hot plugging e.g. USB use special connectors to ensure safe connection.

No responsible engineer would connect a live power rail to use for logic connection without protection and would not use a TRS jack for power!

If you are powering an external circuit any series resistance will compromise its operation (unless the current is minimal). You DON”T put resistors in power rails.

No one can design your circuit for you, even if you published a proper specification.

  • So USB has a way to “ensure safe connection”. I’d be interested to know what methods are used to allow hot plugging of devices? Also the series resistance in this case just takes away from the total range I can read at the ADC. It’s not like a IC that needs a certain voltage, it’s just a potentiometer connected via a TRS cable. Lots of similar devices do this, so I assume there is a standard way. That’s what I’m asking for… – fret 2 hours ago
  • USB has connectors designed for the purpose. TRS jacks ARE NOT Despite other answers NOTHING you can do will help if you wire 3.3V to a TRS jack because the tip and ring make contact with Gnd on insertion. – Milliways 17 mins ago

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