Potentiometer or momentary switch, how to better recognise what was connected?

Question

I have a working circuit and I'm looking for cues on how to improve it.

I'm building a device with a user controlled small stepper motor. Speed of the motor can be controlled by a pedal connected via audio-jack.

I need to properly recognise a type of the connected pedal and handle its value in the MCU.

There are 3 types of pedals that can be connected:

• Type A - simple momentary switch, user can switch motor on or off.
• Type B - pedal with a (10k) potentiometer, user can control speed with her foot.
• "Type C" - nothing is connected, empty socket

I have a circuit that works using two ADC pins on atmega328p, A5 reads pedal type, A1 reads its value, combination of readings from both pins allows me to control connected motor.

Question: how I can improve, simplify or make it more resilient?

I would be very grateful for tips from more experienced colleagues.

Answer 1

1. If you know it is an active device, you test 1st with a DMM voltmeter.
2. Since you know this is passive, you start with a DMM Ohmmeter which is 10uA to 1mA CC and measure voltage. An analog meter in Ohmmeter scale would be more responsive but less accurate.

How you implement this circuit is irrelevant unless you have tolerance specs for these options and may include 4) None of the above

Some audio drivers have this feature for Windows when enabled, that can auto-detect any audio port as having ;

1. a speaker connected, by Low impedance to gnd,
2. an electret microphone, by midscale DC voltage from external bias to V+
3. or open circuit, detecting V+

Using the same approach you can define 3 states;

1. A switch with a 9X resistor across it to get 90% of V+ when open and 10% V with 10% series R when the switch is closed.
2. A slider pot with 20% series R's on either side that controls the V+ bias confined output voltage range with the chosen standard pot value that does not conflict the voltage in states 1. or 3. 15% V+ < Vx < 85% V+
3. Open circuit V=V+ from external R bias.

^{simulate this circuit – Schematic created using CircuitLab}

Something like this can detect the 3 states with voltage-current or by either sweeping with a DAC to detect a zero-crossing for the Voltage result or using an ADC to measure the voltage with V+ bias and series R.

Answer 2

use an out-of-range signal for the button. use the AVR's internal pull-up to probe for a connected device.

^{simulate this circuit – Schematic created using CircuitLab}

you can detect which is connected by enabling the internal pull-up on the analogue pin.

if nothing is connected the voltage will go high and stay there, the potentiometer voltage will change slightly, the button will go to a voltage mid-way somewhere. so turn the pull up off again after probing.

This will add software complexity: it becomes more important to re-probe to detect what's connected and to de-bounce the inputs, but for best reliability your original plan would have needed that too.

It also means that you can use 3-wire plugs which gives more options, especially if you only need to use commercial grade connectors.