Pressing a physical button, but digitally using an Arduino in an existing circuit

Question

I have a pair of speakers that by default select their input as bluetooth and I have to press a physical button (every time I power them up) to switch their input to auxiliary.

So I'm thinking of a small arduino based project where every time the arduino starts up (I will wire it to get 5V when the speakers start) it will simulate a button press. The software part is easy, when the arduino starts, just output the correct voltage on one of the pins, connected to the switch.

Trouble is that I need to press the switch just once (if I press it more than once, the inputs will cycle) and I cannot really understand the electrical schematic.

So as you can see there are 5 switches, all connected to the pin 45 of the MCU. I want to simulate a single press of the SW10/INPUT button.

Conceptually I think the pin 45 measures the voltage (since every button is series with a different resistor) in order to find out which button was pressed. So, is the AD_KEY line connected to the MCU between pin 45 and the 3,3K resistor? Just like a voltage divider?

If yes, does this mean that on startup, and only on startup, it is sufficient to output 1.2V [a] from the arduino to the AD_KEY line (which leads to pin 45, where the speaker's MCU will read) in order to simulate a button press?

Electronically, is there a way I can do this just once at the beginning, without wasting an mcu/arduino (even if just a micro arduino) here?

[a]: If we're talking about a voltage divider then when SW109/INPUT is pressed we have: $$ V_{pin45} = V_{in}\frac{R80}{R80 + R1} => $$ $$ V_{pin45} = 3.3V\frac{2K\Omega}{2K\Omega + 3.3K\Omega} = 1.24V $$

Answer 1

Even simpler is just to have the Arduino or any other chip to connect the AD pin via 2k2 resistor to ground momentarily, just like what the button does.

You only need to have an open-drain output, which can be implemented in Arduino software, or with a suitable logic gate or comparator that has an open-drain output, or even with a transistor.

The timing circuit can also be an Arduino, or a 555, or simple RC time constant with schmitt trigger inverter gate or comparator.

Answer 2

I think it would be better to use a 2N7000 MOSFET in parallel with the switch rather than trying to use the Arduino output directly.

Otherwise accidentally setting the output high would put 5V on a 3.3V chip, which could damage it. Cheap insurance for a single part. The output voltage will also be closer to zero when on, so probably more noise/drift margin.

Arduino Nanos from China are around $2 and the transistor is almost free, so it's a fairly cheap and easy solution.

Answer 3

To simplify it you need a 3,3 volt Arduino (There are many variants to choose from) and a 2k ohm resistor.

Connect the 2k ohm resistor to a digital port and the other end of the resistor to "AD KEY" and also connect Arduinos GND to the amplifiers GND. And I believe you could supply a Arduino from the Amplifiers 3,3 volt rail.

Remember to set the digital pin to "floating state" when its not used, otherwise will it influence the voltage on "AD KEY".

pinMode(7, INPUT);

And to make a program that only run once:

void setup(){
 // Code you want to run once
}

void loop(){
 // No code
}

And take a look at ATtiny MCUs for your project, they can be programmed with Arduino IDE and the smallest is just a 8pin MCU = small, not power hungry and cheap.

Ref.: https://create.arduino.cc/projecthub/arjun/programming-attiny85-with-arduino-uno-afb829

https://www.youtube.com/watch?v=gXXdoeu7yWw

Answer 4

A simple window comparator/inverter solution:

^{simulate this circuit – Schematic created using CircuitLab}

The LM393 is an open collector device; both comparators must be off for the transistor Q1 to be turned on. The voltage thresholds \$V_\mathrm{L}\$ and \$V_\mathrm{H}\$ are set to \$\frac{1}{3}V_+\$ and \$\frac{2}{3}V_+\$ respectively. As the voltage across C1 rises between those two thresholds, both comparators are off. The voltage now applied to Q1's base allows it to conduct, pulling the switch connection to ground, simulating your button press. You can alter C1/R4 time constant, \$\tau\$, and/or the threshold levels to tune your inital delay, and pulse width. With the values given here, the initial delay before the pulse is 40 ms, and the pulse width is 70 ms. These can be calculated as follows:

\$t_\mathrm{delay} = - \tau \ln (1 - \frac{V_\mathrm{L}}{V_+})\$

\$\mathrm{Pulse\ length} = \tau \ln (\frac{V_+ - V_\mathrm{L}}{V_+ - V_\mathrm{H}})\$

Not as flexible as a microcontroller, but might just be cheaper..!

Answer 5

Don Lancaster's CMOS Cookbook may have the simplest most elegant solution. A cmos 4093 plus a capacitor PUR, power up reset circuit. Fast, cheap and lovely.