What's the best way to operate a remote control circuit board?

Question

What would be the best way of using an Arduino to operate the pictured remote control circuit board? My initial instinct would be to leave the batteries in the remote and use a separate transistor for each of the five switches, connecting wires (solder or electrical tape) to the two sides of each button and the base to an Arduino output pin. I'm certainly not an expert, so I wanted to confirm whether (a) that would work and (b) if there's a much better way. The control is normally powered by two AAA batteries.

Edit: There's also a small capacitor on the underside of the circuit board, if that changes anything.

Thanks in advance for your help

Note: I've tried to simply record and emit the IR signals from the remote using my Arduino, but have thus far been unsuccessful, so want another way of using the Arduino to control my fan: Unable to control fan with IR circuit despite capturing and reproducing the IR transmissions

Answer 1

All the right sides of the buttons are connected to the negative of the battery (that is ground). So you connect the remotes GND to the Arduinos GND.

Next solder five wires to the left side of the buttons. You can do this by scraping away a bit of the green solder mask that's on top of the trace going to that button pad. Or find out which pin on the IC goes to the button, and solder a wire to that leg.

I wouldn't mess with the black stuff (graphite?) on top of button pad. That way you can always return the remote to it's original function.

Then connect those wires to the Collectors of NPN transistors. The Emitters all connect to GND. The Bases (via a resistor) to the Arduino pins.

Alternatively, you can connect them directly to the Arduino pins. But you'd have to be careful to never set the pin to HIGH, as the remote won't like 5V on it's input pins while it is itself running on only 3V. You "press" the button by setting the pin to OUTPUT. After a "press" you set it back to input.

To prevent accidentally supplying 5V to the button pin, you can also use diodes (instead of transistors described above). Put the diode between the Arduino pin, and the button wire, pointing towards the Arduino. That way current can only go towards the Arduino. Then you can safely use digitalWrite.