Heres how you can take an electric piano toy apart and control the sound using Arduino

Step 1: Open Your Toy Piano

First Step: open your toy piano, what you should see is a speaker attached to a board holding three AG-10 batteries two capacitors, and a resistor.

Once you have the guts of the piano you can remove the batteries, you won't need them. And if you have the same piano as me from around 1995 they will be dead anyways.

Step 2: Add Power

Step 2: Once you have removed the batteries from your piano you can solder a red wire to the positive end near the capacitor and a black wire to the negative end.

Once you have done this you can plug these ends into the Arduino Uno.

Red going to the 5V terminal

Black Going to the Ground terminal.

You've now got power once you plug your arduino into your computer, No need for any batteries.

Step 3: Add Relay

Step Three: Solder Wires to the First Terminal controlling each note then any other two terminals. This will allow you to have control over two notes. After That you could solder wires to every note terminal and control every note to play a song or melody.

Going from the piano to the relay you connect to terminal 1 and 4, 2 and 3 go straight into the arduino at terminal 11 and 12.

you can connect these relays in series with a relay on each note of the piano. then plug each wire into the Arduino.

Step 4: Add Code

Step 4: Now to actually control these notes you need to tell Arduino what terminals the reals and notes are connected to, in this case terminal 11 and 12.

Now add this into arduino and you will be able to control the delay, how long the notes sound and when 11 or 12 sounds. Very cool.