If you have an old-school toy piano you may encounter a problem that it needs a large number of AA batteries to operate. My Bontempi GT 509 piano required 6 AA cells. Buying 6 alkaline AA cells seems *very* wasteful for me and having to charge as many NiMHs was a significant pain.
So I figured that I should be able to somehow power this from a 18650 LiION cell with a cheap chinese battery module. After confirming with a lab power supply that the piano can work when running directly from 5V the project became even simpler - I could just use an off-the-shelf USB power bank :)
Step 1: Required Parts
1 x Bontempi GT 509 electronic toy piano, price: free (you already have it :)
1 x Telforceone 2000 mAh USB powerbank, price: 4 USD
1 x USB cable with a USB-A plug, it doesn't matter what plug is on the other end, price: free (look in your junk box)
1 x USB cable with micro-usb male and female plugs, price: 2 USD on aliexpress.com
2 x 33nH coil, price: free (look in your junk box)
1 x 2800 uF low voltage (6.3V is OK) capacitor
, price: free (look in your junk box, old PC motherboards are a good source of these)
Step 2: Disassemble the GT 509
Remove all the screws from the back and separate the front and back covers. Take note of the small power supply board (the one with the power switch and LED). It will be used later.
Step 3: Change the Original Positive Wire
Unsolder the original positive battery wire and replace it with a long wire running to the power board.
Solder the long positive wire to an unused pad on the power connected to the L4 coil and marked as the anode of the D4 diode. See the last photo for a marked place.
Step 4: Solder the USB Cable and Filtering Components
Strip the USB cable leaving only the red (+5V) and black (ground) wires. Solder these to the power board as indicated by the photo (red wire on the side of "L4" coil, black wire on the side of "L5" coil).
Add some filtering components: L4 and L5 (33 nH coils) and a 2800uF 6.3V capacitor between the +5V rail and ground at the bottom. Remember the electrolytic capacitor polarity, the "band" on the capacitor case marks the ground.
Try to put the case together to make sure, that the capacitor is not sticking out too much.
Step 5: Intermission: Why Filter?
The filtering components are required to get rid of audible noise caused by the powerbank switching converter. As can be seen on the scope traces and heard on the video this noise is very annoying. Soldering the filtering components removes the low frequency part which is around 2kHz leaving only the high frequency ripple which is beyond audible range.
Step 6: Add the Charging Cable
Put both plugs into the powerbank and see if they both fit and does the charging function work. As you can see in my photo there is not always enough room for both plugs to be comfortably seated at the same time.
Step 7: Glue the Powerbank Into the Case
Use doublesided tape to glue the powerbank in the case in a place of your choosing. Make sure that the case can still be closed after that.
Make a small hole in the back of the case and secure the charging cable with hot glue.