My car CD player doesn't play mp3 files, so I bought a flash memory mp3 player and a cassette-adapter. The problem is that the player doesn't allow external power, even by USB, so I made my own voltage regulator and a fake battery adapter, and now I'm able to power the player from the 12V plug of the car.

Electronic schematic

This is a common voltage regulator, it converts 12V to 1.5VCC using a LM317. The plug J1 must be connected to the 12V plug of the car.

It includes a power-on LED.

Output voltage is regulated by potentiometer R3, and it is connected to a pair of metalic contacts placed on the sides of a wood fake AAA battery, where the player fits and it is powered as if an AAA battery were plugged-in.

A 100nF capacitor must be connected to the LM317 input, but I only had available 56nF capacitors, so I placed two of them in parallel.

The wires that powers the player are soldered to pins J2 and J3.

Testing the circuit

Building the PCB

The potentiometer must be fixed to 100Ω before being soldered to the board, and once the circuit is running it must be adjusted until the regulator outputs exactly 1.5V.

Finished circuit