LM317/LM117, 0.1uF cap between input and ground, 240 Ohm resistor between output and adjust, 330 Ohm (336 Ohm) from adjust to ground.

Depending on current draw (if below ~5 mA) you might need an extra ballast load for proper regulation. 600 Ohm from output to ground will draw 5mA...

Optional 1uF cap between output and ground.

Should fit in the battery compartment without any problems.

Spec sheet




There are probably much easier ways to get a 2.5 to 3.2 (about all regulation needed considering it's a battery powered thing...) voltage from the car, but the above is pretty general if someone needs other voltages and good regulation.

/Michael