This voltage doubler circuit diagram is very simple and require few external components . This voltage doubler use a 555 timer intergrated circuit wired as an astable mutivibrator  .

This voltage doubler circuit can deliver only up to 50mA output current and above that current limit the output voltage will be reduced. The actual output voltage will be around 22V for a 12V DC input and also the output voltage will be a bit unstable. Anyway, for low current applications this circuit is well enough.