It is forbidden to use a recording device on Shabbat. It goes without saying that one may not activate such a device on Shabbat. But even if the device was turned on before Shabbat, or if it was set on a timer that activated it automatically on Shabbat, one may not speak into the recorder on Shabbat. This is the ruling of Hacham Ovadia Yosef, in his work Yehave Da’at.
Hacham Ovadia notes, however, that speaking into a recording device does not constitute Ketiba (writing). It is forbidden on Shabbat for other reasons, involving the use of electricity, but it is not considered as though one "writes" his voice onto the tape, not even indirectly ("Ke’le’ahar Yad"). This point is significant with regard to Hol Ha’mo’ed, when Halacha imposes certain restrictions on writing. Since recording one’s voice does not constitute "writing," it is not subject to the restrictions on writing that apply during Hol Ha’mo’ed. The prohibition of "Hab’ara" (kindling a flame), which applies on Shabbat, does not apply on Hol Ha’mo’ed, and thus using electricity is not forbidden on Hol Ha’mo’ed. For this reason, one may use a recording device on Hol Ha’mo’ed.
Summary: One may not use a recording device on Shabbat under any circumstances, even if it was activated by a timer or before Shabbat, but one may use such a device on Hol Ha’mo’ed.
