Undecidability of bounded security protocols

A magnetic head lifting device for a tape recorder is disclosed in which the turning force of a capstan shaft is used as a drive source for the head lifting device. The device includes a rotary body having indented portions in a peripheral edge, a ring attached to a capstan shaft and engageable with the nonindented portions of the peripheral edge of the rotary body, a rotary body urging member, a rotary body stopping member, an electromagnet, an antiexcessive rotation member positioned by the electromagnet to positively prevent excessive rotation of the rotary body, and a stepped surface cam associated with said rotary body for lifting the magnetic head in response to the rotation of the rotary body. The head lifting device provides a positive engagement and disengagement between the capstan shaft and the rotary body and prevents any malfunction due to an excessive rotation of the rotary body by the capstan shaft. The device is miniaturized since only a small capacity electromagnet is required.