Adjustment device switchable between a rotatable state or a locked state

An adjustment device, comprising:

• a base, comprising an accommodating groove and a plurality of ratchet teeth arranged on an inner side wall of the accommodating groove;
• a ratchet wheel assembly, rotatably accommodated in the accommodating groove and comprising a body, an elastic arm, and a pawl, wherein the elastic arm is arranged on a side of the body and comprises a first connecting segment and a second connecting segment connected to each other, the first connecting segment is connected to the body, the second connecting segment is connected to the pawl, and the pawl is adapted to engage with the ratchet teeth;
• a fixing rod, movably extending between the body and the elastic arm of the ratchet wheel assembly, wherein the pawl is fixed to one of the ratchet teeth when the fixing rod is arranged between the second connecting segment and the body, so that the ratchet wheel assembly is positioned on the base, and the ratchet wheel assembly is adapted to rotate relative to the base when the fixing rod is arranged between the first connecting segment and the body;
• and
• a knob assembled to the base and connected to the ratchet wheel assembly, wherein the knob comprises a guide hole having a first end portion and a second end portion, the fixing rod movably extends through the guide hole, the fixing rod is arranged at the first end portion of the guide hole when the fixing rod moves to a position between the second connecting segment and the body, and the fixing rod is arranged at the second end portion of the guide hole when the fixing rod moves to a position between the first connecting segment and the body.