The invention relates to a tunnel form comprising: two vertical forming walls joined by a horizontal wall in two parts, and at the base of the form, means for adjusting the height of the position of the top edge of the tunnel form relative to the reference surface, and means of support at least indirectly with the reference surface so that it can be extracted from the tunnel after production. The transmitter provides a transmitted signal corresponding to the digital signal to a signal path. The cam set includes a first cam, a second cam, and a first one-way clutch. The light emitting device also includes a third semiconductor layer of a second conductivity type formed overlying the active region.