  (abridged) We used the IRAM 30-m to perform a sensitive wideband survey of 30
protoplanetary disks in the Taurus Auriga region. We simultaneously observed
HCO$^+$(3-2), HCN(3-2), C$_2$H(3-2), CS(5-4), and two transitions of SO. We
combine the results with a previous survey which observed $^{13}$CO (2-1),
CN(2-1), two o-H$_2$CO lines and one of SO. We use available interferometric
data to derive excitation temperatures of CN and C$_2$H in several sources. We
determine characteristic sizes of the gas disks and column densities of all
molecules using a parametric power-law disk model. Our study is mostly
sensitive to molecules at 200-400 au from the stars. We compare the derived
column densities to the predictions of an extensive gas-grain chemical disk
model, under conditions representative of T Tauri disks. This survey provides
20 new detections of HCO$^+$ in disks, 18 in HCN, 11 in C$_2$H, 8 in CS and 4
in SO. HCO$^+$ is detected in almost all sources, and its J=3-2 line is
essentially optically thick, providing good estimates of the disk radii. The
other transitions are (at least partially) optically thin. Variations of the
column density ratios do not correlate with any specific property of the star
or disk. Disks around Herbig Ae stars appear less rich in molecules than those
around T Tauri stars, although the sample remains small. SO is only found in
the (presumably younger) embedded objects, perhaps reflecting an evolution of
the S chemistry due to increasing depletion with time. Overall, the molecular
column densities, and in particular the CN/HCN and CN/C$_2$H ratios, are well
reproduced by gas-grain chemistry in cold disks. This study provides a census
of simple molecules in disks of radii $> 200-300$ au. Extending that to smaller
disks, or searching for less abundant or more complex molecules requires a much
more sensitive facility, i.e. NOEMA and ALMA.
