Context . Diffuse interstellar clouds present an active chemistry despite their relatively low density and the ubiquitous presence of far-UV radiation. Aims . To provide constraints on the chemical processes responsible for the observed columns of organic species, we used the NOEMA interferometer to observe the sight line toward NRAO150 (B0355+508) in the 2 mm spectral window. Methods . We targeted the low excitation lines of ortho H 2 CO (2 1,1 –1 1,0 ) and para H 2 CO (2 0,2 –1 0,1 ) as well as the nearby transitions of CS (3–2) and c-C 3 H 2 (3 1,2 –2 2,1 ), (4 1,4 –3 0,3 ), and (2 2,0 –1 1,1 ). We combined these data with previous observations of the same sight line to determine the excitation conditions, column densities, and abundances relative to H 2 in the different velocity components. We performed non-LTE radiative transfer calculations including collision cross sections with ortho and para H 2 and with electrons. New collision cross sections with electrons were computed for ortho and para formaldehyde. Results . All targeted lines were detected. The c-C 3 H 2 line profiles are very similar to those of HCO + and CCH, while the CS absorption features are narrower and mostly concentrated in two main velocity components at V LSR = −17.2 and −10.4 km s −1 . H 2 CO absorption lines present an intermediate pattern with absorption in all velocity components but larger opacities in the two main velocity components. The ortho-to-para ratios of H 2 CO and c-C 3 H 2 are consistent with the statistical value of three. While the excitation temperature of all c-C 3 H 2 velocity components is consistent with the Cosmic Microwave Background (CMB), the two strong components detected in CS show a clear excess over the CMB indicating that CS resides at higher densities than other species along this particular sightline, n (H 2 ) ~ 2500 cm −3 while n (H 2 ) < 500 cm −3 for the other velocity components. We detected faint absorption from o-H 2 13 CO and C 34 S allowing us to derive isotopic ratios: o-H 2 CO/o-H 2 13 CO = 61 ± 12 and C 32 S/C 34 S = 24 ± 6. The excitation of the 1 1,0 −1 1,1 line of formaldehyde at 4.8 GHz is sensitive to the electron fraction and its excitation temperature is predicted to be lower than the CMB at low and moderate electron fractions ( x ( e ) < 6 × 10 −5 ), and to rise above the CMB at high electron fractions ( x ( e ) > 10 −4 ).